Chapter 12: Patterns of Heredity and Human Geneticsimages.pcmac.org/SiSFiles/Schools/AL/FayetteCounty/...Patterns of Heredity and Human Genetics What You’ll Learn You will compare

Patterns of Heredity and Human Genetics

What You’ll Learn■ You will compare the inheri-

tance of recessive and dominant traits in humans.

■ You will analyze the inheri-tance of incompletely domi-nant and codominant traits.

■ You will determine the inher-itance of sex-linked traits.

Why It’s ImportantThe transmission of traits fromgeneration to generation affectsyour appearance, your behavior,and your health. Understandinghow these traits are inherited isimportant in understanding thetraits you may pass on to afuture generation.

Before starting the chapter,make a list of some physicalcharacteristics that may appearin your family members. As youread the chapter, note whetheryour family members havedominant or recessive traits dis-cussed in the text.

To find out more about humangenetics, visit the GlencoeScience Web site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

12ChapterChapter

It is difficult to imagine howthe information for such var-ied traits as eye or hair colorand athletic talent could becontained in the nucleic acidscomposing this chromosome.

314

Magnification: 4500�

BIOLOGY

http://science.glencoe.com

Section

Making a PedigreeAt some point, you have probably

seen a family tree, either for yourfamily or for someone else’s. A familytree traces a family name and variousfamily members through successivegenerations. Through a family tree,you can trace your cousins, aunts,uncles, grandparents, and great-grandparents.

Pedigrees illustrate inheritanceGeneticists often need to map the

inheritance of genetic traits fromgeneration to generation. A pedigreeis a graphic representation of geneticinheritance. At a glance, it looks verysimilar to any family tree.

A pedigree is made up of a set ofsymbols that identify males and

females, individuals affected by thetrait being studied, and family rela-tionships. Some commonly usedsymbols are shown in Figure 12.1. Acircle represents a female; a square

12.1 MENDELIAN INHERITANCE OF HUMAN TRAITS 315

s you learn about traits, you willsee that some, such as tonguerolling or a widow’s peak hair-

line, are relatively harmless. Other traitsproduce devastating disorders and evendeath. All of these traits demonstrate howgenes are inherited, and this iswhat you need to learn. Thedisorders caused by genetictransmission of traits arethe motivation that drivesscientists to do research todiscover treatments andcures.

SECTION PREVIEW

ObjectivesInterpret a pedigree.Determine humangenetic disorders thatare caused by inheri-tance of recessive alleles.Predict how a humantrait can be determinedby a simple dominantallele.

Vocabularypedigreecarrierfetus

12.1 Mendelian Inheritance of Human Traits

Tongue rolling (above) andwidow’s peak hairline (inset)

Figure 12.1 Geneticists use thesesymbols to make andanalyze a pedigree.

Male Parents

Siblings

Knownheterozygotesfor recessiveallele

DeathMating

Female

Affectedmale

Affectedfemale

A

represents a male. Shaded circles and squares represent individualsshowing the trait being studied.Unshaded circles and squares desig-nate individuals that do not show thetrait. A half-shaded circle or squarerepresents a carrier, a heterozygousindividual. A horizontal line connect-ing a circle and a square indicatesthat the individuals are parents, and avertical line connects a set of parentswith their offspring. Each horizontalrow of circles and squares in a pedi-gree designates a generation, withthe most recent generation shown atthe bottom. The generations areidentified in sequence by Romannumerals, and each individual isgiven an Arabic number. You canpractice using these symbols to makea pedigree in the MiniLab on thispage.

Analyzing a pedigreeAn example of a pedigree for a fic-

titious rare, recessive disorder inhumans is shown in Figure 12.2.This disorder could be any of severalrecessive disorders in which the dis-order shows up only if the affectedperson carries two recessive allelesfor the trait. Follow the pedigree asyou read how to analyze it.

Suppose individual III-1 in thepedigree wants to know the likeli-hood of passing on this allele to herchildren. By studying the pedigree,the individual will be able to deter-mine the likelihood that she carriesthe allele. Notice that informationcan also be gained about other mem-bers of the family by studying thepedigree. For example, you knowthat I-1 and I-2 are both carriers ofthe recessive allele for the traitbecause they have produced II-3,who shows the recessive phenotype.If you drew a Punnett square for themating of individuals I-1 and I-2, you

316 PATTERNS OF HEREDITY AND HUMAN GENETICS

Illustrating a Pedigree The pedigree method of studying a trait in a family uses records of phenotypes extending for two or more generations. Studies of pedigrees can be used to yield a great deal of genetic information about a related group.

Procedure! Working with a partner, choose one human trait, such as

attached and free-hanging earlobes or tongue rolling,that interests both of you.

@ Using either your or your partner’s family, collect informa-tion about your chosen trait. Include whether each indi-vidual is male or female, does or does not have the trait,and the relationship of the individual to others in thefamily.

# Use your information to draw a pedigree for the trait.$ Try to determine how your trait is inherited.

Analysis1. What trait did you study? Can you determine from your

pedigree what the apparent inheritance pattern of thetrait is?

2. How is the study of inheritance patterns limited by pedi-gree analysis?

MiniLab 12-1MiniLab 12-1 Analyzing Information

1

1

1

1 2 3 4 5

2 3 4

2 3 4 5

2

I

II

III

?

IV

Figure 12.2 This pedigreeshows how arare, recessiveallele is trans-mitted fromgeneration togeneration.

Sample pedigree

would find, according to Mendeliansegregation, that the ratio ofhomozygous dominant to heterozy-gous to homozygous recessive geno-types among their children would be1: 2: 1. Of those genotypes possiblefor the members of generation II,only the homozygous recessive geno-type will express the trait, which isthe case for II-3.

You can’t tell the genotypes of II-4and II-5, but they have a normal phe-notype. If you look at the Punnettsquare you made, you can see thatthe probability of II-4 and of II-5being a carrier is each two out ofthree because they can have only twopossible genotypes—homozygousnormal and heterozygous. Thehomozygous recessive genotype isnot a possibility in these individualsbecause neither of them shows theaffected phenotype.

Because none of the children ingeneration III are affected andbecause the recessive allele is rare, itis reasonably safe to assume that II-1is not a carrier. You know that indi-vidual II-2 must be a carrier like herparents because she has passed on therecessive allele to subsequent genera-tion IV. Because individual III-1 hasone parent who is heterozygous andthe other parent who is assumed tobe homozygous normal, III-1 mostlikely has a one-in-two chance ofbeing a carrier. If her parent II-1 hadbeen heterozygous instead ofhomozygous normal, III-1’s chancesof being a carrier are increased totwo in three.

Simple RecessiveHeredity

Most genetic disorders are causedby recessive alleles. Many of thesealleles are relatively rare, but a few are common in certain ethnic


What are the chances?Using a Punnett squareallows you to calculatethe chance that off-spring will be born with certain traits. Inorder to do this, how-ever, you must firstknow the genotype ofthe parents and wheth-er the trait that is beingdescribed is dominant orrecessive.

AnalysisThe following traits and their alleles are to be used in

solving problems.

Thinking Critically1. What are the chances that a child will be born with six fin-

gers ifa. both parents are heterozygous?b. one parent has five fingers, the other is heterozygous?c. both parents have five fingers?

2. How many children in a family of four will be able to rolltheir tongues ifa. one parent is a nonroller and the other is a homozy-

gous roller?b. both parents are rollers and both are homozygous?c. both parents are rollers and each of them has a parent

who cannot roll his or her tongue?3. A child is born with attached earlobes but both parents

have hanging earlobes.a. What are the genotypes and phenotypes for the

parents?b. What is the genotype and phenotype for the child?

Problem-Solving Lab 12-1Problem-Solving Lab 12-1

groups. You can practice calculatingthe chance that offspring will be bornwith some of these genetic traits in the Problem-Solving Lab above.

Trait Recessive alleleDominant allele

Number of fingers

Tongue rolling

Earlobes

d = five

t = cannot roll

a = attached

D = six

T = can roll

A = hanging

Table 12.1 Human traits

Polydactyly—having six fingers

Applying Concepts

1

1

1

1

2 3

2 3 4

2

I

II

III

IV

Cystic fibrosis Cystic fibrosis (CF) is a fairly

common genetic disorder amongwhite Americans. Approximately onein 20 white Americans carries therecessive allele, and one in 2000 chil-dren born to white Americans inher-its the disorder. Due to a defectiveprotein in the plasma membrane,cystic fibrosis results in the formationand accumulation of thick mucus inthe lungs and digestive tract. Physicaltherapy, special diets, and new drugtherapies have continued to raise theaverage life expectancy of CFpatients.

Tay-Sachs diseaseTay-Sachs (tay saks) disease is a

recessive disorder of the central ner-vous system. In this disorder, a reces-sive allele results in the absence of anenzyme that normally breaks down alipid produced and stored in tissuesof the central nervous system.Therefore, this lipid fails to break

down properly and accumulates inthe cells. The allele for Tay-Sachs isespecially common in the UnitedStates among Ashkenazic Jews,whose ancestors came from easternEurope. Figure 12.3 shows a typicalpedigree for Tay-Sachs disease.

Phenylketonuria Phenylketonuria (fen ul keet un

YOOR ee uh), also called PKU, is arecessive disorder that results fromthe absence of an enzyme that con-verts one amino acid, phenylalanine,to a different amino acid, tyrosine.Because phenylalanine cannot bebroken down, it and its by-productsaccumulate in the body and result insevere damage to the central nervoussystem. The PKU allele is most com-mon among people whose ancestorscame from Norway or Sweden.

A homozygous PKU newbornappears healthy at first because itsmother’s normal enzyme level pre-vented phenylalanine accumulationduring development. However, oncethe infant begins drinking milk,which is rich in phenylalanine, theamino acid accumulates and mentalretardation occurs. Today, a PKUtest is normally performed on allinfants a few days after birth. Infantsaffected by PKU are given a diet thatis low in phenylalanine until theirbrains are fully developed. With thisspecial diet, the toxic effects of thedisorder can be avoided.

Ironically, the success of treatingphenylketonuria infants has resultedin a new problem. If a female who ishomozygous recessive for PKUbecomes pregnant, the high phenyl-alanine levels in her blood can dam-age her fetus—the developing baby.This problem occurs even if the fetusis heterozygous and would be pheno-typically normal. You may have


Figure 12.3 A study of familieswho have childrenwith Tay-Sachs dis-ease shows typicalpedigrees for traitsinherited as simplerecessives. Note thatthe trait appears toskip generations, acharacteristic of arecessive trait.

noticed PKU warnings on cans ofdiet soft drinks. Because most dietdrinks are sweetened with an artifi-cial sweetener that contains phenyl-alanine, a pregnant woman who ishomozygous recessive must limit herintake of diet foods.

Simple DominantHeredity

Unlike the inheritance of recessivetraits in which a recessive allele mustbe inherited from both parents for aperson to show the recessive pheno-type, many traits are inherited just asthe rule of dominance predicts.Remember that in Mendelian inheri-tance, a single dominant allele inher-ited from one parent is all that isneeded for a person to show thedominant trait.

Simple dominant traitsTongue rolling is one example of a

simple dominant trait. If you can rollyour tongue, you’ve inherited thedominant allele from at least one of your parents. A Hapsburg lip is

shown in Figure 12.4along with earlobetypes, another dominanttrait that is determinedby simple Mendelianinheritance. Having ear-lobes that are attached to thehead is a recessive trait (ff), whereasheterozygous (Ff) and homozygousdominant (FF) individuals have ear-lobes that hang freely.

There are many other human traitsthat are inherited by simple dominantinheritance. Figure 12.5 shows oneof these traits—hitchhiker’s thumb,the ability to bend your thumb tip


Figure 12.5Hitchhiker’s thumb isa dominant trait.

Figure 12.4 The allele F for freelyhanging earlobes (a) is dominant to theallele f for attachedearlobes (b). TheHapsburg lip, a pro-truding lower lip thatresults in a half-openmouth, has been tracedback to the fourteenthcentury through por-traits of members ofthe Hapsburg Dynastyof Europe (c).

a b

c

Section AssessmentSection AssessmentUnderstanding Main Ideas1. In your own words, define the following

symbols used in a pedigree: a square, a circle, anunshaded circle, a shaded square, a horizontalline, and a vertical line.

2. Describe one genetic disorder that is inherited asa recessive trait.

3. How are the cause and onset of symptoms ofHuntington’s disease different from those of PKUand Tay-Sachs disease?

4. Describe one trait that is inherited as a dominantallele. If you carried that trait, would you neces-sarily pass it on to your children?

Thinking Critically5. Suppose that a child with free-hanging earlobes

has a mother with attached earlobes. Can a manwith attached earlobes be the child’s father?

6. Interpreting Scientific Illustrations Make a pedigree for three generations of a family that shows at least one member of each gen-eration who demonstrates a particular trait.Would this trait be dominant or recessive? Formore help, refer to Thinking Critically in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Huntington’s disease Huntington’s disease is a lethal

genetic disorder caused by a rare dom-inant allele. It results in a breakdownof certain areas of the brain. Noeffective treatment exists.

Ordinarily, a dominant allele withsuch severe effects would result indeath before the affected individualcould have children and pass the alleleon to the next generation. But becausethe onset of Huntington’s disease usu-ally occurs between the ages of 30 and50, an individual may have childrenbefore knowing whether he or shecarries the allele. A genetic test hasbeen developed that allows individu-als to check their DNA. Although thistest allows carriers to decide whetherthey want to have children and riskpassing the trait on to future genera-tions, it also places a tremendousburden on them in knowing they willdevelop the disease. For this reason,some people may choose not to betested. The pedigree in Figure 12.6shows a typical pattern of occurrenceof Huntington’s disease in a family.

Notice that every child of anaffected individual has a 50 percentchance of being affected and then a 50percent chance of passing the defec-tive allele to his or her own child.


Figure 12.6 This is a typical pedigree for a simple, dominant inheritanceof Huntington’s disease. This particular chart shows thedisorder in each generation and equally distributed amongmales and females.

1

1

1 2 3 4 5

2 53 4

2

I

II

III

backward more than 30 degrees. Astraight thumb is recessive. Otherdominant traits in humans includealmond-shaped eyes (round eyes arerecessive), thick lips (thin lips arerecessive), and the presence of hairon the middle section of your fingers.

Section

Complex Patterns of Inheritance

Patterns of inheritance that areexplained by Mendel’s experimentsare often referred to as simpleMendelian inheritance—the inheri-tance controlled by dominant andrecessive paired alleles. However,many inheritance patterns are morecomplex than those studied byMendel. As you will learn, most traitsare not simply dominant or recessive.The BioLab at the end of this chapterinvestigates a type of inheritance thatdoesn’t even involve chromosomes.

Incomplete dominance:Appearance of a third phenotype

When inheritance follows a pat-tern of dominance, heterozygous andhomozygous dominant individualsboth have the same phenotype.When traits are inherited in anincomplete dominance pattern,however, the phenotype of the het-erozygote is intermediate betweenthose of the two homozygotes. Forexample, if a homozygous red-flow-ered snapdragon plant (RR) is crossedwith a homozygous white-floweredsnapdragon plant (R'R'), all of the F1 offspring will have pink

12.2 WHEN HEREDITY FOLLOWS DIFFERENT RULES 321

ariations in the pattern ofinheritance explained by

Mendel became known soonafter his work was discovered. Whatdo geneticists do when observed pat-terns of inheritance, such as kernelcolor in this ear of corn, do not appearto follow Mendel’s laws? They oftenuse a strategy of piecing together bitsof a puzzle until the basis for theunfamiliar inheritance pattern isunderstood.

SECTION PREVIEW

ObjectivesDistinguish betweenincompletely dominantand codominant alleles.Compare multipleallelic and polygenicinheritance.Analyze the pattern ofsex-linked inheritance.Summarize how internal and externalenvironments affectgene expression.

Vocabularyincomplete dominancecodominant allelesmultiple allelesautosomesex chromosomesex-linked traitpolygenic inheritance

12.2 When Heredity FollowsDifferent Rules

The genetics of Indian corn (above)is often like a puzzle (inset).

V

flowers, as shown in Figure 12.7.The intermediate pink form of thetrait occurs because neither allele of the pair is completely dominant.Note that the letters R and R', ratherthan R and r , are used to show that these alleles are incompletelydominant.

The new phenotype occurs becausethe flowers contain a colored pig-ment. The R allele codes for anenzyme that produces a red pigment.The R' allele codes for a defectiveenzyme that makes no pigment.Because the heterozygote has onlyone copy of the R allele, its flowersproduce only half the amount of redpigment that the flowers of the redhomozygote produce, and theyappear pink. The R'R' homozygotehas no normal enzyme, produces nored pigment, and appears white.

Note that the segregation of allelesis the same as in simple Mendelianinheritance. However, because nei-ther allele is dominant, the plants of

the F1 generation all have pink flow-ers. When pink-flowered F1 plantsare crossed with each other, the off-spring in the F2 generation appear ina 1: 2: 1 phenotypic ratio of red topink to white flowers. This resultsupports Mendel’s law of independentassortment, which states that the al-leles are inherited independently.

Codominance: Expression of both alleles

In chickens, black-feathered andwhite-feathered birds are homozy-gotes for the B and W alleles, respec-tively. Two different uppercase lettersare used to represent the alleles incodominant inheritance.

One of the resulting heterozygousoffspring in a breeding experimentbetween a black rooster and a whitehen is shown in Figure 12.8. Youmight expect that heterozygous chick-ens, BW, would be black if the patternof inheritance followed Mendel’s lawof dominance, or gray if the trait were


Figure 12.7 A Punnett square of snapdragon colorshows that the red snapdragon ishomozygous for the allele R, and thewhite snapdragon is homozygous for theallele R'. All of the pink snapdragons areheterozygous, or RR'.

RR'

R'

R'

R R

RR'

RR' RR'

All pink flowers

White(R'R')

Red(RR)

RR

R

R'

R R'

RR'

RR' R'R'

1 red: 2 pink: 1 white

Allpink

Red White

Pink(RR')

Pink(RR')

incompletely dominant. Notice, how-ever, that the heterozygote is neitherblack nor gray. Instead, all of the off-spring are checkered; some feathersare black and other feathers are white.In such situations, the inheritance pat-tern is said to be codominant.Codominant alleles cause the pheno-types of both homozygotes to be pro-duced in heterozygous individuals. Incodominance, both alleles areexpressed equally.

Multiple phenotypes from multiple alleles

Although each trait has only twoalleles in the patterns of heredity youhave studied thus far, it is commonfor more than two alleles to control atrait in a population. This is under-standable when you recall that a newallele can be formed any time a muta-tion occurs in a nitrogen base some-where within a gene. Although onlytwo alleles of a gene can exist withina diploid cell, multiple alleles for a

single gene can be studied in a popu-lation of organisms.

Traits controlled by more than twoalleles have multiple alleles. Thepigeons pictured in Figure 12.9show the effects of multiple allelesfor feather color. Three alleles of asingle gene govern their feathercolor, although each pigeon can haveonly two of these alleles. The num-ber of alleles for any particular trait isnot limited to three, and there areinstances in which more than 100


Figure 12.8When a certain vari-ety of black chicken iscrossed with a whitechicken, all of the off-spring are checkered.Both feather colorsare produced bycodominant alleles.

Figure 12.9 In pigeons, a single gene that con-trols feather color has three alleles.An enzyme that activates the pro-duction of a pigment is controlledby the B allele. This enzyme is lack-ing in bb pigeons.

The dominant BA

allele producesash-red coloredfeathers.

AA

The B allele produces wild-type blue feathers. B is domi-nant to b but recessive to BA.

BB

The allele b pro-duces a chocolate-colored feather andis recessive to bothother alleles.

CC

alleles are known to exist for a singletrait! You can learn about anotherexample of multiple alleles in theProblem-Solving Lab shown here.

Sex determination Recall that in humans the diploid

number of chromosomes is 46, or 23pairs. There are 22 pairs of matchinghomologous chromosomes calledautosomes. Homologous autosomeslook exactly alike. The 23rd pair ofchromosomes differs in males andfemales. These two chromosomes,which determine the sex of an indi-vidual, are called sex chromosomes.In humans, the chromosomes thatcontrol the inheritance of sex charac-teristics are indicated by the letters Xand Y. If you are a human female,XX, your 23rd pair of chromosomesare homologous and look alike, asshown in Figure 12.10A. However,if you are a male, XY, your 23rd pairof chromosomes look different.Males, which have one X and one Ychromosome, produce two kinds ofgametes, X and Y, by meiosis.Females have two X chromosomes,so they produce only X gametes.Figure 12.10B shows that after fertilization, a 1: 1 ratio of males to females is expected. Because fertil-ization is governed by the laws of


How is coat color in rabbits inherited? Coat color in rab-bits is inherited as a series of multiple alleles. This means thatthere can be more than just two alleles for a single gene. Inthe case of coat color in rabbits, there are four alleles, andeach one is expressed with a different phenotype.

AnalysisExamine Table 12.2. Use this information to answer the

questions. Remember, each rabbit can have only two allelesfor coat color.

Thinking Critically1. List all possible genotypes for a

a. dark gray-coated rabbit (there are 4).b. chinchilla rabbit (there are 3).c. Himalayan rabbit (there are 2).d. white rabbit (there is 1).

2. Predict the phenotype for a rabbit with a chcch and with aCch genotype. Explain your answer.

3. Would it be possible to obtain white rabbits if one parentis white and the other is chinchilla? Explain your answer.

4. Would it be possible to obtain chinchilla rabbits if oneparent is Himalayan and the other is white? Explain.

5. A chinchilla rabbit is mated with a Himalayan. Some off-spring are white. What are the parents’ genotypes?

Problem-Solving Lab 12-2Problem-Solving Lab 12-2 Predicting

Figure 12.10The sex chromo-somes in humans arecalled X and Y.

Phenotype Pattern of inheritanceAllele

Dark gray coat

Chinchilla

Himalayan

White

dominant to all other alleles

dominant to Himilayan and to white

dominant to white

recessive

C

cch

ch

c

Table 12.2 Coat color in rabbits

Half the offspring of any matingbetween humans will have two Xchromosomes, XX, which is female.The other half of the offspring willhave one X and one Y chromosome,XY, which is male.

BB

The sex chromo-somes are namedfor the lettersthey resemble.

AAX X X Y

MaleFemale XXFemale

XXFemale

XXFemale

X

X Y

X

XYMale

XYMale

XYMale

probability, the ratio usually is notexactly 1: 1 in a small population.

Sex-linked inheritanceDrosophila (droh SAHF uh luh),

commonly known as fruit flies,inherit sex chromosomes in the sameway as humans do. Traits controlledby genes located on sex chromo-somes are called sex-linked traits.The alleles for sex-linked traits arewritten as superscripts of the X or Ychromosome. Because the X and Ychromosomes are not homologous,the Y chromosome has no corre-sponding allele to one on the X chro-mosome and no superscript is used.Also remember that any allele on theX chromosome of a male will not bemasked by a corresponding allele onthe Y chromosome.

In 1910, Thomas Hunt Morgandiscovered traits linked to sex chro-mosomes. Morgan noticed one daythat one male fly had white eyesrather than the usual red eyes. Hecrossed the white-eyed male with a

homozygous red-eyed female. All ofthe F1 offspring had red eyes, indi-cating that the white-eyed trait isrecessive. Then Morgan allowed theF1 flies to mate among themselves.According to simple Mendelianinheritance, if the trait were reces-sive, the offspring in the F2 genera-tion would show a 3: 1 ratio of red-eyed to white-eyed flies. As you cansee in Figure 12.11, this is whatMorgan observed. However, he alsonoticed that the trait of white eyesappeared only in male flies.

Morgan hypothesized that the red-eye allele was dominant and thewhite-eye allele was recessive. Healso reasoned that the gene for eyecolor was located on the X chromo-some and was not present on the Ychromosome. In heterozygousfemales, the dominant allele for redeyes masks the recessive allele forwhite eyes. In males, however, a sin-gle recessive allele is expressed as awhite-eyed phenotype. When Morgancrossed a heterozygous red-eyed


Figure 12.11 Morgan crossed a white-eyed male fruit flywith a normal homozygous red-eyed female(a). He then allowed the F1 flies to mate (b).The superscripts R and r are the dominant andrecessive alleles for eye color in fruit flies.

a b

XRXr

Xr Y

XR

XR

XRY

XRXr XRY

F1 All red eyed

White-eyedmale (XrY)

Red-eyedfemale (XRXR)

XRXR

XR Y

XR

Xr

XRY

XRXr XrY

F2

Females:all red eyed

Males:1/2 red eyed1/2 white eyed

female with a white-eyed male, halfof all the males and half of all thefemales inherited white eyes. Theonly explanation of these results isMorgan’s hypothesis: The allele foreye color is carried on the X chromo-some and the Y chromosome has noallele for eye color.

Traits dependent on genes that follow the inheritance pattern of a sex chromosome are called sex-linked traits. Eye color in fruit flies is an example of an X-linked trait. Y-linked traits are passed only frommale to male.

Polygenic inheritanceSome traits, such as skin color and

height in humans, and cob length incorn, vary over a wide range. Suchranges occur because these traits aregoverned by many different genes.Polygenic inheritance is the inheri-tance pattern of a trait that is controlled by two or more genes.The genes may be on the same chro-mosome or on different chromo-somes, and each gene may have twoor more alleles. For simplicity,uppercase and lowercase letters are

used to represent the alleles, as theyare in Mendelian inheritance. Keepin mind, however, that the allele rep-resented by an uppercase letter is notdominant. All heterozygotes areintermediate in phenotype.

In polygenic inheritance, eachallele represented by an uppercaseletter contributes a small, but equal,portion to the trait being expressed.The result is that the phenotypesusually show a continuous range ofvariability from the minimum valueof the trait to the maximum value.

Suppose, for example, that stemlength in a plant is controlled bythree different genes: A, B, and C.Each gene is on a different chromo-some and has two alleles, which canbe represented by uppercase or low-ercase letters. Thus, each diploidplant has a total of six alleles for stemlength. A plant that is homozygousfor short alleles at all three gene loca-tions (aabbcc) might grow to be only 4cm tall, the base height. A plant thatis homozygous for tall alleles at allthree gene locations (AABBCC)might be 16 cm tall. The differencebetween the tallest possible plant and

the shortest possible plant is12 cm, or 2 cm per each ofthe six tall alleles. You couldsay that each allele repre-sented by an uppercase lettercontributes 2 cm to the totalheight of the plant.

Suppose a 16-cm-tall plantwere crossed with a 4-cm-tallplant. In the F1 generation,all the offspring would beAaBbCc. If each of the threetall genes A, B, and C con-tributed 2 cm of height tothe base height of 4 cm, theplants would be 10 cm tall (4cm + 6 cm)—intermediate inheight. If they are allowed tointerbreed, the F2 offspring

Figure 12.12 In this example ofpolygenic inheritance,three genes each havetwo alleles that con-tribute to the trait.When the distributionof plant heights isgraphed, a bell-shapedcurve is formed.Intermediate heightsoccur most often.

Stem length (cm)

Expe

cted

num

ber o

f F2 p

lant

s

4 6 8 10 12 14 16

5

10

0

15

20

Stem Length Variation in a Plant Polygenic for the Trait

OriginWORDWORD

polygenicFrom the Greekwords polys, mean-ing “many,” andgenos, meaning“kind.” Polygenicinheritance involvesmany genes.


will show a broad range of heights. APunnett square of this trihybrid crosswould show that 10-cm-tall plantsare most often expected, and thetallest and shortest plants are seldomexpected. Notice in Figure 12.12that when these results are graphed,the shape of the graph confirms theprediction of the Punnett square.

EnvironmentalInfluences

Even when you understand domi-nance and recessiveness and you havesolved the puzzles of the other pat-terns of heredity, the inheritance pic-ture is not complete. The geneticmakeup of an organism at fertiliza-tion determines only the organism’spotential to develop and function. Asthe organism develops, many factorscan influence how the gene isexpressed, or even whether the geneis expressed at all. Two such influ-ences are the organism’s external andinternal environments.

Influence of external environmentSometimes, individuals known to

have a particular gene fail to expressthe phenotype specified by that gene.Temperature, nutrition, light, chemi-cals, and infectious agents all can

influence gene expression. In certainbacteria, temperature has an effect onthe expression of color, as shown inFigure 12.13. External influencescan also be seen in leaves. Leaves ona tree can have different sizes andshapes depending on the amount oflight they receive.

Influence of internal environmentThe internal environments of

males and females are differentbecause of hormones and structuraldifferences, Figure 12.14. For exam-ple, traits such as horn size in moun-tain sheep, male-pattern baldness inhumans, and feather color in pea-cocks are expressed differently in the

327

Figure 12.13 Serratia marcescens isa bacterium that formsbrick-red growth onsolid media at 25oC.When the same bacte-ria are grown at 30oC,the growth is creamcolored.

Figure 12.14 The horns of a ram(male) are muchheavier and morecoiled than those ofa ewe (female)although their geno-types are identical.

sexes, as you can see in Figure 12.15.These differences are controlled bydifferent hormones, which are deter-mined by different sets of genes.

An organism’s age also affects genefunction. The nature of such a pat-tern is not well understood, but it isknown that the internal environmentof an organism changes with age.

You can now see that you mustlearn how genes interact with eachother and with the environment toform a more complete picture ofinheritance. Mendel’s idea that hered-ity is a composite of many individualtraits still holds. Later researchershave filled in more details of Mendel’sgreat contributions.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. A cross between a purebred animal with red hairs

and a purebred animal with white hairs producesan animal that has both red hairs and white hairs.What type of inheritance pattern is involved?

2. In a cross between individuals of a species oftropical fish, all of the male offspring have long tail fins, and none of the females possessthe trait. Mating of the F1 fish fails to producefemales with the trait. Explain a possible inheri-tance pattern of the trait.

3. A red-flowered sweet pea plant is crossed with awhite-flowered sweet pea plant. All of the off-spring are pink. What is the inheritance patternbeing expressed?

4. The color of wheat grains shows a wide vari-ability between red and white with multiple

phenotypes. What type of inheritance pattern isbeing expressed?

Thinking Critically5. Armadillos always have four offspring that have

identical genetic makeup. Suppose that, within alitter, each young armadillo is found to have adifferent phenotype for a particular trait. Howcould you explain this phenomenon?

6. Forming a Hypothesis An ecologist observesthat a population of plants in a meadow hasflowers that may be red, yellow, white, pink, orpurple. Hypothesize what the inheritance pat-tern might be. For more help, refer to PracticingScientific Methods in the Skill Handbook.


Figure 12.15Some traits areexpressed differentlyin the sexes.

Human male-patternbaldness, prematurebalding that occurs in a characteristic pattern,affects males but notfemales.

CC

The plumage of thefemale peahen is dullby comparison.

BB

The plumage ofthe male peacockis highly decoratedand colored.

AA

Section

Codominance in Humans

Remember that in codominance,the phenotypes of both homozygotesare produced in the heterozygote.One example of this type of inheri-tance in humans is the disordersickle-cell anemia.

Sickle-cell anemiaSickle-cell anemia is a major health

problem in the United States and inAfrica. In the United States, it ismost common in black Americanswhose families originated in Africaand in white Americans whose fami-lies originated in the countries

surrounding the Mediterranean Sea.About one in 12 African Americans, amuch larger proportion than in mostpopulations, is heterozygous for thedisorder.

In an individual who is homozy-gous for the sickle-cell allele, theoxygen-carrying protein hemoglobindiffers by one amino acid from nor-mal hemoglobin. This defectivehemoglobin forms crystal-like struc-tures that change the shape of the redblood cells. The abnormal red bloodcells are shaped like a sickle, or half-moon. The change in shape occurs inthe body’s narrow capillaries after thehemoglobin releases oxygen to thecells. Abnormally shaped blood cells,

12.3 COMPLEX INHERITANCE OF HUMAN TRAITS 329

For decades, movies and lit-erature have portrayedtimes when people could

genetically program their futureoffspring to have specific charac-teristics. You have probablythought about how you wouldchange certain of your featuresif given the opportunity. As sci-entists study the inheritance oftraits such as height andeye color, they are dis-covering that thepassing of thesetraits can be verycomplex.

SECTION PREVIEW

ObjectivesCompare codominance,multiple allelic, sex-linked, and polygenic patterns of inheritance in humans.Distinguish among conditions in which extra autosomal or sexchromosomes exist.

Vocabularykaryotype

12.3 Complex Inheritance of Human Traits

Eye color and heightare complex traits.

Figure 12.16, slow blood flow, blocksmall vessels, and result in tissuedamage and pain. Because sickledcells have a shorter life span thannormal red blood cells, the personsuffers from anemia, a condition inwhich there is a low number of redblood cells.

Individuals who are heterozygousfor the allele produce both normaland sickled hemoglobin, an exampleof codominance. They produceenough normal hemoglobin that theydo not have the serious health prob-lems of those homozygous for theallele and can lead relatively normallives. Individuals who are heterozy-gous are said to have the sickle-celltrait because they can show somesigns of sickle-cell anemia if theavailability of oxygen is reduced.

Multiple Alleles in Humans

Traits that are governed by simpleMendelian heredity have only twoalleles. However, you have learnedthat more than two alleles of a geneare possible for certain traits. TheABO blood group is a classic example

of a single gene that has multiplealleles in humans. How many allelesdoes this gene have? Read the InsideStory to answer this question.

Multiple alleles govern blood typeHuman blood types, listed in

Table 12.3, are determined by thepresence or absence of certain mole-cules on the surfaces of red bloodcells. As the determinant of bloodtypes A, B, AB, and O, the gene I hasthree alleles: IA, IB, and i.

The importance of blood typingDetermining blood type is neces-

sary before a person can receive ablood transfusion because the redblood cells of incompatible bloodtypes could clump together, causingdeath. Blood typing can also be help-ful in solving cases of disputedparentage. For example, if a child hastype AB blood and its mother hastype A, a man with type O bloodcould not possibly be the father. Butblood tests cannot prove that a cer-tain man definitely is the father; theyindicate only that he could be. DNAtests are necessary to determineactual parenthood.


Figure 12.16 In sickle-cell anemia,the gene for hemo-globin produces aprotein that is differ-ent by one aminoacid. This hemoglo-bin crystallizes whenoxygen levels arelow, forcing the redblood cells into asickle shape (a). Anormal red blood cellis disc shaped (b).

a bMagnification: 90 000� Magnification: 90 000�


The ABO Blood Group

The gene for blood type, gene I, codes for a mem-brane protein found on the surface of red blood

cells. Each of the three alleles codes for a different pro-tein. Your immune system recognizes the red blood cellsas belonging to you. If cells with a different protein enteryour body, your immune system will attack them.

Critical Thinking If you inherit ii from your parents,what is your blood type?

Magnification: 21 600�

Red blood cells

INSIDESSTORTORYY

INSIDE

Phenotype A The IA allele is dominant toi, so inheriting eitherthe IAi alleles or the IA IA alleles from your two parentswill give you type Ablood. Surface proteinA is produced.

11

Phenotype B The IB allele is also dominantto i. To have type B blood, you must inheritthe IB allele from one par-ent and either another IB

allele or the i allele fromthe other. Surface proteinB is produced.

22

Phenotype O The i allele isrecessive and produces no sur-face molecule. Therefore, ifyou are homozygous ii, your

blood cells have nosurface proteins

and you haveblood type O.

44

Genotypes Surface Proteins Phenotypes

IAIA or IAi A A

IBIB or IBi B B

IAIB A and B AB

ii none O

Table 12.3 Human blood types

Phenotype AB The IA and IB alleles arecodominant to each other. This means that ifyou inherit the IA allele from one parent andthe IB allele from the other, your red bloodcells will produce both surface proteins andyou will have type AB blood.

33

Surface protein A

I A I A

orIAi

Surface protein B

I B I B

orIBi

Surface protein B

Surface protein A

I A I B

i i

Sex-Linked Traits in Humans

Several human traits are deter-mined by genes that are carried on thesex chromosomes; most of these genesare located on the X chromosome.The pattern of sex-linked inheritanceis explained by the fact that males,who are XY, pass an X chromosometo each of their daughters and a Ychromosome to each son. Females,who are XX, pass one of their X chro-mosomes to each child, Figure 12.17.If a son receives an X chromosomewith a recessive allele from hismother, he will express the recessivephenotype because he has no chanceof inheriting from his father a domi-nant allele that would mask theexpression of the recessive allele.

Two traits that are governed by X-linked inheritance in humans arecertain forms of color blindness andhemophilia. Determine whetherDuchenne’s muscular dystrophy issex-linked by reading the Problem-Solving Lab on this page.

Red-green color blindnessPeople who have red-green color

blindness can’t differentiate these two


How is Duchenne’smuscular dystrophyinherited? Muscular dystrophy is oftenthought of as a single disorder, but it is actuallya group of genetic disor-ders that produce muscu-lar weakness, progressivedeterioration of musculartissue, and loss of coordi-nation. Different forms ofmuscular dystrophy canbe inherited as a domi-nant autosomal, a reces-sive autosomal, or a sex-linked disorder. These three patterns of inheritanceappear different from one another when a pedigree is made.One rare form of muscular dystrophy, called Duchenne’s mus-cular dystrophy, affects three in 10 000 American males.

AnalysisThe pedigree shown here represents the typical inheri-

tance pattern for Duchenne’s muscular dystrophy. Refer to Figure 12.1 if you need help interpreting the symbols.Analyze the pedigree to determine the pattern of inheri-tance. Is this an autosomal or a sex-linked disorder?

Thinking CriticallyIf individual IV-1 had a daughter and a son, what would

be the probability that the daughter is a carrier? That the soninherited the disorder?

Problem-Solving Lab 12-3Problem-Solving Lab 12-3 Drawing a Conclusion

Figure 12.17 If a trait is X-linked,males pass the X-linked allele to all oftheir daughters butto none of their sons(a). Heterozygousfemales have a 50percent chance ofpassing a recessive X-linked allele to eachchild (b).

1

1

1 2 3 4

2 3 4 5

2

1 2 3 4 5

I

II

III

IV

Typical pedigree

XY XYAA BB

FemaleMale

Sperm Eggs

XY

Male

XY

Male

XX

Female

XXXX

XX Y X X

Female

XXXX

Eggs SpermXX YXXX

Female Male

XYXY

Male

XY

Male

XX

Female

XXXX

Female

XX

XYa b

colors. Red-green color blindnesswas first described in a boy whocould not be trained to harvest onlythe ripe, red apples from his father’sorchard. Instead, he chose greenapples as often as he chose red.

Other more serious problems canresult from this disorder, such as theinability of color-blind people toidentify red and green traffic lights bycolor. Color blindness is caused bythe inheritance of either of two reces-sive alleles at two gene sites on the Xchromosome that affect red and greenreceptors in the cells of the eyes.

Hemophilia: An X-linked disorderDid you ever wonder about why a

cut stops bleeding so quickly? Thishuman adaptation is essential. If yourblood didn’t have the ability to clot atall, any cut could take a long time tostop bleeding. Of greater concernwould be internal bleeding resultingin a bruise, which a person may notimmediately notice.

Hemophilia A is an X-linked disor-der that causes just such a problemwith blood clotting. About one malein every 10 000 has hemophilia, butonly about one in 100 million femalesinherits the same disorder. Why?Males inherit the allele for hemo-philia on the X chromosome fromtheir carrier mothers. A single reces-sive allele for hemophilia will causethe disorder in males. Females wouldneed two recessive alleles to inherithemophilia. The family of QueenVictoria, pictured in the Social StudiesConnection at the end of this chapter,is the best-known study of hemo-philia A, also called royal hemophilia.

Hemophilia A can be treated withblood transfusions and injections ofFactor VIII, the blood-clottingenzyme that is absent in peopleaffected by the condition. However,both treatments are expensive. New

methods of DNA technology arebeing used to develop a cheapersource of the clotting factor.

Polygenic Inheritance in Humans

Think of all the traits you inher-ited from your parents. Althoughmany of your traits were inheritedthrough simple Mendelian patternsor through multiple alleles, manyother human traits are determined bypolygenic inheritance. These kinds oftraits usually represent a range ofvariation that is measurable. TheMiniLab shown here examines one ofthese traits—the color variations inhuman eyes.


Detecting Colors and Patterns inEyes Human eye color, like skincolor, is determined by polygenicinheritance. You can detect severalshades of eye color, especially if youlook closely at the iris with a magni-fying glass. Often, the pigment isdeposited so that light reflects fromthe eye, causing the iris to appearblue, green, gray, or hazel (brown-green). In actuality, the pigment maybe yellowish or brown, but not blue.

Procedure! Use a magnifying glass to observe the patterns and colors

of pigment in the eyes of five classmates.@ Use colored pencils to make drawings of the five irises.# Describe your observations in your journal.

Analysis1. How many different pigments were you able to detect in

each eye?2. From your data, do you suspect that eye color might not

be inherited by simple Mendelian rules? Explain.3. Suppose that two people have brown eyes. They have two

children with brown eyes, one with blue eyes, and onewith green eyes. What pattern might this suggest?

MiniLab 12-2MiniLab 12-2

Hazel eye color

Observing and Inferring

Skin color: A polygenic trait

In the early 1900s, theidea that polygenic inheritance

occurs in humans was first testedusing data collected on skin color.Scientists found that when light-skinned people mate with dark-skinned people, their offspring haveintermediate skin colors. When thesechildren produce the F2 generation,the resulting skin colors range fromthe light-skin color to the dark-skincolor of the grandparents (the P1 gen-eration), with most children having anintermediate skin color. As Figure12.18 shows, the variation in skincolor indicates that between threeand four genes are involved.

Changes inChromosome Numbers

You have been reading about traitsthat are caused by one or severalgenes on chromosomes. What wouldhappen if an entire chromosome orpart of a chromosome were missingfrom the complete set? What if cells

had an extra chromosome? As youhave learned, abnormal numbers ofchromosomes usually, but not always,result from accidents of meiosis.Many abnormal phenotypic effectsresult from such mistakes.

Unusual numbers of autosomesYou know that a human usually has

23 pairs of chromosomes, or 46 chro-mosomes altogether. Of these 23pairs of chromosomes, 22 pairs areautosomes. Humans who have anunusual number of autosomes all aretrisomic—that is, they have three of aparticular autosome instead of justtwo. In other words, they have 47chromosomes. Recall that trisomyusually results from nondisjunction,which occurs when paired homolo-gous chromosomes fail to separateproperly during meiosis.

To identify an abnormal number ofchromosomes, a sample of cells isobtained from an individual or froma fetus. Metaphase chromosomes arephotographed, and the chromosomepictures are then enlarged, cut apart,and arranged in pairs on a chart


b

OriginWORDWORD

autosomeFrom the Greekwords autos, mean-ing “self,” and soma,meaning “body.” Anautosome is a chro-mosome that is nota sex chromosome.

Num

ber o

f ind

ivid

uals

Classes of skin color

4 genes

Observeddistributionof skin color

Expected distribution

3 genes

1 gene

I II III IV V VI VII VIII IX

Genes Involved in Skin Color

a

Figure 12.18This graph (b) shows theexpected distribution ofhuman skin color if con-trolled by one, three, orfour genes. The observeddistribution of skin color(a) closely matches thedistribution shown byfour genes.

according to length and location ofthe centromere, as Figure 12.19shows. This chart of chromosomepairs is called a karyotype, and it isvaluable in pinpointing unusual chro-mosome numbers in cells.

Down syndrome: Trisomy 21Most disorders of chromosome

number that occur in humans causesymptoms so severe that the develop-ing fetus dies, often before thewoman even realizes she is pregnant.Fortunately, these disorders occuronly rarely. Down syndrome is theonly autosomal trisomy in whichaffected individuals survive to adult-hood. It occurs in about one in 700live births.

Down syndrome is a group ofsymptoms that results from trisomyof chromosome 21. Individuals whohave Down syndrome have at leastsome degree of mental retardation.The incidence of Down syndromebirths is higher in older mothers,especially those over 40.

Unusual numbers of sex chromosomes

Many abnormalities in the numberof sex chromosomes are known to

exist. An X chromosome may bemissing (designated as XO) or theremay be an extra one (XXX or XXY).There may also be an extra Y chro-mosome (XYY), as you can see byexamining Figure 12.19. Any indi-vidual with at least one Y chromo-some is a male, and any individualwithout a Y chromosome is a female.Most of these individuals lead normallives, but they cannot have childrenand some have varying degrees ofmental retardation.


Figure 12.19 This karyotypedemonstrates XYYsyndrome, wheretwo Y chromosomesare inherited in addi-tion to an X chromo-some.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. Why are sex-linked traits such as red-green color

blindness and hemophilia more commonly found in males than in females? Explain your answer in terms of the X chromosome.

2. In addition to revealing chromosome abnormalities,what other information would a karyotype show?

3. What would the genotypes of parents have to be for them to have a color-blind daughter? Explain.

4. Describe a genetic trait in humans that is inherited as codominance. Describe the phenotypes of the two homozygotes and that of the heterozygote. Why is this trait an example of codominance?

Thinking Critically5. A man is accused of fathering two children, one

with type O blood and another with type Ablood. The mother of the children has type Bblood. The man has type AB blood. Could he bethe father of both children? Explain your answer

6. Making and Using Tables Construct a table ofthe traits discussed in this section. For columnheads, use Trait, Pattern of inheritance, andCharacteristics. For more help, refer toOrganizing Information in the Skill Handbook.



What is the pattern of cytoplasmic inheritance?

T he mitochondria of all eukaryotes and the chloroplasts of plants andalgae contain DNA. This DNA is not coiled into chromosomes, but

it still carries genes that control genetic traits. Many of the mitochondrialgenes control steps in the respiration process.

The DNA in chloroplasts controls traits such as chlorophyll produc-tion. Lack of chlorophyll in some cells causes the appearance of whitepatches in a leaf. This trait is known as variegated leaf. In this BioLab,you will carry out an experiment to determine the pattern of cytoplasmicinheritance of the variegated leaf trait in Brassica rapa.

YOUR OWNDESIGN

YOUR OWNDESIGN

ProblemWhat inheritance pattern does the

variegated leaf trait in Brassica show?

HypothesesConsider the possible evidence you

could collect that would answer theproblem question. Among the peoplein your group, form a hypothesis thatyou can test to answer the question,and write the hypothesis in yourjournal.

ObjectivesIn this BioLab, you will:■ Determine which crosses of Brassica

plants will reveal the pattern ofcytoplasmic inheritance.

■ Analyze data from Brassica crosses.

Possible MaterialsBrassica rapa seeds, normal

and variegatedpotting soil and trayspaintbrushesforcepssingle-edge razor bladelight sourcelabels

Safety PrecautionsAlways wear goggles in the lab.

Handle the razor blade with extremecaution. Always cut away from you.Wash your hands with soap and waterafter working with plant material.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

PLAN THE EXPERIMENTPLAN THE EXPERIMENT

1. Decide which crosses will beneeded to test your hypothesis.

2. Keep the available materials inmind as you plan your proce-dure. How many seeds will youneed?

3. Record your procedure, andlist the materials and quantitiesyou will need.

4. Assign a task to each member ofthe group. One person shouldwrite data in a journal, anothercan pollinate the flowers, whilea third can set up the planttrays. Determine who will setup and clean up materials.

5. Design and construct a datatable for recording your obser-vations.

Check the PlanDiscuss the following points

with other group members todecide the final procedure for yourexperiment.

1. What data will you collect, andhow will data be recorded?

2. When will you pollinate theflowers? How many flowerswill you pollinate?

3. How will you transfer pollenfrom one flower to another?

4. How and when will you collectthe seeds that result from yourcrosses?

5. What variables will have to becontrolled? What controls willbe used?

6. When will youend the experi-ment?

7. Make sure yourteacher hasapproved yourexperimentalplan before youproceed further.

8. Carry out yourexperiment.

1. Checking Your Hypothesis Didyour data support your hypothe-sis? Why or why not?

2. Interpreting Observations Whatis the inheritance pattern of var-iegated leaves in Brassica?

3. Making Inferences Explain whygenes in the chloroplast areinherited in this pattern.

4. Drawing Conclusions Whichparent is responsible for passingthe variegated trait to its offspring?

5. Making Scientific IllustrationsDraw a diagram tracing the

inheritance of this trait throughcell division.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE


Going FurtherGoing Further

Project Make crosses between normalBrassica plants and genetically dwarfed,mutant Brassica plants to determine theinheritance pattern of the dwarf mutation.

To find out more about inheritance of traits, visit

the Glencoe Science Web site.science.glencoe.com

BIOLOGY


One of the most famous examples of a pedigree demonstrating inheritance

of a sex-linked trait is the family of QueenVictoria of England and hemophilia.

Queen Victoria had four sons and five daugh-ters. Her son Leopold had hemophilia and

died as a result of a minor fall. Two of her daugh-ters, Alice and Beatrice, were carriers for the traitand passed the disorder to royal families in Spain,Prussia, and Russia over four generations.

The Spanish royal family Victoria’s daughterBeatrice, a carrier for the trait, married PrinceHenry of Battenberg, a descendent of Prussianroyalty. Two of their sons inherited the trait,both dying before the age of 35. Her daughter,Victoria, was a carrier and married King AlfonsoXIII of Spain, thus transmitting the allele to theSpanish royal family. Two of their sons died ofhemophilia, also by their early thirties.

The Prussian royal family Alice, another ofVictoria’s daughters, married Louis IV of Hesse,part of the Prussian royal family and related toPrince Henry of Battenberg. One of Alice’s sons,Frederick, died at the age of three from hemo-philia. One of her daughters, Irene, continued topass the trait to the next generation of Prussianroyalty by giving it to two of her sons.

The Russian royal family Irene’s sister and Queen Victoria’s granddaughter, Alix(Alexandra), married Czar Nicholas II of Russia.Four healthy daughters were born, but the firstmale heir, Alexis, showed signs of bleeding andbruising at only six weeks of age. Having abrother, an uncle, and two cousins who had suf-fered from the disorder and died at early ages,you can imagine the despair Alix felt for her sonand the future heir. In desperation, the familyturned to Rasputin, a man who claimed to havehealing abilities and used Alexis’ illness for his

own political power. The series of events sur-rounding Alexis and his hemophilia played a rolein the downfall of the Russian monarchy.

The British throne today Queen Elizabeth II,the current English monarch, is descended from Queen Victoria’s eldest son, Edward VII.Because he did not inherit the trait, he could notpass it on to his children. Therefore, the Britishmonarchy today does not carry the recessiveallele for hemophilia, at least not inherited fromQueen Victoria.


ConnectionSocial StudiesSocial Studies

Connection Queen Victoria andRoyal Hemophilia

If you were the child of a female carrier for a sex-linked trait such as hemophilia, what wouldbe your chances of carrying the trait?

To find out more about hemophilia, visit the Glencoe

Science Web site. science.glencoe.com

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

Queen Victoria and her family

BIOLOGY


Chapter 12 AssessmentChapter 12 Assessment

SUMMARYSUMMARY

Section 12.1

Section 12.2

Section 12.3

Main Ideas■ A pedigree is a family tree of inheritance.■ Most human genetic disorders are inherited

as rare recessive alleles, but a few are inher-ited as dominant alleles.

Vocabularycarrier (p. 316)fetus (p. 318)pedigree (p. 315)

MendelianInheritance ofHuman Traits

Main Ideas■ Alleles can be incompletely dominant or

codominant. ■ There may be many alleles for one trait or

many genes that interact to produce a trait.■ Inheritance patterns of genes located on sex

chromosomes are due to differences in thenumber and kind of sex chromosomes inmales and in females.

■ The expression of some traits is affected bythe internal and external environments ofthe organism.

Vocabularyautosome (p. 324)codominant alleles

(p. 323)incomplete dominance

(p. 321)multiple alleles (p. 323)polygenic inheritance

(p. 326)sex chromosome (p. 324)sex-linked trait (p. 325)

Main Ideas■ The majority of human traits are controlled

by multiple alleles or by polygenic inheri-tance. The inheritance patterns of thesetraits are highly variable.

■ Sex-linked traits are determined by inheri-tance of sex chromosomes. X-linked traitsare usually passed from carrier females totheir male offspring. Y-linked traits arepassed only from male to male.

■ Mistakes in meiosis, usually due to nondis-junction, may result in an abnormal numberof chromosomes. Autosomes or sex chromo-somes can be affected.

Vocabulary karyotype (p. 335)

CHAPTER 12 ASSESSMENT 339

1. If a trait is X-linked, males pass the X-linkedallele to ________ of their daughters.a. all c. noneb. half d. 1/4

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 2. Stem length demonstrates a range of pheno-types. This is an example of ________.a. autosomal dominantb. autosomal recessivec. sex-linkaged. polygenic inheritance

When HeredityFollowsDifferent Rules

ComplexInheritance ofHuman Traits


3. Two parents with normal phenotypes have adaughter with a genetically inherited disor-der. This is an example of a(n) ________ trait.a. autosomal dominant c. sex-linkedb. autosomal recessive d. polygenic

4. Which of the follow-ing disorders would be inherited accord-ing to the pedigree shown here?a. Tay-Sachs diseaseb. sickle-cell anemiac. cystic fibrosisd. Huntington’s disease

5. Which of the following disorders is likely tobe inherited by more males than females?a. Huntington’s diseaseb. Down syndromec. hemophiliad. cystic fibrosis

6. Infants with PKU cannot break down theamino acid ________.a. tyrosine c. methionineb. lysine d. phenylalanine

7. A karyotype reveals ________.a. an abnormal number of genesb. an abnormal number of chromosomesc. polygenic traitsd. multiple alleles for a trait

8. A mother with blood type IBi and a fatherwith blood type IAIB have children. Which ofthe following genotypes would be possiblefor their children?a. AB c. Bb. O d. a and c are correct

9. Normally, lethal autosomal dominant traitsare eliminated from a population becausethey ________.a. have a late onsetb. have an early onsetc. don’t produce phenotypes that affect a

carrier’s healthd. aren’t dominant

10. Whose chromosomes determine the sex ofoffspring in humans?a. mother’s c. both parents’b. father’s d. neither parents’

11. A single individual carries ________ alleles for a trait.

12. ________ is a disorder that results from trisomyof chromosome 21.

13. Most sex-linked traits are passed from motherto ________.

14. The normal sex chromosomes of human malesare ________, and the normal sex chromo-somes of females are ________.

15. To analyze ________, geneticists make a chartof chromosomes called a(n) ________.

16. ________ during meiosis might result inmonosomy or trisomy.

17. The inheritance pattern that occurs equallyin both sexes and skips generations is________.

18. The genotype of the individ-ual represented by this pedigree symbol is ________. Use the letters Y and y to represent alleles.

19. Feather colors in pigeons are produced by________ inheritance.

20. If a trait has three different phenotypes, thetrait is inherited by ________ or ________.

21. The brother of a woman’s father has hemo-philia. Her father was unaffected, but sheworries that she may have an affected son.Should she worry? Explain.

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

340 CHAPTER 12 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Get to the Root of Things If you don’t know a word’s meaning, you can stillget an idea of its meaning if you focus on its roots,prefixes, and suffixes. For instance, words that startwith non-, un-, a-, dis-, and in- generally reversewhat the rest of the word means.


CHAPTER 12 ASSESSMENT 341

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The following graph illustrates the numberof flowers produced per plant by a certainplant population.

Interpreting Data Use the graph to answerthe questions that follow.1. How many flowers are produced by

plants that have only dominant genes forflower production?a. 4 c. 16b. 12 d. 28

2. How many flowers are produced byplants that have half the possible num-ber of dominant genes for flower pro-duction?a. 4 c. 16b. 12 d. 28

3. What pattern of inheritance is suggestedby the graph?a. multiple allelesb. incomplete dominancec. polygenic inheritanced. sex-linkage

4. Observing and Inferring From theabove graph, estimate the number ofgene pairs that control the number offlowers in these plants.

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.science.glencoe.com

CD-ROM

which determine the trait of sickle-

cell anemia by

and height by

4.

6.

which determine the

3.

5.

of colorblindness

and

shows

2.

A

1.

22. If a child has type O blood and its mother hastype A, could a man with type B be the father?Why couldn’t a blood test be used to provethat he is the father?

23. Why do certain human genetic disorders,such as sickle-cell anemia and Tay-Sachs dis-ease, occur more frequently among one eth-nic group than another?

24. How can a single gene mutation in a proteinsuch as hemoglobin affect several body systems?

25. Recognizing Cause and Effect Explain why amale with a recessive X-linked trait usuallyproduces no female offspring with the trait.

26. Comparing and Contrasting Compare mul-tiple allelic with polygenic inheritance.

27. Concept Mapping Complete the conceptmap by using the following vocabulary terms: sex-linked trait, autosomes, karyotype,sex chromosomes, polygenic inheritance,codominant alleles.

THINKING CRITICALLYTHINKING CRITICALLY

Numbers of flowers per plantN

umbe

r of p

lant

s4 8 12 16 20 24 28

100

300

200

400

500

Number of Flowers Produced by Plants


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Chapter 12: Patterns of Heredity and Human Geneticsimages.pcmac.org/SiSFiles/Schools/AL/FayetteCounty/...Patterns of Heredity and Human Genetics What You’ll Learn You will compare