Unit 4Unit 4 Unit 4 Genetics Advance Planning file257 Unit Projects Unit Projects Unit 4 Advance Planning Chapter 10 Purchase the following seed types from a biological supply house

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Unit Projects

Unit ProjectsUnit ProjectsUnit Projects

Unit 4Unit 4Advance Planning Chapter 10 ■ Purchase the following seed

types from a biological supplyhouse for the BioLab: purebreeding green tobacco seedsand green-albino tobacco seedmixed in 3:1 ratio (heterozy-gous parents).

■ Purchase Wisconsin FastPlants for the Project and theTech Prep activity.

Chapter 11none

Chapter 12■ Order Brassica rapa seeds for

the BioLab.■ Obtain photos of a red short-

horn bull, a white shorthorncow, and a roan shorthorn cowfor the Discussion.

■ Collect articles and pamphletson genetic disorders for theDisplay.

■ Order tobacco seeds forAlternative Lab 12-1.

■ Obtain slides of normal andsickled blood cells for theMicroscope Activity.

■ Obtain slides of male andfemale body cells for Alter-native Lab 12-2.

Chapter 13■ Order the Chromosome Sim-

ulation Biokit for the QuickDemo in Section 13.2.

■ Purchase cloning kits for theExtension in Section 13.3.

■ Obtain a slide of female cheekcells for the Alternative Lab.

Using the LibraryIntrapersonal Research one diseasecurrently being treated by gene

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InterviewLinguistic Interview someone witha family pedigree that demon-

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Final ReportHave student groups present their find-ings about genetics in reports that couldbe presented to students at your localmiddle school. L3

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GeneticsGeneticsPhysical traits, such as the colors of thesesnapdragons, are encoded in small segmentsof a chromosome called genes, which arepassed from one generation to the next. Bystudying the inheritance pattern of a traitthrough several generations, the probabilitythat future offspring will express that traitcan be predicted.

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UNIT CONTENTSUNIT CONTENTS

Mendel and Meiosis

DNA and Genes

Patterns of Heredity andHuman Genetics

Genetic Technology

UNIT PROJECTUNIT PROJECT

Use the Glencoe Science Web Site for more project

activities that are connected to this unit.www.glencoe.com/sec/science

Unit 4Unit 4

GeneticsBIODIGESTBIODIGEST

Unit ProjectsUnit ProjectsUnit Projects

Unit Projects

Unit 4Unit 4

256

Genetics

Unit OverviewUnit 4 presents an overview ofgenetics and its role in determin-ing the traits of organisms.Chapter 10 introduces geneticsthrough a short historical presen-tation of the work of GregorMendel. Meiosis is then intro-duced and discussed. In Chapter11, students learn about thestructure of DNA and how it isreplicated. The processes of tran-scription and translation areexplained. Various kinds of muta-tions are described. In Chapter12, students examine non-Mendelian patterns of heredityand the principles of genetics asapplied to humans. Finally, selec-tive breeding, DNA technology,and the Human Genome Projectare discussed in Chapter 13.

Introducing the UnitHave students look at the pictureof the snapdragons and describethe traits they see. Then havethem look around the room anddescribe the traits they see ineach other. Just as the snapdrag-ons are all the same kind of plantwith different colored flowers, soare all the students alike, yet withdifferent traits. Unit 4 will dis-cuss how genes determine anorganism’s physical traits, as wellas how those traits are inherited.

Human GeneticsHave students do one of the projects forthis unit as described on the GlencoeScience Web Site. As an alternative, stu-dents can do one of the projectsdescribed on these two pages.

Make a GraphLogical-Mathematical Measure theheight of every student in the class

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ModelingKinesthetic Create several chro-matids by tying two socks together.

Now model what happens to them dur-ing meiosis.

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http://www.glencoe.com/sec/science

258B

Mendel and MeiosisMendel and Meiosis

TransparenciesReproducible MastersSection

Mendel’s Laws of Heredity

Meiosis

Section 10.1

Section 10.2

Teacher Classroom Resources

Reinforcement and Study Guide, pp. 43-44Critical Thinking/Problem Solving, p. 10BioLab and MiniLab Worksheets, p. 43Tech Prep Applications, pp. 17-18Content Mastery, pp. 49-50, 52

Reinforcement and Study Guide, pp. 45-46Concept Mapping, p. 10BioLab and MiniLab Worksheets, pp. 45-48Laboratory Manual, pp. 69-74Content Mastery, pp. 49, 51-52 L1

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Section Focus Transparency 24Basic Concepts Transparency 14Reteaching Skills Transparency 16

Section Focus Transparency 25Basic Concepts Transparency 15Reteaching Skills Transparency 17

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LSAssessment Resources Additional Resources

Spanish ResourcesEnglish/Spanish AudiocassettesCooperative Learning in the Science ClassroomLesson Plans/Block Scheduling

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Chapter Assessment, pp. 55-60MindJogger VideoquizzesPerformance Assessment in the Biology ClassroomAlternate Assessment in the Science ClassroomComputer Test BankBDOL Interactive CD-ROM, Chapter 10 quiz

Chapter 10 OrganizerChapter 10 Organizer

Activities/FeaturesObjectivesSection

Mendel’s Laws of HeredityNational Science EducationStandards UCP.1-3, UCP.5;A.1, A.2; G.1-3 (3 sessions, 2 blocks)

MeiosisNational Science EducationStandards UCP.1-3; C.1,C.2; E.1, E.2; F.6; G.1-3(3 sessions, 2 blocks)

1. Analyze the results obtained by GregorMendel in his experiments with gardenpeas.

2. Predict the possible offspring of agenetic cross by using a Punnett square.

3. Analyze how meiosis maintains a con-stant number of chromosomes within aspecies.

4. Infer how meiosis leads to variation in aspecies.

5. Relate Mendel’s laws of heredity to theevents of meiosis.

MiniLab 10-1: Looking at Pollen, p. 260Problem-Solving Lab 10-1, p. 268Internet BioLab: How can phenotypes andgenotypes of plants be determined? p. 280Math Connection: A Solution from Ratios,p. 282

Problem-Solving Lab 10-2, p. 270MiniLab 10-2: Modeling Crossing Over, p. 274Inside Story: Genetic Recombination, p. 277

Section 10.2

Section 10.1

MATERIALS LIST

BioLabp. 280 potting soil, small flowerpotsor seedling flats, 2 groups of tobaccoseeds, hand lens, light source, ther-mometer, plant watering bottle

MiniLabsp. 260 flower, microscope, microscopeslide, coverslip, dropper, waterp. 274 clay, twist ties (2), pencil

Alternative Labp. 272 9 sheets of unlined paper orposter board (30-cm square), longlength of yarn, paper clips, string,toothpicks, tape or glue, scissors

Quick Demosp. 265 black beans (300), white beans(100), paper cupsp. 272 none

Need Materials? Contact Carolina Biological Supply Company at 1-800-334-5551or at http://www.carolina.com

258A

Refer to pages 4T-5T of the Teacher Guide for an explanation of the National Science Education Standards correlations.

Key to Teaching StrategiesKey to Teaching Strategies

Level 1 activities should be appropriatefor students with learning difficulties.Level 2 activities should be within theability range of all students.Level 3 activities are designed for above-average students.ELL activities should be within the abilityrange of English Language Learners.

Cooperative Learning activitiesare designed for small group work.These strategies represent student prod-ucts that can be placed into a best-workportfolio.These strategies are useful in a blockscheduling format.

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The following multimedia resources are available from Glencoe.

Biology: The Dynamics of LifeCD-ROM

Exploration: Trait InheritanceAnimation: Meiosis

Videodisc ProgramMeiosis

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The Infinite VoyageThe Geometry of Life

The Secret of Life SeriesHeredity in Mendel’s PeasDominant vs. RecessiveSegregationSex and the Single Gene: Cell DevelopmentIndependent AssortmentMeiosis Ia, Ib, IIa, IIb

http://www.carolina.com

Section

Why Mendel SucceededGregor Mendel carried out the

first important studies of heredity,the passing on of characteristics from parents to offspring. Althoughpeople had noticed for thousands ofyears that family resemblances wereinherited from generation to genera-tion, a complete explanation requiredthe careful study of genetics—the branch of biology that studiesheredity. Characteristics that areinherited are called traits. Mendelwas the first person to succeed inpredicting how traits would be trans-ferred from one generation to thenext. How was he able to solve thisproblem of heredity?

Mendel chose his subject carefully

Mendel studied many plantsbefore deciding to use the garden peain his experiments. Garden pea plantsreproduce sexually, which means theyhave two distinct sex cells—male andfemale. Sex cells are called gametes.

In peas, both male and femalegametes are in the same flower. Themale gamete is in the pollen grain,which is produced by the anther. Thefemale gamete is in the ovule, which islocated in the pistil. The transfer ofthe male pollen grains to the pistil of aflower is called pollination. The unit-ing of male and female gametes, in aprocess called fertilization, occurs

10.1 MENDEL’S LAWS OF HEREDITY 259

An Austrian monastery in the mid-nineteenth century might seem an

unusual place to begin your searchfor the answer to why offspringresemble their parents. Yet, it wasin this community of scholars thatyoung Gregor Mendelbegan to breed gardenpea plants so that hecould study theinheritance of theircharacteristics.

SECTION PREVIEW

ObjectivesAnalyze the resultsobtained by GregorMendel in his experi-ments with gardenpeas.Predict the possible offspring of a geneticcross by using a Punnettsquare.

Vocabularyhereditygeneticstraitgametepollinationfertilizationhybridalleledominantrecessivelaw of segregationphenotypegenotypehomozygousheterozygouslaw of independent

assortment

10.1 Mendel’s Laws of Heredity

OriginWORDWORD

heredity From the Latinword hered-, mean-ing “heir.” Hereditydescribes thegenetic qualitiesyou receive fromyour ancestors.

Gregor Mendeland pea plants

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Section 10.1

BIOLOGY: The Dynamics of Life SECTION FOCUS TRANSPARENCIES

Use with Chapter 10,Section 10.1

What possible combinations can result from combiningone coin from each group?

What is the ratio of the possible combinations?

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Transparency PredictingCombinations24

Group 1

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PrepareKey ConceptsStudents are led through mono-and dihybrid crosses, applyingMendel’s laws of segregation andindependent assortment.

Planning■ Purchase smooth and wrinkled

peas for the Getting StartedDemo.

■ Purchase fresh or preservedflowers for MiniLab 10-1.

■ Gather soil and pots for theBioLab, the Project, and theTech Prep activity. Locatethermometers, hand lenses,and possible light banks.

■ Purchase black and whitebeans for the Quick Demo.

■ Gather candy and buttons forthe Reinforcement.

1 FocusBellringer Before presenting the lesson, display Section Focus Trans-parency 24 on the overhead pro-jector and have students answerthe accompanying questions.

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Assessment PlannerAssessment PlannerPortfolio Assessment

Assessment, TWE, p. 262 Portfolio, TWE, pp. 264, 274, 275 BioLab, TWE, p. 281

Performance AssessmentMiniLabs, TWE, pp. 260, 274Problem-Solving Lab, TWE, p. 270Assessment, TWE, p. 276 MiniLabs, SE, pp. 260, 274BioLab, SE, p. 281

Alternative Lab, TWE, p. 272-273Knowledge Assessment

Assessment, TWE, p. 267 Alternative Lab, TWE, p. 272-273Section Assessments, SE, p. 268, 279 Chapter Assessment, SE, p. 283-285

Skill AssessmentProblem-Solving Lab, TWE, p. 268Assessment, TWE, p. 279

258 MENDEL AND MEIOSIS

Mendel and Meiosis

What You’ll Learn■ You will identify the basic

concepts of genetics.■ You will examine the process

of meiosis.

Why It’s ImportantGenetics explains why you haveinherited certain traits fromyour parents. If you understandhow meiosis occurs, you can seehow these traits were passedon to you.

Traits of PeasExamine a pea seed. How manytraits can you identify? Make alist of these traits.

To find outmore about

genetics, visit the GlencoeScience Web Site.www.glencoe.com/sec/science

10

GETTING STARTEDGETTING STARTED

ChapterChapter

Organisms usuallyresemble their parentsbecause they inheritcertain characteristicsfrom them. Thesecharacteristics, alsocalled traits, are de-termined by geneticinformation on chromo-somes such as thoseshown in the inset photo.

258

Theme DevelopmentThe theme of nature of scienceis developed within the chapter asMendel’s laws of segregation andindependent assortment areexplained. Mendel’s laws areshown to be supported by currentknowledge of meiosis. The themeof homeostasis is illustrated bythe knowledge that diploid chro-mosome numbers are maintainedwhen gametes join at fertiliza-tion. The gametes are formed asthe result of meiosis, and theirchromosome numbers are halfthe diploid number.

Chapter 10Chapter 10

MultipleLearningStyles

Look for the following logos for strategies that emphasize different learning modalities.

Kinesthetic Meeting IndividualNeeds, pp. 260, 271; Tech Prep,

p. 265; Reinforcement, p. 266; QuickDemo, p. 272

Visual-Spatial Getting StartedDemo, p. 258; Quick Demo,

p. 265; Meeting Individual Needs, pp. 265, 267; Biology Journal, p. 266;

Activity, p. 268; Portfolio, p. 274;Visual Learning, p. 277

Intrapersonal Meeting Indivi-dual Needs, p. 260Linguistic Enrichment, p. 261;Portfolio, pp. 264, 275; Biology

Journal, p. 276; Tech Prep, p. 278Logical-Mathematical ConceptDevelopment, p. 266

GETTING STARTED DEMOGETTING STARTED DEMO

Visual-Spatial Show stu-dents wrinkled and

smooth pea seeds. Have stu-dents compare and contrastthe physical traits of the peas.Encourage them to speculateabout less obvious traits suchas biochemical or physiologicalcharacteristics. The wrinklednature of the seeds is due to alack of well-formed starchgrains and lower water reten-tion.

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If time does not permit teaching the entire chapter,use the BioDigest at the endof the unit as an overview.


he worked with were from popula-tions of plants that had been tall formany generations and had always pro-duced tall offspring. Such plants aresaid to be true breeding for tallness.Likewise, the short plants he workedwith were true breeding for shortness.

Mendel’s MonohybridCrosses

What did Mendel do with the talland short pea plants he so carefullyselected? He crossed them to producenew plants. Mendel referred to theoffspring of this cross as hybrids. Ahybrid is the offspring of parents thathave different forms of a trait, such astall and short height. Mendel’s firstexperiments are called monohybridcrosses because mono means “one”and the two parent plants differed bya single trait—height.

The first generationMendel selected a six-foot-tall pea

plant that came from a population ofpea plants, all of which were over sixfeet tall. He cross-pollinated this tallpea plant with a short pea plant thatwas less than two feet tall and whichcame from a population of pea plantsthat were all short. When he plantedthe seeds from this cross, he foundthat all of the offspring grew to be astall as the taller parent. In this firstgeneration, it was as if the shorterparent had never existed!

The second generationNext, Mendel allowed the tall

plants in this first generation to self-pollinate. After the seeds formed, heplanted them and counted more than1000 plants in this second genera-tion. Mendel found that three-fourths of the plants were as tall asthe tall plants in the parent and firstgenerations. He also found that one-


Figure 10.2 When Mendel crossed true-breeding tallpea plants with true-breeding short peaplants, all the offspring were tall. When heallowed first-generation tall plants to self-pollinate, three-fourths of the offspringwere tall and one-fourth were short.

fourth of the offspring were as shortas the short plants in the parent gen-eration. In other words, in the secondgeneration, tall and short plantsoccurred in a ratio of approximatelythree tall plants to one short plant,Figure 10.2. The short trait hadreappeared as if from nowhere!

The original parents, the true-breeding tall and short plants, areknown as the P1 generation. The Pstands for "parent." The offspring ofthe parent plants are known as the F1

Short pea plant

All tall pea plants

3 tall :1 short

Tall pea plant

F2

F1

P1

261

when the male gamete in the pollengrain meets and fuses with the femalegamete in the ovule. After the ovuleis fertilized, it matures into a seed.

The reproductive parts of the peaflower are tightly enclosed in petals,preventing the pollen of other flow-ers from entering. As a result, peasnormally reproduce by self-pollina-tion; that is, the male and femalegametes come from the same plant.In many of Mendel’s experiments,this is exactly what he wanted. Whenhe needed to breed—or cross—oneplant with another, Mendel openedthe petals and removed the anthersfrom a flower, Figure 10.1a. He thendusted the pistil with pollen from theplant he wished to cross it with,Figure 10.1b, and covered the flowerwith a small bag to prevent pollen inthe air from landing on the pistil.This process is called cross-pollina-tion. By using this technique, Mendelcould be sure of the parents in hiscross. You can observe anthers andtheir pollen grains in the MiniLab onthis page.

Mendel was a careful researcherMendel carefully controlled his

experiments and the peas he used. Hestudied only one trait at a time to con-trol variables, and he analyzed his datamathematically. The tall pea plants


Looking at Pollen Pollengrains are formed within themale anthers of flowers. Whatis their role? Pollen containsthe male gametes or spermcells needed for fertilization.This means that pollen grainscarry the hereditary unitsfrom male parent plants tofemale parent plants. Thepollen grains that Mendeltransferred from the antherof one pea plant to the pistil of another plant carried the hereditary traits that he so carefully observed in the next generation.

Procedure! Examine a flower. Using the diagram as a guide, locate

the stamens of your flower. There are usually several sta-mens in each flower.

@ Remove one stamen and locate the enlarged end—theanther.

# Add a drop of water to a microscope glass slide. Place theanther in the water. Add a coverslip. Using the eraser endof a pencil, tap the coverslip several times to squash theanther.

$ Observe under low power. Look for numerous small roundstructures. These are pollen grains.

Analysis1. Provide an estimate of the number of pollen grains pres-

ent in an anther.2. Describe the appearance of a single pollen grain.3. Explain the role of pollen grains in plant reproduction.

MiniLab 10-1MiniLab 10-1 Observing and Inferring

Removeanthers

(male part)

TransferPollenPistil

(female part)

a b

Anthers

Cross-pollination

Pollengrains

Figure 10.1 In his experiments,Mendel often had totransfer pollen fromone plant to anotherplant with differenttraits. This is calledmaking a cross.

Pistil

Anther

Stamens

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2 Teach

Purpose Students will observe pollengrains in plants.

Process Skillsobserve and infer

Teaching Strategies■ Preserved or fresh flowermaterial may be used. Preservedmaterial is available from biologi-cal supply houses.■ Have students wash theirhands after handling flowers andflower parts.■ Forceps may be used to re-move stamens from the flower. ■ If coverslips or slides breakduring the squashing process,have students place them in acontainer for broken glass andstart over with new material.

Expected ResultsStudents will observe that eachanther contains thousands ofsmall pollen grains.

Analysis1. Numbers should be in the

several thousands.2. Student answers will vary

depending on species used.Pollen grains are microscopiccells, often round in shape.

3. Pollen grains contain themale reproductive cellsneeded for fertilization. Theyprovide the chromosomesand genes of the male parent.

Performance Have stu-dents make a diagram of severalpollen cells under high power.Ask them to determine the size ofpollen cells in micrometers.Pollen size should be included on their diagrams. If students havenot done any previous measuring,refer them to MiniLab 7-1. Usethe Performance Task AssessmentList for Using Math in Science inPASC, p. 29. L2

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AssessmentAssessment

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MiniLab 10-1MiniLab 10-1

MMEETING EETING IINDIVIDUAL NDIVIDUAL NNEEDS EEDS MEETING INDIVIDUAL NEEDS

Visually Impaired Kinesthetic Obtain a large flowermodel. Allow those students who are

visually impaired to handle the model. Direct their attention to the stamen and pistil. Then, have students examine an actual flower after having studied themodel.

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Gifted Intrapersonal Have students do re-search to determine how to prepare a

solution for pollen tube germination. Havethem prepare the solution and watch pollentube germination under the microscope.Lilies and members of the mustard familyare good choices for this activity.

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VIDEODISCThe Secret of LifeHeredity in

Mendel’s Peas

The Infinite Voyage: TheGeometry of Life, Introduction3 min. 30 sec.

CD-ROMBiology: The Dynamicsof Life

Exploration: Trait InheritanceDisc 2

Genetic Influence of IdenticalTwins (Ch. 1), 5 min. 30 sec.

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EnrichmentLinguistic Some researchershave doubted Mendel’s hon-

esty, despite the accuracy of hisconclusions. One claim is thatMendel deliberately excludeddata from traits that do not inde-pendently assort because he hadonly one chance in 6000 of ran-domly selecting one gene on eachof seven chromosomes. Anotherclaim is that Mendel fabricatedsome of his data in order to makethem fit expected ratios.Countering these claims areother analyses of Mendel’s datathat concluded he actually had aone in three chance of choosingonly traits that independentlyassort, and a suggestion thatMendel intended to presentsomething like a demonstrationrather than an exact experiment.Have interested students researchthis topic and draw their ownconclusions in a report for theirportfolios. L3

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Resource ManagerResource ManagerSection Focus Transparency 24 and

MasterBioLab and MiniLab Worksheets,

p. 42Basic Concepts Transparency 14 and

Master

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in Mendel’s pea plants, the allele fortall plants is dominant to the allelefor short plants. Plants that had oneallele for tallness and one for short-ness were tall because the allele fortallness is dominant to the allele forshortness. Expressed another way,the allele for short plants is recessiveto the allele for tall plants. Pea plantsthat had two alleles for tallness weretall, and those that had two alleles forshortness were short. You can see inFigure 10.4 how the rule of domi-nance explained the resulting F1 gen-eration.

When recording the results ofcrosses, it is customary to use thesame letter for different alleles of thesame gene. An uppercase letter isused for the dominant allele, and alowercase letter for the recessiveallele. The dominant allele is alwayswritten first. So the allele for tallnessis written as T, and the allele forshortness as t, as it is in Figure 10.4.

The law of segregationNow recall the results of Mendel’s

cross between F1 tall plants, when thetrait of shortness reappeared. Toexplain this result, Mendel formu-lated the first of his two laws ofheredity. He concluded that each tallplant in the F1 generation carriedone dominant allele for tallness andone unexpressed recessive allele forshortness. It received the allele fortallness from its tall parent and theallele for shortness from its shortparent in the P1 generation. Becauseeach F1 plant has two different al-leles, it can produce two differenttypes of gametes— “tall” gametesand “short” gametes. During fertil-ization, these gametes randomly pairto produce four combinations of alleles. This conclusion, illustrated inFigure 10.5 on the next page, iscalled the law of segregation.

Figure 10.4 The rule of dominance explains the resultsof Mendel’s cross between P1 tall and shortplants (a). Tall pea plants are about six feettall, whereas short plants are less than twofeet tall (b).

Tall plant

a

All tall plantsT t

Short plantT T t

tT

t

F1

b

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round yellow purpleaxial(side) green inflated tall

wrinkled green whiteterminal

(tips) yellow constricted short

Seedshape

Dominanttrait

Recessivetrait

Seedcolor

Flowercolor

Flowerposition

Podcolor

Podshape

Plantheight

generation. The F stands for “filial”—son or daughter. When you cross twoF1 plants with each other, their off-spring are called the F2 generation—the second filial generation. Youmight find it easier to understandthese terms if you look at your ownfamily. Your parents are the P1 gener-ation. You are the F1 generation, andany children you might have in thefuture would be the F2 generation.

Mendel did similar monohybridcrosses with a total of seven pairs oftraits, studying one pair of traits at atime. These pairs of traits are shownin Figure 10.3. In every case, hefound that one trait of a pair seemedto disappear in the F1 generation,only to reappear unchanged in one-fourth of the F2 plants.

The rule of unit factorsMendel concluded that each

organism has two factors that controleach of its traits. We now know that

these factors are genes and that theyare located on chromosomes. Genesexist in alternative forms. We callthese different gene forms alleles (uh LEELZ). For example, each ofMendel’s pea plants had two alleles ofthe gene that determined its height.A plant could have two alleles fortallness, two alleles for shortness, orone allele for tallness and one forshortness. An organism’s two allelesare located on different copies of achromosome—one inherited fromthe female parent and one from themale parent.

The rule of dominance Remember what happened when

Mendel crossed a tall P1 plant with ashort P1 plant? The F1 offspringwere all tall. In other words, only onetrait was observed. In such crosses,Mendel called the observed traitdominant and the trait that disap-peared recessive. We now know that


OriginWORDWORD

allele From the Greekword allelon, mean-ing "of each other."Genes exist in alter-native forms calledalleles.

Figure 10.3 Mendel chose seventraits of peas for hisexperiments. Eachtrait had two clearlydifferent forms; nointermediate formswere observed.

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P R O J E C TExperimental CrossesSeeds of a plant called Brassica rapa areavailable from Carolina Biological SupplyCompany under the name of Wisconsin FastPlants. These plants, grown from seed, com-plete their life cycle in 30 to 35 days. Theyare ideal for use in the classroom becausewithin this short time, the plants flower and

form seeds for the next generation. Geneticstudies can be carried out using differenttraits, such as petal-less flowers or hairystems. Students will find it easy to carry outthe pollinating between plants. These plantsare an ideal experimental organism forgenetic studies.

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VIDEODISCThe Secret of LifeDominant vs. Recessive

Segregation

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Revealing MisconceptionsPoint out to students that thedominant trait is not necessarilythe more common or desirabletrait. For example, in humans,Huntington’s disease and hyper-cholesterolemia (having danger-ously high levels of bloodcholesterol) are both dominanttraits and both are rare.

Visual LearningFigure 10.4 Have students writeout the three important writtenconventions that are describedwith this diagram. Use the sameletter for different alleles of the samegene; use uppercase letters for domi-nant alleles and lowercase letters forrecessive alleles; and always write thedominant allele first. L1

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Note Internet addresses that you find useful in the spacebelow for quick reference.

Portfolio Have studentsimagine that they are GregorMendel and that they have justformulated the laws of dominanceand segregation. Have them writea short article describing theirfindings for the only science jour-nal in existence. They shouldremember that the year is 1863.

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P1

F1

F2

Round yellow Wrinkled green

All roundyellow

9Roundyellow

3Roundgreen

3Wrinkledyellow

1Wrinkled

green

Dihybrid cross round yellow ×× wrinkled green

homozygous recessive for the trait ofheight.

An organism is heterozygous (hetuh roh ZI gus) for a trait if its twoalleles for the trait differ from eachother. Therefore, the tall plant thathad one allele for tallness and oneallele for shortness (Tt) is heterozy-gous for the trait of height.

Now look at Figure 10.5 again.Can you identify the phenotype andgenotype of each plant? Is eachhomozygous or heterozygous? Youcan practice determining genotypesand phenotypes in the BioLab at theend of this chapter.

Mendel’s DihybridCrosses

Mendel performed another set ofcrosses in which he used peas thatdiffered from each other in two traitsrather than only one. Such a crossinvolving two different traits is calleda dihybrid cross because di means“two.” In a dihybrid cross, will thetwo traits stay together in the nextgeneration or will they be inheritedindependently of each other?

The first generationMendel took true-breeding pea

plants that had round yellow seeds(RRYY) and crossed them with true-breeding pea plants that had wrinkledgreen seeds (rryy). He already knewthat when he crossed plants that pro-duced round seeds with plants thatproduced wrinkled seeds, all theplants in the F1 generation producedseeds that were round. In otherwords, just as tall plants were domi-nant to short plants, the round-seeded trait was dominant to thewrinkled-seeded trait. Similarly,when he crossed plants that producedyellow seeds with plants that pro-duced green seeds, all the plants

in the F1 generation produced yel-low seeds—yellow was dominant.Therefore, Mendel was not surprisedwhen he found that the F1 plants ofhis dihybrid cross all had the twodominant traits of round and yellowseeds, as Figure 10.6 shows.

The second generationMendel then let the F1 plants pol-

linate themselves. As you mightexpect, he found some plants thatproduced round yellow seeds andothers that produced wrinkled greenseeds. But that’s not all. He alsofound some plants with round greenseeds and others with wrinkled yel-low seeds. When Mendel sorted andcounted the plants of the F2 genera-tion, he found they appeared in a def-inite ratio of phenotypes—9 roundyellow: 3 round green: 3 wrinkledyellow: 1 wrinkled green. To explainthe results of this dihybrid cross,Mendel formulated his second law.


OriginWORDWORD

heterozygous From the Greekwords heteros,meaning “other,”and zygotos, mean-ing “joinedtogether.” A trait isheterozygous whenan individual hastwo different allelesfor that trait.

Figure 10.6 When Mendel crossed true-breeding plants with round yellowseeds and true-breeding plants with wrinkled green seeds, theseeds of all the offspring were round and yellow. When the F1plants were allowed to self-pollinate, they produced four different kinds of plants in the F2 generation.

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Phenotypes andGenotypes

Mendel showed that tall plants arenot all the same. Some tall plants,when crossed with each other, yieldedonly tall offspring. These wereMendel’s original P1 true-breedingtall plants. Other tall plants, whencrossed with each other, yielded bothtall and short offspring. These werethe F1 tall plants in Figure 10.5 thatcame from a cross between a tallplant and a short plant.

Two organisms, therefore, canlook alike but have different underly-ing gene combinations. The way anorganism looks and behaves makesup its phenotype (FEE nuh tipe). Thephenotype of a tall plant is tall,regardless of the genes it contains.

The gene combination an organismcontains is known as its genotype(JEE nuh tipe). The genotype of a tallplant that has two alleles for tallnessis TT. The genotype of a tall plantthat has one allele for tallness andone allele for shortness is Tt.

You can see that you can’t alwaysknow an organism’s genotype simplyby looking at its phenotype. Anorganism is homozygous (hoh muhZI gus) for a trait if its two alleles forthe trait are the same. The true-breeding tall plant that had two al-leles for tallness (TT) would behomozygous for the trait of height.Because tallness is dominant, a TTindividual is homozygous dominantfor that trait. A short plant wouldalways have two alleles for shortness(tt). It would, therefore, always be


OriginWORDWORD

phenotype From the Greekwords phainein,meaning “to show,”and typos, meaning“model.” The visi-ble characteristics ofan organism makeup its phenotype.

genotype From the Greekwords gen or geno,meaning “race,”and typos, meaning“model.” Thegenetic characteris-tics of an organismmake up its geno-type.

Tall plant

3 1

TallT T

TallT t

TallT t

Shortt t

Tall plant

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Law of segregation Tt ×× Tt cross

T t T t

Figure 10.5 Mendel’s law of segrega-tion explains the results ofhis cross between F1 tallplants. He concluded thatthe two alleles for eachtrait must separate whengametes are formed. Aparent, therefore, passeson at random only oneallele for each trait to eachoffspring.

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Using Science TermsIt is sometimes easier for studentsto remember the meaning of theterm genotype by reversing theword so it becomes “type ofgene.” When stated this way, stu-dents can associate the term geno-type with its definition. Phenomeans “to show.” Thus, the phe-notype shows the type of trait orhow it appears.

Concept Development■ Ask students to supply the cor-

rect term—genotype or phe-notype—to the followingexamples: (a) LL, (b) blondhair, (c) dimpled chin, (d) blueeyes, (e) Dd, (f) ss, (g) whiteand green leaves. a, e, and f aregenotypes; b, c, d, and g are phe-notypes.

■ Have students provide the fol-lowing information for thisexample: G = green pea pod, g = yellow pea pod. (a) Givethe phenotypes of plants withthese genotypes: Gg, GG, andgg. (b) Use the terms homozy-gous or heterozygous to describeeach of the three examplesabove. (a) green, green, yellow;(b) heterozygous, homozygous,homozygous.

Selective BreedingKinesthetic The selectivebreeding of plants is

crucial to agriculture. Studentscan try this for themselves bygrowing Wisconsin Fast Plantsand selecting for a desiredtrait. For example, students can select for “hairiness” andby crossing two hairy plants,produce offspring that willhave even more hairs. Duringthis activity, students will haveto plant and care for theirplants as well as perform thecross-pollinations.

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Quick DemoQuick Demo

Visual-Spatial Mix 300black beans with 100

white beans. Give each studenta small cup or bag of beans.Explain that the color of thebeans represents an inheritedtrait. Ask students to speculateas to which trait is dominantand what the genotypes forcolor would be for the beans.The black color is dominant andthe black beans would be eitherBB or Bb. The white beanswould be bb.

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Learning Disabled/EnglishLanguage Learners

Visual-Spatial To help students predictgenes within each gamete for a dihybrid

cross and to illustrate the law of independentassortment, provide them with the followingmnemonic: 1 and 3 and 1 and 4, 2 and 3 and 2and 4. Have students write the numbers 1-4across the top of a set of parental alleles in a

dihybrid cross. It would look like this.1 2 3 4R r Y y

To obtain the correct gamete combinationsaccording to the law of independent assort-ment, students should match alleles 1 and 3(RY), 1 and 4 (Ry), then match 2 and 3 (rY), 2and 4 (ry).

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VIDEODISCVIDEOTAPEThe Secret of Life

Sex and the Single Gene: CellDevelopment

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Martian Traits Linguistic Have students imagine thatthey have encountered their first

Martian. Assume that Martian traits areinherited exactly the same way as Earthling

traits. Have students provide examples offive Martian traits through the use of a dia-gram and a written statement describing thepossible genotypes and phenotypes for thesefive traits.

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The BioLab at theend of the chaptercan be used at thispoint in the lesson.

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Reteaching Skills Trans-parency 16 and Master

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phenotypes. Looking at the Punnettsquare for this cross in Figure 10.7B,you can see that three-fourths of theoffspring are expected to be tallbecause they have at least one domi-nant allele. One-fourth are expectedto be short because they lack a domi-nant allele. Of the tall offspring, one-third will be homozygous dominant(TT) and two-thirds will be heterozy-gous (Tt). Note that whereas thegenotype ratio is 1TT: 2Tt: 1tt, thephenotype ratio is 3 tall: 1 short. Youcan practice doing calculations suchas Mendel did in the Math Connectionat the end of this chapter.

Dihybrid crossesWhat happens in a Punnett square

when two traits are considered?Think again about Mendel’s crossbetween pea plants with round yel-low seeds and plants with wrinkledgreen seeds. All the F1 plants pro-duced seeds that were round and yel-low and were heterozygous for eachtrait (RrYy). What kind of gameteswill these F1 plants form?

Mendel explained that the traitsfor seed shape and seed color wouldbe inherited independently of eachother. This means that each F1 plantwill produce gametes containing thefollowing combinations of genes withequal frequency: round yellow (RY),round green (Ry), wrinkled yellow(rY), and wrinkled green (ry). APunnett square for a dihybrid crosswill then need to be four boxes oneach side for a total of 16 boxes, asFigure 10.8 shows.

ProbabilityPunnett squares are good for

showing all the possible combina-tions of gametes and the likelihoodthat each will occur. In reality, how-ever, you don’t get the exact ratio of

results shown in the square. That’sbecause, in some ways, genetics islike flipping a coin—it follows therules of chance.

When you toss a coin, it landseither heads up or tails up. The prob-ability or chance that an event willoccur can be determined by dividingthe number of desired outcomes bythe total number of possible out-comes. So the probability of gettingheads when you toss a coin would beone in two chances, written as 1: 2 or1/2. A Punnett square can be used todetermine the probability of getting apea plant that produces round seedswhen two plants that are heterozygous(Rr) are crossed. Because this Punnett


Gametes from RrYy parentRY

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Ry rY ry

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Punnett Square of Dihybrid Cross

Gam

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round yellow

round green

wrinkled yellow

wrinkled green

F1 cross: RrYy × RrYyFigure 10.8 A Punnett square for a dihybrid crossbetween heterozygous pea plants withround yellow seeds shows clearly thatthe offspring fulfill Mendel’s observedratio of 9 round yellow: 3 round green: 3wrinkled yellow: 1 wrinkled green.

Visual LearningFigure 10.8 Ask students howmany different genotypes andphenotypes resulted from thiscross? 9 genotypes and 4 phenotypes

3 AssessCheck for UnderstandingHave students describe the rela-tionship between or among thefollowing terms.

a. pollination—fertilization b. allele—dominant—recessive c. genotype—phenotype d. homozygous—heterozygous e. monohybrid—dihybrid

ReteachHave students provide an exam-ple of each relationship providedin Check for Understanding.

ExtensionHave students list the genotypesand phenotypes resulting from (a)an RrYy plant cross-pollinated byan RRyy plant; (b) an rrYy plantcross-pollinated by a RrYy plant.(a) genotypes: RrYy, RRyy, RRYy,Rryy; phenotypes: 1/2 round yellow,1/2 round green; (b) genotypes:RrYY; RrYy, Rryy, rrYY, rrYy,rryy; phenotypes: 3 round yellow; 3wrinkled yellow; 1 round green; 1wrinkled green

Knowledge Have studentsexplain what each of the followingrepresents in a Punnett square: (a)the letters written at the top andside of the square; (b) the letterswritten within each box; (c) theboxes. (a) gametes; (b) genotype ofan offspring; (c) possible offspringP

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Heterozygoustall parent

Heterozygoustall parent

T

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The law of independent assortment

Mendel’s second law states thatgenes for different traits—for exam-ple, seed shape and seed color—areinherited independently of eachother. This conclusion is known asthe law of independent assortment.When a pea plant with the genotypeRrYy produces gametes, the alleles Rand r will separate from each other(the law of segregation) as well asfrom the alleles Y and y (the law ofindependent assortment), and viceversa. These alleles can then recom-

bine in four different ways. If thealleles for seed shape and color wereinherited together, only two kinds ofpea seeds would have been produced:round yellow and wrinkled green.

Punnett SquaresIn 1905, Reginald Punnett, an

English biologist, devised a short-hand way of finding the expectedproportions of possible genotypes inthe offspring of a cross. This methodis called a Punnett square. It takesaccount of the fact that fertilizationoccurs at random, as Mendel’s law ofsegregation states. If you know thegenotypes of the parents, you can usea Punnett square to predict the possi-ble genotypes of their offspring.

Monohybrid crossesConsider the cross between two F1

tall pea plants, each of which has thegenotype Tt. Half the gametes ofeach parent would contain the Tallele, and the other half would con-tain the t allele. A Punnett square forthis cross is two boxes tall and twoboxes wide because each parent canproduce two kinds of gametes for thistrait. The two kinds of gametes fromone parent are listed on top of thesquare, and the two kinds of gametesfrom the other parent are listed onthe left side, Figure 10.7A. It doesn’tmatter which set of gametes is on topand which is on the side, that is,which parent contributes the T andwhich contributes the t. Refer to thePunnett square in Figure 10.7B todetermine the possible genotypes ofthe offspring. Each box is filled inwith the gametes above and to theleft side of that box. You can see thateach box then contains two alleles—one possible genotype.

After the genotypes have beendetermined, you can determine the


Figure 10.7 This Punnett square predicts the resultsof a monohybrid cross between twoheterozygous tall pea plants.

T

TT Tt

Tt

T

t

t tt

You can see thatthere are three different possiblegenotypes—TT, Tt,and tt—and that Tt can result fromtwo different combinations.

BB

The gametes thateach parent formsare listed on thetop and left sideof the Punnettsquare.

AA

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Concept DevelopmentLogical-Mathematical Havestudents construct Punnett

squares and solve these problems,giving the genotypic and pheno-typic ratios expected. (a) Homo-zygous tall plant bred to ahomozygous short plant. All off-spring will be tall and heterozygous.(b) Homozygous tall plant bredto a heterozygous tall plant. Alloffspring will be tall, half being TTand half being Tt. (c) Hetero-zygous tall plant bred to ahomozygous short plant. Half willbe tall and half will be short; all talloffspring will be Tt and all short off-spring will be tt.

ReinforcementKinesthetic Provide studentswith two large and two

small round pieces of candy plustwo large and two small buttons.Advise them that the buttons andcandy represent two differentgenes and the large-sized objectsrepresent dominant alleles. Askthem to prepare two sets ofobjects (alleles) for two traits intwo parents, making both parentsheterozygous for both traits.Then have students arrange theirparental gene sets into indepen-dently assorted gametes of twoalleles each. Advise them thatthey cannot have two candies ortwo buttons in their final groups.Have them explain how this illus-trates the law of independentassortment.

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BIOLOGY JOURNAL BIOLOGY JOURNAL

Punnett SquareVisual-Spatial Have students con-struct a Punnett square to illustrate

the probable bean parents from the QuickDemo on the previous page. Ask studentsto write a prediction of what the outcomewould be if two white beans werecrossed.

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Learning Disabled/EnglishLanguage Learners

Visual-Spatial Provide students withblank Punnett square outlines. Project

a similar copy onto a screen with an over-head projector. Lead students through thesteps showing a cross between two Tt par-ents. Reinforce how the letters placed to the

side and across the top represent all thepossible gametes for each parent and how this illustrates the law of segregation.Reinforce the significance of the foursquares and what the letter combinationswithin them represent. Provide studentswith a variety of problems (TT � tt, Tt � tt,Tt � TT, tt � tt).

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VIDEODISCThe Secret of LifeIndependent Assortment

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CD-ROMBiology: The Dynamics of LifeAnimation: Punnett Square Disc 2

Exploration: Punnett Square Disc 2


Critical Thinking/ProblemSolving, p. 10

Reinforcement and StudyGuide, pp. 43-44

Content Mastery, p. 50 L1

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267

How did Mendel analyze his data?In addition to crossing tall and short pea plants, Mendel crossed plants that formed round seeds with plants that formed wrinkled seeds. He found a 3: 1 ratio of round-seeded plants to wrinkled-seeded plants in the F2 generation.

AnalysisMendel’s actual results

in the F2 generation are shown to the right.1. Calculate the actual

ratio of round-seeded plants to wrinkled-seeded plants. To do this, divide the number of round-seeded plants by the number of wrinkled-seeded plants (round to the nearest hundredth). Your answer tells you how manymore times round-seeded plants resulted than wrinkled-seeded plants.

2. To express your answer as a ratio, write the number fromstep 1 followed by a colon and the numeral 1.

Thinking Critically1. What was the actual ratio Mendel observed for this cross? 2. How does Mendel’s observed ratio compare with the

expected 3: 1 ratio? 3. Why was the actual ratio different from the expected

ratio?

Problem-Solving Lab 10-1Problem-Solving Lab 10-1 AnalyzingInformation

square shows three plants with roundseeds out of four total plants, theprobability is 3/4, as Figure 10.9shows. Yet, if you calculate the frac-tion of round-seeded plants fromMendel’s actual data in the Problem-Solving Lab on this page, you will seethat slightly less than three-fourths ofthe plants were round-seeded. It isimportant to remember that theresults predicted by probability aremore likely to be seen when there is alarge number of offspring.


Section AssessmentSection Assessment

Understanding Main Ideas1. What structural features of pea plant flowers

made them suitable for Mendel’s genetic studies?2. What are the genotypes of a homozygous and a

heterozygous tall pea plant?3. One parent is homozygous tall and the other

parent is heterozygous tall. Make a Punnettsquare to determine what fraction of their offspring is expected to be heterozygous.

4. How many different gametes can an RRYy parentform? What are they?

Thinking Critically5. In garden peas, the allele for yellow peas is

dominant to the allele for green peas. Suppose

you have a plant that produces yellow peas, but you don’t know whether it is homozygousdominant or heterozygous. What experimentcould you do to find out? Draw a Punnett squareto help you.

6. Observing and Inferring The offspring of across between a purple-flowered plant and awhite-flowered plant are 23 plants with purpleflowers and 26 plants with white flowers. Usethe letter P for purple and p for white. What arethe genotypes of the parent plants? Explain yourreasoning. For more help, refer to ThinkingCritically in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

RR

R

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Rr

Rr

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Figure 10.9The probability that the offspring from amating of two heterozygotes will show adominant phenotype is 3 out of 4, or 3/4.

Kind of Plants

Number of Plants

Round-seeded

Wrinkled-seeded

5474

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Mendel’s results

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PurposeStudents will convert andexpress two sets of related

numbers as a ratio.

Process Skillsuse and interpret data, calculate

BackgroundThe round seeds contain anabundance of well-formed starchgrains and have higher waterretention than the wrinkledseeds. A lack of well-formedstarch grains and lower waterretention account for the wrin-kled appearance.

Teaching Strategies■ Have students work in groups.Place students with poor mathskills with students working atgrade level.■ Provide students with anexample if they are having diffi-culty in determining a ratio.

Thinking Critically1. The ratio was 2.96 : 1.2. The observed ratio is slightly

lower than expected. 3. The observed and expected

ratios may differ slightly dueto chance.

Skill Provide students withother examples of traits anddata. Have them calculateratios. Use the Perfor-

mance Task Assessment List forUsing Math in Science in PASC,p. 29.

4 CloseActivity

Visual-Spatial Have studentsillustrate Mendel’s law of

segregation using as much of aPunnett square as needed. Do thesame with the law of independentassortment. L2

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Problem Solving Lab 10-1Problem Solving Lab 10-1

Section AssessmentSection AssessmentSection Assessment1. They are self-pollinating, and male

flower parts can be easily removed toallow for cross-pollination.

2. homozygous tall = TT, heterozygous = Tt3. One-half of all offspring will be heterozy-

gous. (Note: It will not matter if thehomozygous parent is homozygous dom-inant or recessive.)

4. two; RY and Ry

5. Cross the unknown yellow plant with arecessive green parent. If the offspringare all yellow, the unknown genotype ishomozygous yellow. If half the offspringare yellow and half green, the unknowngenotype is heterozygous.

6. Genotypes of parents are Pp for the pur-ple-flowered plant and pp for the white-flowered plant. Purple is dominant.

Section

10.2 MEIOSIS 269

Genes, Chromosomes,and Numbers

Organisms have tens of thousandsof genes that determine individualtraits. Genes do not exist free in thenucleus of a cell; they are lined up onchromosomes. Typically, a thousandor more genes are arranged on achromosome.

Diploid and haploid cellsIf you examined the nucleus in a

cell of one of Mendel’s pea plants,you would find it had 14 chromo-somes—seven pairs. In the body cellsof animals and most plants, chromo-

somes occur in pairs. One chromo-some in each pair came from themale parent, and the other camefrom the female parent. A cell withtwo of each kind of chromosome iscalled a diploid cell and is said tocontain a diploid, or 2n, number ofchromosomes. This pairing supportsMendel’s conclusion that organismshave two factors—alleles—for eachtrait. One allele is located on each ofthe paired chromosomes.

Organisms produce gametes thatcontain one of each kind of chromo-some. A cell with one of each kind ofchromosome is called a haploid celland is said to contain a haploid, or n,

Mendel’s study of inheritancewas based on careful obser-vations of pea plants, but

pieces of the hereditary puzzle werestill missing. Modern technologiessuch as high-power microscopes allowus a glimpse of things that Mendelcould only imagine. You can now lookinside a cell to see the chromosomes onwhich the traits described by Mendelare carried. You can also examinethe process by which these traitsare transmitted to the nextgeneration.

SECTION PREVIEW

ObjectivesAnalyze how meiosismaintains a constantnumber of chromo-somes within a species.Infer how meiosis leadsto variation in a species.Relate Mendel’s laws ofheredity to the eventsof meiosis.

Vocabularydiploidhaploidhomologous

chromosomemeiosisspermeggzygotesexual reproductioncrossing overgenetic recombinationnondisjunction

10.2 Meiosis

Metaphase chromosomes(top) and a plant cell inearly anaphase of meiosis

Magnification:1100�

Magnification: 2300�

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Section 10.2

BIOLOGY: The Dynamics of Life SECTION FOCUS TRANSPARENCIES

Use with Chapter 10,Section 10.2

What is the number of chromosomes in each body cell ofthese fruit flies?

How many chromosomes must each body cell of normaloffspring have?

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PrepareKey ConceptsThis section develops the con-cepts of diploid and haploidchromosome numbers and ho-mologous chromosomes. Eventsthat occur during meiosis areillustrated, and their role ingenetic recombination is ex-plained.

Planning ■ Gather pipe cleaners or jelly-

beans for the Meeting Indivi-dual Needs.

■ Gather materials for the Alter-native Lab.

■ Purchase modeling clay orPlasticene for MiniLab 10-2.

■ Collect pictures of karotypesfor the Display.

■ Prepare line drawings for thePortfolio.

1 FocusBellringer Before presenting the lesson, display Section Focus Trans-parency 25 on the overhead pro-jector and have students answerthe accompanying questions.

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Let’s look at the seven pairs ofhomologous chromosomes inMendel’s peas. These chromosomepairs are numbered 1 through 7. Eachpair contains certain genes located atspecific places on the chromosome.Chromosome 4 contains the genesfor three of the traits that Mendelstudied. Many other genes can befound on this chromosome as well.

Every pea plant has two copies ofchromosome 4. It received one fromeach of its parents and will give oneat random to each of its offspring.Remember, however, that the twocopies of chromosome 4 in a peaplant may not necessarily have iden-tical alleles. Each can have one of thedifferent alleles possible for eachgene. The homologous chromo-somes diagrammed in Figure 10.10show both alleles for each of threetraits. Thus, the plant represented bythese chromosomes is heterozygousfor each of the traits.

Why meiosis?When cells divide by mitosis, the

new cells have exactly the same num-ber and kind of chromosomes as theoriginal cells. Imagine if mitosis werethe only means of cell division. Eachpea plant parent, which has 14 chro-mosomes, would produce gametesthat contained a complete set of 14chromosomes. That means that eachoffspring formed by fertilization ofgametes would have twice the num-ber of chromosomes as each of itsparents. The F1 pea plants wouldhave cell nuclei with 28 chromo-somes, and the F2 plants would havecell nuclei with 56 chromosomes.The nuclei would certainly becrowded! What do you think theseplants might look like?

Clearly, there must be anotherform of cell division that allows off-spring to have the same number of

chromosomes as their parents. Thiskind of cell division, which producesgametes containing half the numberof chromosomes as a parent’s bodycell, is called meiosis (mi OH sus).Meiosis occurs in the specializedbody cells of each parent that pro-duce gametes.

Meiosis consists of two separatedivisions, known as meiosis I andmeiosis II. Meiosis I begins with onediploid (2n) cell. By the end of meio-sis II, there are four haploid (n) cells.These haploid cells are called sexcells—gametes. Male gametes arecalled sperm. Female gametes arecalled eggs. When a sperm fertilizesan egg, the resulting cell, called azygote (ZI goht), once again has thediploid number of chromosomes.

10.2 MEIOSIS 271

Chromosome 4

a

Terminal

Tall

Inflated

Axial

Short

Constricted

T

l

t

i

A

Homologous chromosome 4

Figure 10.10Each chromosome 4 in garden peas contains genes forflower position, height, and pod shape. Flower positioncan be either axial (flowers located along the stems) orterminal (flowers clustered at the top of the plant).Plant height can be either tall or short. Pod shape canbe either inflated or constricted.

OriginWORDWORD

meiosis From the Greekword meioun,meaning “to dimin-ish.” Meiosis is cell division thatresults in a gametecontaining half thenumber of chromo-somes of its parent.

271

number of chromosomes. This factsupports Mendel’s conclusion thatparent organisms give one factor, orallele, for each trait to each of theiroffspring.

Each species of organism containsa characteristic number of chromo-somes. Table 10.1 shows the diploidand haploid numbers of chromo-somes of some species. Note thelarge range of chromosome numbers.Note also that the chromosomenumber of a species is not related tothe complexity of the organism.

Homologous chromosomesThe two chromosomes of each

pair in a diploid cell help determinewhat the individual organism lookslike. These paired chromosomes arecalled homologous chromosomes(huh MAHL uh gus). Each of a pair of homologous chromosomes hasgenes for the same traits, such as podshape. On homologous chromo-somes, these genes are arranged inthe same order, but because there aredifferent possible alleles for the samegene, the two chromosomes in ahomologous pair are not always iden-tical to each other. Identify thehomologous chromosomes in theProblem-Solving Lab.

Organism Body Cell (2n) Gamete (n)

Fruit fly 8 4

Garden pea 14 7

Corn 20 10

Tomato 24 12

Leopard frog 26 13

Apple 34 17

Human 46 23

Chimpanzee 48 24

Dog 78 39

Adder’s tongue fern 1260 630

Table 10.1 Chromosome Numbers of Some Common Organisms

Can you identify homologous chromosomes?Homologous chromosomes are paired chromosomes havinggenes for the same trait located at the same place on thechromosome. The gene itself, however, may have differentalleles, producing different forms of the trait.

AnalysisThe diagram below shows chromosome 1 with four

different genes present. These genes are represented by theletters F, g, h, and J. Possible homologous chromosomes ofchromosome 1 are labeled 2-5. Examine the five chromo-somes and the genes they contain to determine which ofchromosomes 2-5 are homologous with chromosome 1.

Thinking Critically1. Could chromosome 2 be homologous with chromosome

1? Explain why.2. Could chromosome 3 be homologous with chromosome 1?

Explain why.3. Could chromosome 4 be homologous with chromosome

1? Explain why.4. Could chromosome 5 be homologous with chromosome 1?

Explain why.

Problem-Solving Lab 10-2Problem-Solving Lab 10-2 Interpreting ScientificIllustrations

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Leopardfrog

Adder’stongue fern

Corn

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2 Teach

PurposeStudents will compare alleles onhomologous chromosomes.

Process Skillsinterpret scientific diagrams,apply concepts, compare andcontrast, think critically

Teaching Strategies■ Review the meaning of allelesand homologous chromosomes.

Thinking Critically1. 2 is homologous with 1.

Alleles may or may not beidentical, as long as they arepositioned at the same loca-tion on matching chromo-somes.

2. 3 is not homologous with 1.Genes must be identical.Gene K is not the same asgene J.

3. 4 is not homologous with 1.Chromosomes must match inphysical size and location ofgenes in order to be homolo-gous. (Sex chromosomes arean exception to the rule ofmatching physical size.)

4. 5 is homologous with 1 forthe same reasons as in ques-tion 1.

Performance Ask studentsto make a diagram of a chromo-some with three marked genelocations. Have them diagram allthe homologous chromosomesthat are possible for this chromo-some. Use the Performance TaskAssessment List for ScientificDrawing in PASC, p. 55. L2

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Problem-Solving Lab 10-2Problem-Solving Lab 10-2

Cultural Diversity

electron microscopic techniques to study meiosis. Obtain a copy of Anderson’s 1972publication, The Meiotic Process: Pairing,Recombination, and Chromosome Movements,and discuss with students the methodologiesused in studying the process of meiosis.

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Learning DisabledKinesthetic Have students use pipecleaners or jelly beans to show why

meiosis is necessary to prevent the numberof chromosomes from doubling in each gen-eration. Students should start with a bodycell containing two pipe cleaners or jellybeans, 2n. After meiosis, the gametes shouldeach contain one, n. After fertilization, the

zygote again has two. Students can thenrepeat the process for several generations,assuming that gametes are formed by mito-sis. In the first generation, the gametes willbe 2n. In the next generation, they will be4n. Students will see that the number ofchromosomes doubles in each generation.

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Everett Anderson Introduce students to the contribution ofAfrican American cell biologist EverettAnderson to the modern understanding ofthe meiotic process. Anderson (1928- ), whoreceived his Ph.D. in 1955, has been one of the leading researchers in developing

CD-ROMBiology: The Dynamicsof Life

Animation: Meiosis Disc 2VIDEODISCBiology: The Dynamicsof Life

Disc 1, Side 1, 1 min. 47 sec.Meiosis (Ch. 30)

!:,Å"

Visual LearningTable 10.1 Ask students whetherthere is any evidence to supportthe idea that plants have fewerchromosomes than animals. Tell students to use examplesfrom the table to support theiranswers. No; apples have 34 chro-mosomes, which is more than fruitflies or frogs have. Ask for the chro-mosome numbers in skin cells ofa leopard frog and a dog, 26 and78, and in root cells of tomatoesand garden peas, 24 and 14.

DiscussionShow students an egg and explainthat it is a gamete. Based on pre-vious information, studentsshould be able to tell how manyalleles are present for each trait.only one allele for each trait Askwhy an organism cannot producegametes by mitosis. Mitosis pro-duces a cell with both members ofeach pair of chromosomes.

10.2 MEIOSIS 273

Figure 10.12Follow the diagrams showing interphase and meiosis as youread about each phase. Compare these diagrams with thoseof mitosis in Chapter 8. Note that after telophase II, meiosisis finished and gametes form. In whatother ways are mitosis and meiosisdifferent?

Meiosis I

Metaphase I

Metaphase II

Prophase I

Prophase II

Interphase

Telophase II

Telophase IAnaphase II

Anaphase I

Meiosis II





Magnification: 640�Magnification: 470�




Using an AnalogyTo reinforce the concept ofhomologous chromosomes, sisterchromatids, tetrad formation,crossing over, and anaphase I, trythe following analogy: A magicpair of shoes (left and right) isfound on a shelf (homologouschromosomes in a cell). Theseshoes, being magic, can and doreplicate (interphase replication).Each copy is tied to its originalwith its shoelaces (centromere;both lefts are tied together andboth rights are tied together).Both rights are now called rightsister shoes (sister chromatids).Both lefts are now called left sis-ter shoes (sister chromatids). Allfour shoes line up next to oneanother (tetrad formation).While next to one another, partof one nonsister shoe (a left shoe)exchanges its innersole withanother nonsister shoe (a rightshoe) (crossing over). Right shoesmove away from left shoes to dif-ferent shelves (anaphase I, withhomologous chromosomes sepa-rating and going to two differentcells. Both rights are still tiedtogether and both lefts are stilltied together). To improve theanalogy, locate two identical pairsof shoes to demonstrate the ev-ents as they are described.

5. Model interphase, prophase I,metaphase I, anaphase I, and telophaseI, prophase II, metaphase II, anaphaseII, and telophase II.

Expected ResultsStudents will gain an understanding ofmeiosis when visualizing each phase.Analysis

1. What happens to the chromosomenumber during meiosis? reduced by 1/2

2. How many cells are formed duringmeiosis? 4

3. What is the fate of the cells formedduring meiosis? They form either eggor sperm cells.

Knowledge Ask students to writea paragraph explaining the value of mak-ing models such as the ones in this lab.Use the Performance Task AssessmentList for Writing in Science in PASC,p. 87. L2

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273

The zygote then can develop bymitosis into a multicellular organism.The pattern of reproduction thatinvolves the production and subse-quent fusion of haploid sex cells iscalled sexual reproduction. Thisreproductive pattern is illustrated inFigure 10.11.

The Phases of MeiosisDuring meiosis, a spindle forms

and the cytoplasm divides in thesame ways they do during mitosis.However, what happens to the chro-mosomes in meiosis is very different.Figure 10.12 illustrates interphaseand the phases of meiosis. Examinethe diagram and photo of each phaseas you read about it.

InterphaseRecall from Chapter 8 that, during

interphase, the cell replicates its chro-mosomes. During interphase thatprecedes meiosis I, the cell also repli-cates its chromosomes. After replica-tion, each chromosome consists of

two identical sister chromatids, heldtogether by a centromere.

Prophase I A cell entering prophase I behaves

in a similar way to one enteringprophase of mitosis. The chromo-somes coil up and a spindle forms.Then, in a step unique to meiosis,each pair of homologous chromo-somes comes together, matched geneby gene, to form a four-part structurecalled a tetrad. A tetrad consists of twohomologous chromosomes, each made up of two sister chromatids. Thechromatids in a tetrad pair tightly. Infact, they pair so tightly that nonsis-ter chromatids from homologouschromosomes sometimes actuallyexchange genetic material in a processknown as crossing over. Crossingover can occur at any location on achromosome, and it can occur at sev-eral locations at the same time. It isestimated that during prophase I ofmeiosis in humans, there is an averageof two to three crossovers for eachpair of homologous chromosomes.


Figure 10.11 In sexual reproduction, the doubling of the chromosomenumber that results from fertil-ization is balanced by the halv-ing of the chromosome numberthat results from meiosis.

Fertilization

Meiosis

Mitosis andDevelopment

Diploid zygote(2n = 46)

Haploid gametes(n = 23)

sperm cell

Multicellular diploid adults

(2n = 46)

Meiosis

egg cell

Alternative Lab Modeling Meiosis

PurposeStudents will observe the changes thatoccur during meiosis.

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Materials 9 sheets of unlined paper or poster boardscut to 30 cm square, long length of yarn,paper clips, long length of string, tooth-picks, tape or glue, scissors ProcedureGive students the following directions.

1. Work in groups of nine students. Eachstudent is to model one phase of inter-phase and meiosis. Each model is tohave a phase name, labels, and an

explanation of the events taking place.2. Represent cell structures as follows:

yarn strands = chromosomes, paperclips = centromeres, string = nuclearmembranes, toothpicks = spindlefibers.

3. Place models on large sheets of paperor poster board.

4. Only one pair of chromosomes is to befollowed through all models. Glue ortape all parts in place.272

Knowledge Have studentstake turns describing the phasesof meiosis. L1

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Visual LearningFigure 10.12 Ask students howthe processes of mitosis andmeiosis are different. Events thatoccur in meiosis but not in mitosisinclude (1) in prophase I, pairs ofhomologous chromosomes formtetrads and crossing over occurs; (2)in metaphase I, homologous chromo-somes line up in pairs rather thanindependently; (3) in anaphase I,homologous chromosomes, not chro-matids, separate; (4) in telophase I,each cell has only one chromosomefrom each pair; and (5) at the end ofmeiosis II, each new cell has the hap-loid number of chromosomes.P

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Quick DemoQuick Demo

Kinesthetic Have eightstudents form two

“tetrads” and walk throughthe phases of meiosis. P

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BioLab and MiniLab Work-sheets, pp. 45-46

Basic Concepts Transparency15 and Master

Reteaching Skills Trans-parency 17 and Master

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The phases of meiosis IIThe newly formed cells in some

organisms undergo a short interphasein which the chromosomes do notreplicate. In other organisms, how-ever, the cells go from late anaphaseof meiosis I directly to metaphase ofmeiosis II, skipping telophase I,interphase, and prophase II.

The second division in meiosisconsists of prophase II, metaphase II,anaphase II, and telophase II. Duringprophase II, a spindle forms in eachof the two new cells and the spindlefibers attach to the chromosomes.The chromosomes, still made up ofsister chromatids, are pulled to thecenter of the cell and line up ran-domly at the equator duringmetaphase II, just as they do in mito-sis. Anaphase II begins as the cen-tromere of each chromosome splits,allowing the sister chromatids to sep-arate and move to opposite poles.Finally, nuclei re-form, the spindlesbreak down, and the cytoplasmdivides during telophase II. Theevents of meiosis II are identical tothose you studied for mitosis.

At the end of meiosis II, four hap-loid sex cells have been formed fromone original diploid cell. Each hap-loid cell contains one chromosomefrom each homologous pair. Thesehaploid cells will become gametes,transmitting the genes they containto offspring.

Meiosis Provides forGenetic Variation

Cells that are formed by mitosis areidentical to each other and to the par-ent cell. Meiosis, however, provides amechanism for shuffling the chromo-somes and the genetic informationthey carry. By shuffling the chromo-somes, genetic variation is produced.

Genetic recombinationHow many different kinds of

sperm can a pea plant produce? Eachcell undergoing meiosis has sevenpairs of chromosomes. Because eachof the seven pairs of chromosomescan line up at the cell’s equator in twodifferent ways, 128 different kinds ofsperm are possible (27 = 128).

10.2 MEIOSIS 275

OriginWORDWORD

pro- From the Greekword pro, meaning“before.”

meta- From the Greekword meta, meaning“after.”

ana- From the Greekword ana, meaning“away, onward.”

telo- From the Greektelos, meaning“end.”

The four phases ofcell division areprophase,metaphase,anaphase, andtelophase.

Figure 10.13 Late in prophase I, the homologous chromosomes come together to formtetrads (a). Arms of nonsister chromatidswind around each other (b), and geneticmaterial may be exchanged (c).

Homologous chromosomesCrossing over in tetrad

Gametes

Tetrad

A A

Sister chromatids Nonsister chromatids

a b

c

B B

a a

b b

A A

B Bb

a

A A a a

B Bb b

b

a

275

This exchange of genetic materialis diagrammed as an X-shaped config-uration in Figure 10.13b. Crossingover results in new combinations ofalleles on a chromosome, as shown inFigure 10.13c. You can practicemodeling crossing over in theMiniLab at the left.

Metaphase IAs prophase I ends, the cen-

tromere of each chromosomebecomes attached to a spindle fiber.The spindle fibers pull the tetradsinto the middle, or equator, of thespindle. This is an important stepunique to meiosis. Note that homol-ogous chromosomes are lined up sideby side as tetrads. In mitosis, on theother hand, homologous chromo-somes line up on the equator inde-pendently of each other.

Anaphase I Anaphase I begins as homologous

chromosomes, each with its twochromatids, separate and move toopposite ends of the cell. This occursbecause the centromeres holding thesister chromatids together do notsplit as they do during anaphase inmitosis. This critical step ensuresthat each new cell will receive onlyone chromosome from each homolo-gous pair.

Telophase IEvents occur in the reverse order

from the events of prophase I. Thespindle is broken down, the chromo-somes uncoil, and the cytoplasmdivides to yield two new cells. Eachcell has only half the genetic infor-mation of the original cell because ithas only one chromosome from eachhomologous pair. However, anothercell division is needed because eachchromosome is still doubled, consist-ing of two sister chromatids.


Modeling CrossingOver Crossing overoccurs during meiosisand involves only thenonsister chromatidsthat are present duringtetrad formation. Theprocess is responsiblefor the appearance ofnew combinations ofalleles in gamete cells.

Procedure! Copy the data table.@ Roll out four long strands of clay at least 10 cm long to

represent two chromosomes, each with two chromatids. # Use the figure above as a guide in joining and labeling

these model chromatids. Although there are four chro-matids, assume that they started out as a single pair ofhomologous chromosomes prior to replication. The figureshows tetrad formation during prophase I of meiosis.

$ First, assume that no crossing over takes place. Model theappearance of the four gamete cells that will result at theend of meiosis. Record your model’s appearance by draw-ing the gametes’ chromosomes and their genes in yourdata table.

% Next, repeat steps 2-4. This time, however, assume thatcrossing over occurs between genes B and C.

Analysis1. Predict and diagram the appearance of the chromosomes

prior to replication.2. Define crossing over and explain when it occurs.3. Compare any differences in the appearance of genes on

chromosomes in gamete cells when crossing over occursand when it does not occur.

4. Crossing over has been compared to “shuffling the deck”in cards. Explain what this means.

5. What would be accomplished if crossing over occurredbetween sister chromatids? Explain your answer.

MiniLab 10-2MiniLab 10-2 Formulating Models

2 Chromosomes with chromatids

Nonsister chromatids

Twist tie

Mark geneswith a pencilpoint

No crossing over Crossing over

Appearance of gamete cells Appearance of gamete cells

Data Table

274

Purpose Students will model the processof crossing over.

Process Skillsapply concepts, compare andcontrast, formulate models, rec-ognize cause and effect, thinkcritically, define operationally

Teaching Strategies■ Plasticene or clay is availablefrom biological supply houses orcraft stores.■ String or twine may be used asa substitute for twist ties.■ Review the concept that gam-etes contain only one chromo-some from each pair. ■ Make sure students wash theirhands after the lab.

Expected ResultsAll gametes will show the samecombination of genes if no cross-ing over occurs and differentcombinations of genes if crossingover does occur.

Analysis1. Only two chromosomes

should be drawn. Thesequence of genes would be CB A on one chromosome andc B a on the other.

2. Crossing over is the exchangeof genetic material betweennonsister chromatids duringprophase I of meiosis.

3. All gamete cells show thesame pattern of genes onchromosomes as in the origi-nal diagram if no crossingover occurs. If crossing overoccurs between B and C,gamete cells with genearrangements C B a and c B Awill be formed as well as cellswith C B A and c B a.

4. Student answers will vary.There is a mixing of the genetraits from their originalorder.

5. There would be no mixing ofgene traits when comparedwith the original chromo-somes because sister chro-matids are identical.

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MiniLab 10-2MiniLab 10-2

PortfolioPortfolio

Modeling the MembraneVisual-Spatial Have student groupsinvent a mechanism for demonstrat-

ing how the plasma membrane of an ani-mal cell pinches together during telophaseI and II of meiosis. Students can use a vari-ety of common objects as models, such asballoons, string, and wire.

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Comparing Mitosis and Meiosis Linguistic Provide students with two sets of simple line drawings, one show-

ing interphase and the phases of mitosis andthe other showing interphase and meiosis.Place comparable phases next to each otherwhen possible (prophase of mitosis next toprophase I of meiosis, and so on). Ask students

to describe in their portfolios the similari-ties and differences between processes.Also have them indicate the type of cellformed at the end of the process—body orgamete—and whether its chromosomenumber would be diploid or haploid. P

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PortfolioPortfolioPerformance Ask students to model

the appearance of gamete cells if crossingover occurred between sister chromatids.This should confirm their answer to question 5. Use the Performance TaskAssessment List for Model in PASC, p. 51.

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VIDEODISCThe Secret of LifeMeiosis Ia

Meiosis Ib

!7;=.B"

!7;=BD"

Using an AnalogyContinue the shoe analogy ordemonstration of meiosis to illus-trate telophase I, metaphase II,and telophase II events. Startwith two right shoes tiedtogether. Both sets are separatefrom each other on differentcloset shelves (two new cellsformed after telophase I). Theshoes untie (centromere splitsafter metaphase II). They moveto different shelves in the closet(two new cells formed intelophase II). Ask students: (a)How many shoes are now on sep-arate shelves (separate cells)? 4(b) How many shoes were pre-sent at the start of this analogy? 2(c) Is the chromosome number ina cell diploid or haploid before acell undergoes meiosis? diploid (d)How many shoes are present oneach shelf at the end of theprocess? 1 (e) How many shoeswere on the original shelf? 2 (f) Isthe chromosome number in eachof the four new cells formed inmeiosis reduced by half? yes

Concept DevelopmentThe chances of two humansbeing born exactly alike is almostan impossibility (except for iden-tical twins). Explain why childrenin the same family can neverthe-less resemble one another ratherclosely.

Sister chromatids

Nonsister chromatids

Genetic Recombination

One source of genetic recombination is a processknown as crossing over, the exchange of genetic

material by nonsister chromatids during meiosis.

Critical Thinking How can the frequency of crossing over be used to map the location of genes on chromosomes?

INSIDESSTORTORYY

INSIDE

New allele combinations Crossing overcauses a shuffling of allele combinations, just as shuffling a deck of cards producesnew combinations of cards dealt in a hand.Rather than the alleles from each parentstaying together on their homologous chro-mosome, new combinations of alleles canform. Thus, variability is increased.

33

44

10.2 MEIOSIS 277

Chiasmata The pair-ing of homologouschromosomes duringsynapsis is precise—they line up with eachother allele by allele.The nonsister chro-matids twist aroundeach other, forming X-shaped regions calledchiasmata (ki az MAH

tuh). The two nonsisterchromatids break andexchange geneticmaterial.

22

Tetrads Duringlate prophase I,homologous chro-mosomes cometogether to form atetrad. This pairingof homologouschromosomes iscalled synapsis.

11

Mapping Geneticists have used the fre-quency of crossing over to map the relativelocation of alleles on chromosomes. Allelesthat are further apart on the chromosomesare more likely to have chiasmata betweenthem than alleles that are close together.

The X-shaped regions are called chiasmata.

Chiasmata

277

IINSIDENSIDESSTORTORYY

INSIDE

PurposeStudents study the process ofcrossing over and its importancein genetic recombination.

Teaching Strategies■ Ask students to describe theprocess of crossing over.

Visual Learning■ Have students examine the

photograph and count the chi-asmata.

■ Visual-Spatial Have stu-dents draw chromosomes

before and after crossing over.The use of colored pencils willhelp students visualize howthis process leads to geneticrecombination.

Critical ThinkingThe relative position of genes ona chromosome can be determinedby the frequency of crossing over.The greater the frequency ofcrossing over, the greater the dis-tance between the two genes.

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In the same way, any pea plant can form 128 different eggs. Becauseany egg can be fertilized by anysperm, the number of different possi-ble offspring is 16 384 (128 � 128).Figure 10.14a shows a simple exam-ple of how genetic recombinationoccurs. You can see that the genecombinations in the gametes varydepending on how each pair ofhomologous chromosomes lines upduring metaphase I, a random process.

These numbers increase greatly asthe number of chromosomes in thespecies increases. In humans, n = 23,so the number of different kinds ofeggs or sperm a person can produceis more than 8 million (223). Whenfertilization occurs, 223 � 223, or 70trillion, different zygotes are possi-ble! It’s no wonder that each individ-ual is unique.

In addition, crossing over canoccur anywhere at random on a chro-mosome. Typically, two or threecrossovers per chromosome occur dur-ing meiosis. This means that an almostendless number of different possiblechromosomes can be produced bycrossing over, providing additionalvariation to that already produced by

the random assortment of chromo-somes. This reassortment of chromo-somes and the genetic informationthey carry, either by crossing over orby independent segregation ofhomologous chromosomes, is calledgenetic recombination. It is a majorsource of variation among organisms.Variation is important to a speciesbecause it is the raw material thatforms the basis for evolution. Howdoes crossing over increase geneticvariability? To answer this question,read the Inside Story on the next page.

Meiosis explains Mendel’s resultsMeiosis provides the physical basis

for explaining Mendel’s results. Thesegregation of chromosomes inanaphase I of meiosis explainsMendel’s observation that each parentgives one allele for each trait at ran-dom to each offspring, regardless ofwhether the allele is expressed. Thesegregation of chromosomes at ran-dom during anaphase I explainsMendel’s observation that factors, orgenes, for different traits are inheritedindependently of each other. Today,Mendel’s laws of heredity form thefoundation of modern genetics.


Figure 10.14 If a cell has two pairsof chromosomes (n = 2), four kinds ofgametes (22 ) are pos-sible, depending onhow the homologouschromosomes line up at the equatorduring meiosis I (a).This event is a matterof chance. Whenzygotes are formedby the union of these gametes, 4 � 4 or 16 possiblecombinations mayoccur (b).

ab

aB

Ab

AB

AB

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

Possible combinations ofchromosomes in zygotes (in boxes)

Poss

ible

arr

ange

men

t of

chro

mos

omes

in e

ggs

Possible arrangement ofchromosomes in sperm

Ab aB ab

ba

Possible gametes

b A b

b

A a

B

B Ba

aA

b

A

B

a

Possible gametes

b bAA a

B

B B a

aA

b

A

B

a

b

276

BIOLOGY JOURNAL BIOLOGY JOURNAL

Gamete Formation Linguistic Ask students to imaginethat they are pea gametes who have

just met their cousin leaf cells from thesame plant. You want to explain to yourcousins how you are different from them.Write a story that describes how you be-came a gamete and how you differ fromyour cousins.

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Internet Address Book

Note Internet addresses that you find useful in the spacebelow for quick reference.

VIDEODISCThe Secret of LifeMeiosis IIa

Meiosis IIb

!7;=8C"

!7;=LE"

Performance Assessmentin the Biology Classroom, p. 13.Investigating Mitosis and Meiosis,Have students carry out this activ-ity to determine which slides in anunlabeled set show mitosis andwhich show meiosis, and whichwere made from animal cells andwhich from plant cells. L2

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Concept Mapping, p. 10

Laboratory Manual, pp. 71-74 L2

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sets of chromosomes—a tetraploid.This can be seen in Figure 10.15

Organisms with more than theusual number of chromosome setsare called polyploids. Polyploidy israre in animals and almost alwayscauses death of the zygote. However,polyploidy frequently occurs inplants. Often, the flowers and fruitsof these plants are larger than nor-mal, and the plants are healthier.Many polyploid plants, such as thesterile banana plant and the day lilyshown in Figure 10.16, are of greatcommercial value.

Meiosis is a complex process, andthe results of an error occurring aresometimes unfortunate. However,mistakes in meiosis can be beneficial,such as those that have occurred inagriculture. Tetraploid (4n) wheat,triploid (3n) apples, and polyploidchrysanthemums all are availablecommercially. You can see that athorough understanding of meiosisand genetics would be very helpful toplant breeders. In fact, plant breedershave learned to artificially producepolyploid plants using chemicals thatcause nondisjunction.

10.2 MEIOSIS 279

Section AssessmentSection Assessment

Understanding Main Ideas1. How are the cells at the end of meiosis different

from the cells at the beginning of meiosis? Usethe terms chromosome number, haploid, anddiploid in your answer.

2. What is the role of meiosis in maintaining a con-stant number of chromosomes in a species?

3. Why are there so many varied phenotypes withina species such as humans?

4. If the diploid number of a plant is 10, how manychromosomes would you expect to find in itstriploid offspring?

Thinking Critically5. How do the events of meiosis explain Mendel’s

law of independent assortment?

6. Interpreting Scientific IllustrationsCompare Figures 10.12 and 8.12. Explain whycrossing over between nonsister chromatids ofhomologous chromosomes cannot occur duringmitosis. For more help, refer to ThinkingCritically in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Figure 10.16The banana plant isan example of atriploid plant (a). Thisday lily is a tetraploidplant (b).

a

b

ExtensionOogenesis and spermatogenesisare the terms used to describeegg and sperm cell formation inhumans through meiosis. Havestudents research how these twoprocesses differ and how they arealike.

Skill Provide students withsimple line diagrams of the vari-ous phases of meiosis. The dia-grams should not be in normalsequence. Have students place thediagrams in proper sequence,name each phase, and describewhat is occurring.

4 CloseDiscussionHave students describe as manydifferences and similarities aspossible between mitosis andmeiosis. Verify the accuracy oftheir descriptions as a class. L2

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Mistakes in MeiosisAlthough the events of meiosis

usually proceed accurately, sometimesan accident occurs and chromosomesfail to separate correctly. The failureof homologous chromosomes to sepa-rate properly during meiosis is callednondisjunction. Recall that duringmeiosis I, one chromosome fromeach homologous pair moves to eachpole of the cell. Occasionally, bothchromosomes of a homologous pairmove to the same pole of the cell.

Trisomy, monosomy, and triploidyIn one form of nondisjunction, two

kinds of gametes result. One has anextra chromosome, and the other ismissing a chromosome. The effectsof nondisjunction are often seen aftergametes fuse. For example, when agamete with an extra chromosome isfertilized by a normal gamete, thezygote will have an extra chromo-some. This condition is called tri-somy (TRI soh mee). In humans, if agamete with an extra chromosomenumber 21 is fertilized by a normalgamete, the resulting zygote has 47chromosomes instead of 46. This

zygote will develop into a baby withDown syndrome.

Although organisms with extrachromosomes often survive, organ-isms lacking one or more chromo-somes usually do not. When a gametewith a missing chromosome fuseswith a normal gamete during fertil-ization, the resulting zygote lacks achromosome. This condition is calledmonosomy. In humans, most zygoteswith monosomy do not survive. If azygote with monosomy does survive,the resulting organism usually doesnot. An example of monosomy that isnot lethal is Turner syndrome, inwhich human females have only asingle X chromosome instead of two.

Another form of nondisjunctioninvolves a total lack of separation ofhomologous chromosomes. Whenthis happens, a gamete inherits acomplete diploid set of chromo-somes, as shown in Figure 10.15.When a gamete with an extra set ofchromosomes is fertilized by a nor-mal haploid gamete, the offspring hasthree sets of chromosomes and istriploid. The fusion of two gametes,each with an extra set of chromo-somes, produces offspring with four


Male parent (2n)

Meiosis

Abnormalgamete (2n)

Nondisjunction

Zygote(4n)

Female parent (2n)

Meiosis

Abnormalgamete (2n)

Nondisjunction

Figure 10.15 Follow the steps tosee how a tetraploidplant such as thischrysanthemum isproduced.

278

DisplayMake a bulletin board display ofpictures of karotypes showingabnormal numbers of chromo-somes.

3 AssessCheck for UnderstandingAsk students to explain how thewords in the following combina-tions are related.

a. diploid—haploid b. homologous chromosomes—

allelec. sperm—egg—zygoted. meiosis—gamete e. crossing over—genetic

recombination

ReteachAsk students to prepare a list ofthe important characteristics ofeach step in the process of meio-sis. Then have them prepare asecond list of all the reasons whymeiosis is important to organismsthat reproduce sexually. L2

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Polyploidy in PlantsLinguistic Have students researchand report on how plant breeders

create polyploid flowers. Students shouldname some polyploid cultivars and indi-cate how many sets of chromosomes eachhas.

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Section AssessmentSection AssessmentSection Assessment1. The chromosome number in a cell at

the end is half the chromosome num-ber in a parent cell. The original cellhas a diploid number of chromosomesand each of the new cells has a hap-loid number.

2. The reduction of chromosome num-bers by half allows for the return tothe constant chromosome number

when a zygote is formed at fertiliza-tion.

3. Crossing over as well as the reassort-ment of the 46 chromosomes bothcontribute to the large number ofphenotypes that are possible.

4. 155. After meiosis, only one member of

each homologous chromosome pair

can be found in a gamete. Thus, nogamete will end up with two homo-logues. Alleles on different chromo-somes will sort independently fromone another.

6. Tetrad formation does not occur dur-ing mitosis. This prevents crossingover from taking place.

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BioLab and MiniLab Work-sheets, pp. 47-48

Reinforcement and StudyGuide, pp. 45-46

Content Mastery, pp. 49, 51-52 L1

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Sharing Your DataSharing Your Data

Find this BioLab on theGlencoe Science Web Site

at www.glencoe.com/sec/science. Brieflydescribe your experimental design. Post yourresults in the table provided.

10.2 MEIOSIS 281

1. Thinking Critically Why was itnecessary to grow plants from theseeds in order to determine thephenotypes of the plants thatformed the seeds?

2. Drawing Conclusions Using theinformation in the introduction,describe how the gene for greencolor (C) is inherited.

3. Making Inferences For the groupof seeds that yielded all greenplants, are you able to determineexactly the genotypes of the par-ents that formed these seeds? Canyou determine the genotype ofeach plant observed? Explain.

4. Making Inferences For thegroup of seeds that yielded somegreen and some albino plants, are

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

PLAN THE EXPERIMENTPLAN THE EXPERIMENT

1. Examine the materials providedby your teacher. As a group,make a list of the possible waysyou might test your hypothesis.

2. Agree on one way that yourgroup could investigate yourhypothesis.

3. Design an experiment that willallow you to collect quantita-tive data. For example, howmany plants do you think youwill need to examine?

4. Prepare a numbered list of direc-tions. Include a list of materialsand the quantities you will need.

5. Make a data table for recordingyour observations.

Check the Plan1. Carefully determine what data

you are going to collect. How many seeds do you think you will need? How long will you carry out the experiment?

2. What variables, if any, will have to be controlled? (Hint: Think about the growing conditions for the plants.)

3. Make sure your teacher has approved your experimental plan before you proceed.

4. Carry out your experiment. Make any needed observa-tions, such as the numbers of green and albino plants in each group, and complete your data table.

5. Go to the Glencoe ScienceWeb Site at the address shownbelow to post your data.

you able to determine exactly thegenotypes of the plants thatformed these seeds? Can youdetermine the genotype of eachplant observed? Explain.

5. Using the Internet Compareyour experimental design withthat of other students. Were yourresults similar? What mightaccount for the differences?

INTERNETINTERNET

4. Yes, both parents must beheterozygous to yield a ratioof 3 green to 1 albino. Forthe offspring genotypes, youcan conclude only that thealbino offspring are cc. Greenare either CC or Cc.

5. Answers may vary. Geneticratios are governed by thelaws of probability. Thelarger the population size, thecloser the calculated valuewill be to the theoretical.

Portfolio Ask students tomake diagrams that show theparental and offspring genotypesand phenotypes for both groupsof seeds used in this experiment.Use the Performance TaskAssessment List for ScientificDrawing in PASC, p. 55.PP

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■ Be sure students mark the seed typeplanted in the flat or pot. Popsicle stickscan serve as markers.

Data and ObservationsSeeds that came from true breeding plantswill produce plants that are all green. Seedsfrom heterozygous parents will produceboth green and albino seedlings in the ratio

of about 3 green to 1 albino. Have studentsreview Problem-Solving Lab 10-1 for helpin calculating the ratio of green to albinoplants.

Sharing Your DataSharing Your Data

To navigateto the Inter-

net BioLabs choose the Biology:The Dynamics of Life icon atGlencoe’s Web Site. Click onthe student site icon, then theBioLabs icon. Students shouldgo to the Glencoe Science WebSite only after they have donethe experiment and collectedtheir data to post their experi-mental design and compare itwith that of other students.Students should be careful thatonly data from identical crossesare pooled.

How can phenotypes and genotypes of plants be determined?

I t’s difficult to predict the traits of plants if all that you see is theirseeds. But if these seeds are planted and allowed to grow, certain traits

will appear. By observing these traits, you might be able to determine thepossible phenotypes and genotypes of the parent plants that produced theseseeds. In this lab, you will determine the genotypes of plants that growfrom two groups of tobacco seeds. Each group of seeds came from differentparents. Plants will be either green or albino (white) in color. Use the fol-lowing genotypes for this cross. CC = green, Cc = green, and cc = albino

INTERNETINTERNET

ProblemCan the phenotypes and genotypes

of the parent plants that producedtwo groups of seeds be determinedfrom the phenotypes of the plantsgrown from the seeds?

HypothesesHave your group agree on a

hypothesis to be tested that willanswer the problem question. Recordyour hypothesis.

ObjectivesIn this BioLab, you will:■ Analyze the results of growing two

groups of seeds.■ Draw conclusions about phenotypes

and genotypes based on thoseresults.

■ Use the Internet to collect and com-pare data from other students.

Possible Materialspotting soilsmall flowerpots or seedling flatstwo groups of tobacco seedshand lenslight sourcethermometerplant-watering bottle

Safety PrecautionsAlways wash your hands after han-

dling plant materials. Always weargoggles in the lab.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

INTERNETINTERNET

Time AllotmentInitial session: one class period;follow-up session: 5 minutes eachday for watering, 20 minutes onlast day for counting

Process Skillshypothesize, observe and infer,collect data

Safety PrecautionsSome seed materials are poison-ous. Do not allow students to eatthe seeds. Have students washtheir hands after the lab.

Alternative MaterialsSeeds can be germinated andobserved in petri dishes. This willeliminate the need for soil, flats,or pots. Place seeds on moistenedpaper towels in the bottom of thedish and keep the dish covered.

Possible Hypotheses■ If the parent plants were true

breeding for green color, thenall offspring will be green.

■ If the parent plants were het-erozygous for green color,then offspring will show anapproximate ratio of 3 green to1 white.

1. Leaf color cannot beobserved in the seed butappears only after the planthas emerged from the seed.

2. The gene for green color is adominant trait.

3. No, one parent may havebeen true breeding for green(CC), the other may havebeen heterozygous (Cc). Thiswould have yielded all greenoffspring. Offspring may beCC or Cc but still appeargreen.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

PREPARATIONPREPARATION

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PLAN THE EXPERIMENTPLAN THE EXPERIMENT

Teaching Strategies■ When supplying seeds to students, makesure that the two types are kept separatefrom each other. Stick seeds onto a piece oftape for dispensing.■ An ideal quantity of seeds to use is 20-30per type.■ Cotyledons (seed leaves) will appear afterabout 10 days.

Possible Procedures■ Students should keep growing conditions

for the two seed groups as constant aspossible. Soil should be kept moist at alltimes. Natural window light should besufficient.

■ Seeds should be planted about 1 cmbelow the soil. Planting is easier if thesoil is moist.


Chapter 10 AssessmentChapter 10 Assessment

SUMMARYSUMMARY

Section 10.1

Section 10.2

Main Ideas■ Genes are located on chromosomes and exist in

alternative forms called alleles. A dominant allelecan mask the expression of a recessive allele.

■ When Mendel crossed pea plants differing inone trait, one form of the trait disappeared until the second generation of offspring. Toexplain his results, Mendel formulated the law of segregation.

■ Mendel formulated the law of indepen-dent assortment to explain that twotraits are inherited independently.

■ Events in genetics are governed by thelaws of probability.

Vocabularyallele (p. 262)dominant (p. 262)fertilization (p. 259)gamete (p. 259)genetics (p. 259)genotype (p. 264)heredity (p. 259)heterozygous (p. 265)homozygous (p. 264)hybrid (p. 261)law of independent

assortment (p. 266)law of segregation

(p. 263)phenotype (p. 264)pollination (p. 259)recessive (p. 262)trait (p. 259)

Mendel’s Lawsof Heredity

Main Ideas■ In meiosis, one diploid (2n) cell produces four

haploid (n) cells, providing a way for offspringto have the same number of chromosomes astheir parents.

■ Mendel’s results can be explained by the distri-bution of chromosomes during meiosis.

■ Random assortment and crossing over duringmeiosis provide for genetic variation among themembers of a species.

■ Mistakes in meiosis may result from nondisjunc-tion, the failure of chromosomes to separateproperly during cell division.

Vocabularycrossing over (p. 272)diploid (p. 269)egg (p. 271)genetic recombination

(p. 276)haploid (p. 269)homologous chromo-

some (p. 270)meiosis (p. 271)nondisjunction (p. 278)sexual reproduction

(p. 272)sperm (p. 271)zygote (p. 271)

Meiosis

CHAPTER 10 ASSESSMENT 283

1. An organism that is true breeding for a traitis said to be ________.a. homozygous c. a monohybridb. heterozygous d. a dihybrid

2. At the end of meiosis, how many haploidcells have been formed from the original cell?a. one c. threeb. two d. four

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 3. When Mendel transferred pollen from onepea plant to another, he was ________ theplants.a. self-pollinating c. self-fertilizingb. cross-pollinating d. cross-fertilizing

4. A short pea plant is ________.a. homozygous recessiveb. homozygous dominantc. heterozygousd. a dihybrid

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Main IdeasSummary statements can be used bystudents to review the major con-cepts of the chapter.

Using the VocabularyTo reinforce chapter vocabulary, usethe Content Mastery Booklet andthe activities in the Interactive Tutorfor Biology: The Dynamics of Life onthe Glencoe Science Web Site.www.glencoe.com/sec/science

1. a2. d3. b4. a

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS


All ChapterAssessment

questions and answers have beenvalidated for accuracy and suitabil-ity by The Princeton Review.

Gregor Mendel was an Austrian monk who experimented with garden peas.

In 1866, he published the results of eight years of experiments. His work was ignored until

1900, when it was rediscovered.

Mendel had three qualities that led to hisdiscovery of the laws of heredity. First, he

was curious, impelled to find out why things hap-pened. Second, he was a keen observer. Third, hewas a skilled mathematician. Mendel was the firstbiologist who relied heavily on statistics for solu-tions to how traits are inherited.

Darwin missed his chance About the same time that Mendel was carry-

ing out his experiments with pea plants, CharlesDarwin was gathering data on snapdragon flow-ers. When Darwin crossed plants that had nor-mal-shaped flowers with plants that had odd-shaped flowers, all the offspring had normal-shaped flowers. He thought the two traits hadblended. When he allowed the F1 plants to self-pollinate, his results were 88 plants with normal-shaped flowers and 37 plants with odd-shapedflowers. Darwin was puzzled by the results anddid not continue his studies with these plants.Lacking Mendel’s statistical skills, Darwin failedto see the significance of the ratio of normal-shaped flowers to odd-shaped flowers in the F2generation. What was this ratio? Was this ratiosimilar to Mendel’s ratio of dominant to recessivetraits in pea plants?

Finding the ratios for four other traits Figure 10.3 on page 262 shows seven traits

that Mendel studied in pea plants. You havealready looked at Mendel’s data for plant heightand seed shape. Now use the data for seed color,flower position, pod color, and pod shape to findthe ratios of dominant to recessive for these traitsin the F2 generation.

Draw Table B in your notebook or journal.Calculate the ratios for the data in Table A andcomplete Table B by following these steps:• Step 1 Divide the larger number by the smaller

number. • Step 2 Round to the nearest hundredth.• Step 3 To express your answer as a ratio, write

the number from step 2 followed by a colonand the number 1.


ConnectionMathMath

Connection A Solution fromRatios

Why were ratios so important in understandinghow dominant and recessive traits are inherited?

To find out more about Mendel’swork, visit the Glencoe Science

Web Site.www.glencoe.com/sec/science

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

Seed Flower Pod Pod Color Position Color Shape

Yellow Lateral Green Inflated6022 651 428 882

Green Terminal Yellow Constricted2001 207 152 299

Table A Mendel’s Results

Seed Flower Pod PodColor Position Color Shape

Calculation

Ratio 3:1yellow:green

Table B Calculating Ratios for Mendel’s Results

6022 2001 = 3.00

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PurposeStudents will gain insight into theimportance of mathematics to thestudy of biology. They will learnhow mathematics aided Mendelin understanding the laws ofheredity.

Teaching Strategies■ Students may be surprised tolearn that a biologist of CharlesDarwin’s stature missed a greatopportunity in his brilliantcareer. Had he interpreted hisdata about snapdragon flowershapes correctly, the whole worldmight have understood the lawsof heredity 40 years earlier.■ Discuss how ratios helpedMendel see that definite factorswere being passed on from par-ents to offspring. He didn’t knowwhat these factors were, nor howthey operated. He knew nothingabout chromosomes and meiosis,yet was able to show how traitswere transmitted because of hismathematical analysis.

Connection to BiologyStudents may say that the ratiosrevealed that a dominant traitshowed up three times moreoften because it was always ableto overcome the effect of a reces-sive trait that accompanied it.

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Connection


Chapter Assessment, pp. 55-60MindJogger VideoquizzesComputer Test BankBDOL Interactive CD-ROM, Chapter

10 quiz

VIDEOTAPEMindJogger Videoquizzes

Chapter 10: Mendel and MeiosisHave students work in groups as they playthe videoquiz game to review key chapterconcepts.




CHAPTER 10 ASSESSMENT 285

25. Relating to the methods of science, why doyou think it was important for Mendel tostudy only one trait at a time during hisexperiments?

26. Observing and Inferring Why is it possibleto have a family of six girls and no boys, butextremely unlikely that there will be a publicschool with 500 girls and no boys?

27. Comparing and Contrasting Comparemetaphase of mitosis with metaphase I ofmeiosis.

28. Recognizing Cause and Effect Why is itsometimes impossible to determine the genotype of an organism that has a dominantphenotype?

29. Observing and Inferring While examining acell in prophase I of meiosis, you observe apair of homologous chromosomes pairingtightly. What is the significance of the placesat which the chromosomes are joined?

30. Concept Mapping Complete the conceptmap by using the following vocabulary terms:recessive, zygote, homozygous, fertilization,heterozygous.

THINKING CRITICALLYTHINKING CRITICALLY

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

In fruit flies, the allele for long wings isdominant to the allele for short wings.

Interpreting Data Study the Punnettsquare and answer the following questions.1. What term is given to the parent fly

whose genotype is shown?a. heterozygous c. recessiveb. homozygous d. haploid

2. What is the phenotype of each parent?a. both dominantb. both recessivec. one dominant and one recessived. unable to tell

3. What is the genotype of each parent?a. WW—Ww c. Ww—Wwb. Ww—ww d. WW—WW

4. What are the phenotypes of the offspring?a. all long wingsb. all short wingsc. mostly long wingsd. half short and half long

5. Interpreting Data Suppose the fruit flyparents in the Punnett square abovewere both heterozygous for an eye colortrait in which R is red and r is white.What genotypes appear in the offspring?What fraction of the offspring will haveshort wings and white eyes?

W

Ww ww

Ww

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w

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produces a

1.

that can be

2.

3.

dominant 5.

4.or

or

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web Site.www.glencoe.com/sec/science

CD-ROM

25. Controlled experiments, such asMendel’s, require that no morethan one variable be manipu-lated at a time. By doing manyexperiments, Mendel was ableto determine the principles thatgovern genetics. These princi-ples would not have been aseasily observable without con-trolled experiments.

26. The probability of a family withsix girls is (1/2)6, but the proba-bility of an entire school of girlswould be (1/2)500.

27. In metaphase of mitosis, all ofthe chromosomes align ran-domly at the equator of the cell.In metaphase I of meiosis, pairsof homologous chromosomesalign at the equator of the cell.

28. If the dominant allele com-pletely masks the recessiveallele, you cannot tell if anorganism with the dominanttrait is homozygous or heterozy-gous for the dominant allele.

29. These are the places wherecrossing over takes place andchromosomal material isexchanged.

30. 1. Fertilization; 2. Zygote; 3.Heterozygous; 4. Homozygous;5. Recessive

THINKING CRITICALLYTHINKING CRITICALLY


5. Which of these shows a dominant trait ingarden peas?a. b. c. d.

6. During what phase of meiosis do sister chromatids separate?a. prophase I c. anaphase IIb. telophase I d. telophase II

7. During what phase of meiosis do homolo-gous chromosomes cross over?a. prophase I c. telophase Ib. anaphase I d. telophase II

8. Recessive traits appear only when an organ-ism is ________.a. matureb. different from its parentsc. heterozygousd. homozygous

9. Mendel’s use of peas was a good choice forgenetic study because ________.a. they produce many offspringb. they are easy to growc. they can be self-pollinatedd. all of the above

10. A dihybrid cross between two heterozygousparents produces a phenotypic ratio of________.a. 3: 1 c. 9: 3: 3: 1b. 1: 2: 1 d. 1: 6: 9

11. If two heterozygous organisms for a singledominant trait mate, the ratio of their off-spring should be about ________.

12. A trait that is hidden in the heterozygouscondition is said to be a ________ trait.

13. An organism that has two different alleles fora trait is called ________.

14. The process that results in Down syndromeis called ________.

15. If a species normally has 46 chromosomes,the cells it produces by meiosis will each have________ chromosomes.

16. Metaphase I of meiosis occurs when________ line up next to each other at thecell’s equator.

17. The stage of meiosis shown here is ________.

18. In the first generation of Mendel’s experi-ments with a single trait, the ________ traitdisappeared, only to reappear in the nextgeneration.

19. A cell that has successfully completed meiosishas a chromosome number called ________.

20. Meiosis results in the direct production of________.

21. Why do you think Mendel’s results are alsovalid for humans?

22. On the average, each human has about sixrecessive alleles that would be lethal ifexpressed. Why do you think that humancultures have laws against marriage betweenclose relatives?

23. Assume that a couple has four children whoare all boys. What are the chances that theirnext child will also be a boy? Explain youranswer.

24. How does separation of homologous chro-mosomes during anaphase I of meiosisincrease variation among offspring?

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

284 CHAPTER 10 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Use the Buddy System Study in a group. A small gathering of peopleworks well because it allows you to draw from abroader base of skills and expertise. Keep it smalland keep on target.

284

5. d6. c7. a8. d9. d

10. c11. 3:112. recessive13. heterozygous14. nondisjunction15. 2316. homologous chromosomes17. telophase I18. recessive19. haploid20. gametes

21. Like plants, humans reproducesexually, have chromosomesand genes, and have traits con-trolled by genes.

22. The likelihood that close rela-tives share the same recessivegenes is greater than in thegeneral population, thus raisingthe risk of a child beinghomozygous for those traits.

23. 50%; the probability of any onechild being a certain sex is unaf-fected by the birth of previouschildren.

24. The order of lining up at theequator during metaphase I ofmeiosis will vary, thus providingadditional variation when thechromosomes separate duringanaphase I.

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

Chapter 10 AssessmentChapter 10 Assessment Chapter 10 AssessmentChapter 10 Assessment

1. a2. c3. b4. d5. Genotypes include WwRr,

WwRR, Wwrr, wwrr,wwRR, wwRr. The geno-type of a fruit fly withshort wings and whiteeyes is wwrr. Two of thesixteen possible out-comes, 1/8, in this Punnettsquare are wwrr.

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Documents

Unit 4Unit 4 Unit 4 Genetics Advance Planning file257 Unit Projects Unit Projects Unit 4 Advance Planning Chapter 10 Purchase the following seed types from a biological supply house