Chapter 11 Introduction to Genetics · Applying Concepts In guinea pigs, the allele for a rough coat (R) is dominant over the allele for a smooth coat (r). A heterozygous guinea pig

Prentice Hall Biology

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Chapter 11 Introduction to Genetics

The varied patterns of stripes on zebras are due to differences in genetic makeup. No two zebras have identical stripe patterns.

Are traits inherited?

Procedure

1. Look at your classmates. Note how they vary in the shape of the front hairline, the space between the twoupper front teeth, and the way in which the earlobes are attached.

2. Make a list of the different forms of these traits that you have observed in the class or among other peopleyou know.

Think About It

1. Inferring Could these traits be inherited? From whom could they be inherited?2. Inferring How is it possible that these traits could be found in a person and his or her biological

grandparents but not in the biological parents?

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Critical Thinking

22. Designing Experiments In sheep, the allele for white wool (A) isdominant over the allele for black wool (a). How would you determinethe genotype of a white ram, or male sheep?

23. Comparing and Contrasting Design and complete a table tocompare and contrast meiosis and mitosis.

24. Applying Concepts In dogs, the allele for short hair is dominant overthe allele for long hair. Two short-haired dogs are the parents of alitter of eight puppies. Six puppies have short hair, and two have longhair. What are the genotypes of the parents?

25. Applying Concepts In guinea pigs, the allele for a rough coat (R) isdominant over the allele for a smooth coat (r). A heterozygous guineapig (Rr) and a homozygous recessive guinea pig (rr) have a total ofnine offspring. The Punnett square for this cross shows a 50 percentchance that any particular offspring will have smooth coats. Explainhow all nine offspring can have smooth coats.

26. Inferring Suppose Mendel crossed two pea plants and got both talland short offspring. What could have been the genotypes of the twooriginal plants? What genotype could not have been present?

27. Comparing and Contrasting Suppose a plant geneticist uses true-breeding plants to make a two-factor cross involving genes that areclosely linked on a chromosome. How would the genetic variation inthe F2 generation probably differ from a cross in which two genesassort independently?

28. Interpreting Graphics Genes that control hair or feather color in

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some animals have different effects in the winter than in the summer.How might such a difference be beneficial to the ptarmigan shownbelow?

29. Formulating Hypotheses Suppose you found out that a matingbetween a black animal and a brown animal produced all blackoffspring. Propose a hypothesis to explain the color of the offspring.

30. Connecting Concepts How might a scientist use a Punnett square indesigning and carrying out an experiment?

Writing in ScienceWrite an explanation of dominant and recessive alleles that would be appropriate to give to an eighth-grade scienceclass. You can assume that the eighth-grade students already know the meanings of gene and allele. (Hint: Useexamples to make your explanation clear.)

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Performance-Based AssessmentCreating Storyboards You are a writer for a TV station. The producer asks you to write a series that takes theviewer on an imaginary voyage back in time. Each week, a famous person in history will be visited. The show isdesigned to provide insight into the work of the person being interviewed and to give the viewers a feel for the eventsof that era. The first person you visit will be Gregor Mendel. Create storyboards that plan one scene for this program.


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Key Concepts

What is the principle ofdominance?

What happens duringsegregation?

Vocabulary genetics fertilization true-breeding trait hybrid gene allele segregation gameteReading Strategy: FindingMain Ideas As you read, findevidence to support thefollowing statement: Mendel'sideas about genetics were thebeginning of a new area ofbiology.

111 The Work of Gregor Mendel

What is an inheritance? To most people, it is money or property left to themby a relative who has passed away. That kind of inheritance is important, ofcourse. There is another form of inheritance, however, that matters evenmore. This inheritance has been with you from the very first day you werealiveyour genes.

Every living thingplant or animal, microbe or human beinghas a set ofcharacteristics inherited from its parent or parents. Since the beginning ofrecorded history, people have wanted to understand how that inheritance ispassed from generation to generation. More recently, however, scientists havebegun to appreciate that heredity holds the key to understanding what makeseach species unique. As a result, genetics, the scientific study of heredity, isnow at the core of a revolution in understanding biology.

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111 The Work of Gregor Mendel (continued)Gregor Mendel's Peas

The work of an Austrian monk named Gregor Mendel was particularly important tounderstanding biological inheritance. Gregor Mendel was born in 1822 in what is nowthe Czech Republic. After becoming a priest, Mendel spent several years studyingscience and mathematics at the University of Vienna. He spent the next 14 yearsworking in the monastery and teaching at the high school. In addition to his teachingduties, Mendel was in charge of the monastery garden. In this ordinary garden, he wasto do the work that changed biology forever.

Mendel carried out his work with ordinary garden peas. He knew that part of eachflower produces pollen, which contains the plant's male reproductive cells, or sperm.Similarly, the female portion of the flower produces egg cells. During sexualreproduction, male and female reproductive cells join, a process known asfertilization. Fertilization produces a new cell, which develops into a tiny embryoencased within a seed. Pea flowers are normally self-pollinating, which means thatsperm cells in pollen fertilize the egg cells in the same flower. The seeds that areproduced by self-pollination inherit all of their characteristics from the single plantthat bore them. In effect, they have a single parent.

When Mendel took charge of the monastery garden, he had several stocks of peaplants. These peas were true-breeding, meaning that if they were allowed to self-pollinate, they would produce offspring identical to themselves. One stock of seedswould produce only tall plants, another only short ones. One line produced only greenseeds, another only yellow seeds. These true-breeding plants were the basis ofMendel's experiments.

Mendel wanted to produce seeds by joining male and female reproductive cells fromtwo different plants. To do this, he had to prevent self-pollination. He accomplishedthis by cutting away the pollen-bearing male parts as shown in the figure at right andthen dusting pollen from another plant onto the flower. This process, which is knownas cross-pollination, produced seeds that had two different plants as parents. Thismade it possible for Mendel to cross-breed plants with different characteristics, andthen to study the results.

Cross-Pollination

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111 The Work of Gregor Mendel (continued)Genes and Dominance

Mendel studied seven different pea plant traits. A trait is a specific characteristic,such as seed color or plant height, that varies from one individual to another. Each ofthe seven traits Mendel studied had two contrasting characters, for example, greenseed color and yellow seed color. Mendel crossed plants with each of the sevencontrasting characters and studied their offspring. We call each original pair of plantsthe P (parental) generation. The offspring are called the F1, or first filial, generation.Filius and filia are the Latin words for son and daughter. The offspring of crossesbetween parents with different traits are called hybrids.

For: Articles on genetics

What were those F1 hybrid plants like? Did the characters of the parent plants blend inthe offspring? Not at all. To Mendel's surprise, all of the offspring had the character ofonly one of the parents, as shown at right. In each cross, the character of the otherparent seemed to have disappeared.

Mendel's F1 Crosses

From this set of experiments, Mendel drew two conclusions. Mendel's first conclusionwas that biological inheritance is determined by factors that are passed from onegeneration to the next. Today, scientists call the chemical factors that determine traitsgenes. Each of the traits Mendel studied was controlled by one gene that occurred intwo contrasting forms. These contrasting forms produced the different characters ofeach trait. For example, the gene for plant height occurs in one form that produces tallplants and in another form that produces short plants. The different forms of a geneare called alleles (uh-LEELZ).

Mendel's second conclusion is called the principle of dominance. The principleof dominance states that some alleles are dominant and others are recessive. Anorganism with a dominant allele for a particular form of a trait will always exhibit thatform of the trait. An organism with a recessive allele for a particular form of a traitwill exhibit that form only when the dominant allele for the trait is not present. InMendel's experiments, the allele for tall plants was dominant and the allele for shortplants was recessive. The allele for yellow seeds was dominant, while the allele forgreen seeds was recessive.

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111 The Work of Gregor Mendel (continued)Segregation

Mendel wanted the answer to another question: Had the recessive alleles disappeared,or were they still present in the F1 plants? To answer this question, he allowed allseven kinds of F1 hybrid plants to produce an F2 (second filial) generation by self-pollination. In effect, he crossed the F1 generation with itself to produce the F2offspring, as shown in the figure at right.

Mendel's F2 Generation

The F1 Cross The results of the F1 cross were remarkable. When Mendel comparedthe F2 plants, he discovered that the traits controlled by the recessive alleles hadreappeared! Roughly one fourth of the F2 plants showed the trait controlled by therecessive allele. Why did the recessive alleles seem to disappear in the F1 generationand then reappear in the F2 generation? To answer this question, let's take a closerlook at one of Mendel's crosses.

Explaining the F1 Cross To begin with, Mendel assumed that a dominant allele hadmasked the corresponding recessive allele in the F1 generation. However, the traitcontrolled by the recessive allele showed up in some of the F2 plants. Thisreappearance indicated that at some point the allele for shortness had been separatedfrom the allele for tallness. How did this separation, or segregation, of alleles occur?Mendel suggested that the alleles for tallness and shortness in the F1 plants segregatedfrom each other during the formation of the sex cells, or gametes (GAM-eetz). Didthat suggestion make sense?

Let's assume, as perhaps Mendel did, that the F1 plants inherited an allele for tallnessfrom the tall parent and an allele for shortness from the short parent. Because theallele for tallness is dominant, all the F1 plants are tall. When each F1 plantflowers and produces gametes, the two alleles segregate from each other so thateach gamete carries only a single copy of each gene. Therefore, each F1 plantproduces two types of gametesthose with the allele for tallness and those withthe allele for shortness.

Look at the figure below to see how alleles separated during gamete formation andthen paired up again in the F2 generation. A capital letter T represents a dominantallele. A lowercase letter t represents a recessive allele. The result of this process is anF2 generation with new combinations of alleles.

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Segregation of Alleles During gamete formation, alleles segregate from each other so that each gametecarries only a single copy of each gene. Each F1 plant produces two types of gametesthose with the allele for tallnessand those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT andTt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant.

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111 The Work of Gregor Mendel

1. Key Concept What are dominant and recessive alleles? 2. Key Concept What is segregation? What happens to alleles during segregation? 3. What did Mendel conclude determines biological inheritance? 4. Describe how Mendel cross-pollinated pea plants. 5. Why did only about one fourth of Mendel's F2 plants exhibit the recessive trait? 6. Critical Thinking Applying Concepts Why were true-breeding pea plants important for Mendel's

experiments?

Using DiagramsUse a diagram to explain Mendel's principles of dominance and segregation. Your diagram should show how thealleles segregate during gamete formation.

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Key Concepts

How do geneticists use theprinciples of probability?

How do geneticists usePunnett squares?

Vocabulary probability Punnett square homozygous heterozygous phenotype genotypeReading Strategy: BuildingVocabulary Before you read,preview the list of newvocabulary words. Predict therelationship between phenotypeand genotype. As you read,check to see if your predictionswere correct.

112 Probability and Punnett Squares

Whenever Mendel performed a cross with pea plants, he carefully categorizedand counted the many offspring. Every time Mendel repeated a particularcross, he obtained similar results. For example, whenever Mendel crossed twoplants that were hybrid for stem height (Tt), about three fourths of theresulting plants were tall and about one fourth were short. Mendel realizedthat the principles of probability could be used to explain the results ofgenetic crosses.

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112 Probability and Punnett Squares (continued)Genetics and Probability

The likelihood that a particular event will occur is called probability. As an exampleof probability, consider an ordinary event like flipping a coin. There are two possibleoutcomes: The coin may land heads up or tails up. The chances, or probabilities, ofeither outcome are equal. Therefore, the probability that a single coin flip will comeup heads is 1 chance in 2. This is 1/2, or 50 percent.

If you flip a coin three times in a row, what is the probability that it will land heads upevery time? Because each coin flip is an independent event, the probability of eachcoin's landing heads up is 1/2. Therefore, the probability of flipping three heads in arow is:

As you can see, you have 1 chance in 8 of flipping heads three times in a row. Thatthe individual probabilities are multiplied together illustrates an important pointpastoutcomes do not affect future ones.

How is coin flipping relevant to genetics? The way in which alleles segregate iscompletely random, like a coin flip. The principles of probability can be usedto predict the outcomes of genetic crosses.

How are dimples inherited?

Materials copy of page from telephone book, calculator

Procedure 1. Write the last 4 digits of any telephone number. These 4

random digits represent the alleles of a gene that determines whether a person will have dimples. Odddigits represent the allele for the dominant trait of dimples. Even digits stand for the allele for the recessivetrait of no dimples.

2. Use the first 2 digits to represent a certain father's genotype. Use the symbols D and d to write hisgenotype, as shown in the example.

3. Use the last 2 digits the same way to find the mother's genotype. Write her genotype.

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4. Use the figure Punnett Square as an example to construct a Punnett square for the cross of these parents.Then, using the Punnett square, determine the probability that their child will have dimples.

5. Determine the class average of the percent of children with dimples.

Analyze and Conclude 1. Applying Concepts How does the class average compare with the result of a cross of two heterozygous

parents?2. Drawing Conclusions What percentage of the children will be expected to have dimples if one parent is

homozygous for dimples (DD) and the other is heterozygous (Dd)?

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112 Probability and Punnett Squares (continued)Punnett Squares

The gene combinations that might result from a genetic cross can be determined bydrawing a diagram known as a Punnett square. The Punnett square shown belowshows one of Mendel's segregation experiments. The types of gametes produced byeach F1 parent are shown along the top and left sides of the square. The possible genecombinations for the F2 offspring appear in the four boxes that make up the square.The letters in the Punnett square represent alleles. In this example, T represents thedominant allele for tallness and t represents the recessive allele for shortness. Punnett squares can be used to predict and compare the genetic variations thatwill result from a cross.

Punnett Square The principles of probability can be used to predict the outcomes of genetic crosses. ThisPunnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

Organisms that have two identical alleles for a particular traitTT or tt in thisexampleare said to be homozygous (hoh-moh-ZY-gus). Organisms that have twodifferent alleles for the same trait are heterozygous (het-ur-oh-ZY-gus). Homozygousorganisms are true-breeding for a particular trait. Heterozygous organisms are hybridfor a particular trait.

All of the tall plants have the same phenotype, or physical characteristics. They donot, however, have the same genotype, or genetic makeup. The genotype of one thirdof the tall plants is TT, while the genotype of two thirds of the tall plants is Tt. Theplants in the figure below have the same phenotype but different genotypes. Click onthe activity at right to gain a greater understanding of the Punnett square.

Punnett Squares

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Phenotypes and Genotypes Although these plants have different genotypes (TT and Tt), they have the samephenotype (tall).

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112 Probability and Punnett Squares (continued)Probability and Segregation

Look again at the figure Punnet Square. One fourth (1/4) of the F2 plants have twoalleles for tallness (TT); 2/4, or 1/2, of the F2 plants have one allele for tallness andone allele for shortness (Tt). Because the allele for tallness is dominant over the allelefor shortness, 3/4 of the F2 plants should be tall. Overall, there are 3 tall plants forevery 1 short plant in the F2 generation. Thus, the ratio of tall plants to short plants is3 : 1. This assumes, of course, that Mendel's model of segregation is correct.

Did the data from Mendel's experiments fit his model? Yes. The predicted ratio3dominant to 1 recessiveshowed up consistently, indicating that Mendel'sassumptions about segregation had been correct. For each of his seven crosses, about3/4 of the plants showed the trait controlled by the dominant allele. About 1/4 showedthe trait controlled by the recessive allele. Segregation did indeed occur according toMendel's model.

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112 Probability and Punnett Squares (continued)Probabilities Predict Averages

Probabilities predict the average outcome of a large number of events. However,probability cannot predict the precise outcome of an individual event. If you flip acoin twice, you are likely to get one head and one tail. However, you might also gettwo heads or two tails. To be more likely to get the expected 50 : 50 ratio, you wouldhave to flip the coin many times.

The same is true of genetics. The larger the number of offspring, the closer theresulting numbers will get to expected values. If an F1 generation contains just three orfour offspring, it may not match Mendelian predicted ratios. When an F1 generationcontains hundreds or thousands of individuals, however, the ratios usually come veryclose to matching expectations.




112 Probability and Punnett Squares

1. Key Concept How are the principles of probability used to predict the outcomes of genetic crosses?

2. Key Concept How are Punnett squares used? 3. What is probability? 4. Define the terms genotype and phenotype. 5. Critical Thinking Problem Solving An F1 plant that is homozygous for shortness is crossed with a

heterozygous F1 plant. What is the probability that a seed from the cross will produce a tall plant? Use aPunnett square to explain your answer and to compare the probable genetic variations in the F2 plants.

Drawing Punnett SquaresImagine that you came upon a tall pea plant similar to those Mendel used in his experiments. How could youdetermine the plant's genotype with respect to height? Draw two Punnett squares to show your answer.

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Key Concepts

What is the principle ofindependent assortment?

What inheritance patternsexist aside from simpledominance?

Vocabulary independent assortment incomplete dominance codominance multiple alleles polygenic traitReading Strategy: FindingMain Ideas Before you read,draw a line down the center ofa sheet of paper. On the leftside, write down the maintopics of the section. On theright side, note supportingdetails and examples.

113 Exploring Mendelian Genetics

After showing that alleles segregate during the formation of gametes, Mendelwondered if they did so independently. In other words, does the segregationof one pair of alleles affect the segregation of another pair of alleles? Forexample, does the gene that determines whether a seed is round or wrinkledin shape have anything to do with the gene for seed color? Must a round seedalso be yellow?

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113 Exploring Mendelian Genetics (continued)Independent Assortment

To answer these questions, Mendel performed an experiment to follow two differentgenes as they passed from one generation to the next. Mendel's experiment is knownas a two-factor cross.

The Two-Factor Cross: F1 First, Mendel crossed true-breeding plants that producedonly round yellow peas (genotype RRYY) with plants that produced wrinkled greenpeas (genotype rryy). All of the F1 offspring produced round yellow peas. This showsthat the alleles for yellow and round peas are dominant over the alleles for green andwrinkled peas. A Punnett square for this cross, shown below, shows that the genotypeof each of these F1 plants is RrYy.

Two-Factor Cross: F1 Mendel crossed plants that were homozygous dominant for round yellow peas with plantsthat were homozygous recessive for wrinkled green peas. All of the F1 offspring were heterozygous dominant for roundyellow peas.

This cross does not indicate whether genes assort, or segregate, independently.However, it provides the hybrid plants needed for the next crossthe cross of F1plants to produce the F2 generation.

The Two-Factor Cross: F2 Mendel knew that the F1 plants had genotypes of RrYy.In other words, the F1 plants were all heterozygous for both the seed shape and seedcolor genes. How would the alleles segregate when the F1 plants were crossed to eachother to produce an F2 generation? Remember that each plant in the F1 generation wasformed by the fusion of a gamete carrying the dominant RY alleles with another

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gamete carrying the recessive ry alleles. Did this mean that the two dominant alleleswould always stay together? Or would they segregate independently, so that anycombination of alleles was possible?

In Mendel's experiment, the F2 plants produced 556 seeds. Mendel compared thevariation in the seeds. He observed that 315 seeds were round and yellow and another32 were wrinkled and green, the two parental phenotypes. However, 209 of the seedshad combinations of phenotypesand therefore combinations of allelesnot found ineither parent. This clearly meant that the alleles for seed shape segregatedindependently of those for seed colora principle known as independentassortment. Put another way, genes that segregate independentlysuch as the genesfor seed shape and seed color in pea plantsdo not influence each other's inheritance.Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio that the Punnettsquare shown below predicts. Mendel had discovered the principle of independentassortment. The principle of independent assortment states that genes fordifferent traits can segregate independently during the formation of gametes.Independent assortment helps account for the many genetic variations observedin plants, animals, and other organisms.

Two-Factor Cross: F2 When Mendel crossed plants that were heterozygous dominant for round yellowpeas, he found that the alleles segregated independently to produce the F2 generation.

Producing True-Breeding SeedsSuppose you work for a company that specializes in ornamental flowers. One spring, you find an ornamentalplant with beautiful lavender flowers. Knowing that these plants are self-pollinating, you harvest seeds from it.You plant the seeds the following season. Of the 106 test plants, 31 have white flowers. Is there a way to developseeds that produce only lavender flowers?

Defining the Problem Describe the problem that must be solved to make the lavender-flowered plants acommercial success.

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Organizing Information The first lavender flower produced offspring with both lavender and white flowerswhen allowed to self-pollinate. Use your knowledge of Mendelian genetics, including Punnett squares, to drawconclusions about the nature of the allele for these lavender flowers.

Creating a Solution Write a description of how you would produce seeds guaranteed to produce 100percent lavender plants. A single plant can produce as many as 1000 seeds.

Presenting Your Plan Prepare a step-by-step outline of your plan, including Punnett squares whenappropriate. Present the procedure to your class.




113 Exploring Mendelian Genetics (continued)A Summary of Mendel's Principles

Mendel's principles form the basis of the modern science of genetics. These principlescan be summarized as follows:

The inheritance of biological characteristics is determined by individual unitsknown as genes. Genes are passed from parents to their offspring.In cases in which two or more forms (alleles) of the gene for a single traitexist, some forms of the gene may be dominant and others may be recessive.In most sexually reproducing organisms, each adult has two copies of eachgeneone from each parent. These genes are segregated from each otherwhen gametes are formed.The alleles for different genes usually segregate independently of oneanother.

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113 Exploring Mendelian Genetics (continued)Beyond Dominant and Recessive Alleles

Despite the importance of Mendel's work, there are important exceptions to most ofhis principles. For example, not all genes show simple patterns of dominant andrecessive alleles. In most organisms, genetics is more complicated, because themajority of genes have more than two alleles. In addition, many important traits arecontrolled by more than one gene. Some alleles are neither dominant norrecessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance A cross between two four o'clock (Mirabilis) plants showsone of these complications. The F1 generation produced by a cross between red-flowered (RR) and white-flowered (WW) plants consists of pink-colored flowers (RW),as shown in the Punnett square at right. Which allele is dominant in this case? Neitherone. Cases in which one allele is not completely dominant over another are calledincomplete dominance. In incomplete dominance, the heterozygous phenotype issomewhere in between the two homozygous phenotypes.

Incomplete Dominance

Codominance A similar situation is codominance, in which both alleles contribute tothe phenotype. For example, in certain varieties of chicken, the allele for blackfeathers is codominant with the allele for white feathers. Heterozygous chickens havea color described as erminette, speckled with black and white feathers. Unlike theblending of red and white colors in heterozygous four o'clocks, black and white colorsappear separately. Many human genes show codominance, too, including one for aprotein that controls cholesterol levels in the blood. People with the heterozygousform of the gene produce two different forms of the protein, each with a differenteffect on cholesterol levels.

Multiple Alleles Many genes have more than two alleles and are therefore said tohave multiple alleles. This does not mean that an individual can have more than twoalleles. It only means that more than two possible alleles exist in a population. One ofthe best-known examples is coat color in rabbits. A rabbit's coat color is determined bya single gene that has at least four different alleles. The four known alleles display apattern of simple dominance that can produce four possible coat colors, as shown inthe figure at right. Many other genes have multiple alleles, including the human genesfor blood type.

Coat Color in Rabbits

Animal PedigreesACanine Family Tree

Polygenic Traits Many traits are produced by the interaction of several genes. Traitscontrolled by two or more genes are said to be polygenic traits, which means having

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many genes. For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Different combinations of alleles for thesegenes produce very different eye colors. Polygenic traits often show a wide range ofphenotypes. For example, the wide range of skin color in humans comes about partlybecause more than four different genes probably control this trait.

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113 Exploring Mendelian Genetics (continued)Applying Mendel's Principles

Mendel's principles don't apply only to plants. At the beginning of the 1900s, theAmerican geneticist Thomas Hunt Morgan decided to look for a model organism toadvance the study of genetics. He wanted an animal that was small, easy to keep in thelaboratory, and able to produce large numbers of offspring in a short period of time.He decided to work on a tiny insect that kept showing up, uninvited, in his laboratory.The insect was the common fruit fly, Drosophila melanogaster.

Morgan grew the flies in small milk bottles stoppered with cotton gauze. Drosophilawas an ideal organism for genetics because it could produce plenty of offspring, and itdid so quickly. A single pair of flies could produce as many as 100 offspring. Beforelong, Morgan and other biologists had tested every one of Mendel's principles andlearned that they applied not just to pea plants but to other organisms as well.

Mendel's principles also apply to humans. The basic principles of Mendelian geneticscan be used to study the inheritance of human traits and to calculate the probability ofcertain traits appearing in the next generation. You will learn more about humangenetics in Chapter 14.

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113 Exploring Mendelian Genetics (continued)Genetics and the Environment

The characteristics of any organism, whether bacterium, fruit fly, or human being, arenot determined solely by the genes it inherits. Rather, characteristics are determined byinteraction between genes and the environment. For example, genes may affect asunflower plant's height and the color of its flowers. However, these samecharacteristics are also influenced by climate, soil conditions, and the availability ofwater. Genes provide a plan for development, but how that plan unfolds also dependson the environment.




113 Exploring Mendelian Genetics

1. Key Concept Explain what independent assortment means. 2. Key Concept Describe two inheritance patterns besides simple dominance. 3. What is the difference between incomplete dominance and codominance? 4. Why are fruit flies an ideal organism for genetic research? 5. Critical Thinking Comparing and Contrasting A geneticist studying coat color in animals crosses a male

rabbit having the genotype CC with a female having genotype Ccch. The geneticist then crosses a ccch malewith a Ccc female. In which of the two crosses are the offspring more likely to show greater geneticvariation? Use Punnett squares to explain your answer.

Problem SolvingConstruct a genetics problem to be given as an assignment to a classmate. The problem must test incompletedominance, codominance, multiple alleles, or polygenic traits. Your problem must have an answer key thatincludes all of your work.




Key Concepts

What happens during theprocess of meiosis?

How is meiosis differentfrom mitosis?

Vocabulary homologous diploid haploid meiosis tetrad crossing-overReading Strategy: UsingVisuals Before you read,preview the figure Meiosis. Asyou read, note what happens ateach stage of meiosis.

114 Meiosis

Gregor Mendel did not know where the genes he had discovered werelocated in the cell. Fortunately, his predictions of how genes should behavewere so specific that it was not long before biologists were certain they hadfound them. Genes are located on chromosomes in the cell nucleus.

Mendel's principles of genetics require at least two things. First, eachorganism must inherit a single copy of every gene from each of its parents.Second, when an organism produces its own gametes, those two sets of genesmust be separated from each other so that each gamete contains just one setof genes. This means that when gametes are formed, there must be a processthat separates the two sets of genes so that each gamete ends up with just oneset. Although Mendel didn't know it, gametes are formed through exactlysuch a process.

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114 Meiosis (continued)Chromosome Number

As an example of how this process works, let's consider the fruit fly, Drosophila. Abody cell in an adult fruit fly has 8 chromosomes, as shown in the drawing below.Four of the chromosomes came from the fruit fly's male parent, and 4 came from itsfemale parent. These two sets of chromosomes are homologous (hoh-MAHL-uh-guhs), meaning that each of the 4 chromosomes that came from the male parent has acorresponding chromosome from the female parent.

Fruit-Fly Chromosomes These chromosomes are from a fruit fly (Drosophila). Each of the fruit fly's body cellshas 8 chromosomes.

A cell that contains both sets of homologous chromosomes is said to be diploid,which means two sets. The number of chromosomes in a diploid cell is sometimesrepresented by the symbol 2N. Thus for Drosophila, the diploid number is 8, whichcan be written 2N = 8. Diploid cells contain two complete sets of chromosomes andtwo complete sets of genes. This agrees with Mendel's idea that the cells of an adultorganism contain two copies of each gene.

By contrast, the gametes of sexually reproducing organisms, including fruit flies andpeas, contain only a single set of chromosomes, and therefore only a single set ofgenes. Such cells are said to be haploid, which means one set. For Drosophila, thiscan be written as N = 4, meaning that the haploid number is 4.

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114 Meiosis (continued)Phases of Meiosis

How are haploid (N) gamete cells produced from diploid (2N) cells? That's wheremeiosis (my-OH-sis) comes in. Meiosis is a process of reduction division inwhich the number of chromosomes per cell is cut in half through the separationof homologous chromosomes in a diploid cell.

Meiosis Overview

Meiosis usually involves two distinct divisions, called meiosis I and meiosis II. By theend of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Thefigure at right shows meiosis in an organism that has a diploid number of 4 (2N = 4).

Meiosis

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Meiosis I Prior to meiosis I, each chromosome is replicated. The cells then begin todivide in a way that looks similar to mitosis. In mitosis, the 4 chromosomes line upindividually in the center of the cell. The 2 chromatids that make up each chromosomethen separate from each other.

Animal Cell Meiosis I

In prophase of meiosis I, however, each chromosome pairs with its correspondinghomologous chromosome to form a structure called a tetrad. There are 4 chromatidsin a tetrad. This pairing of homologous chromosomes is the key to understandingmeiosis.

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As homologous chromosomes pair up and form tetrads in meiosis I, they exchangeportions of their chromatids in a process called crossing-over. Crossing-over, shownin the figure at right, results in the exchange of alleles between homologouschromosomes and produces new combinations of alleles.

Crossing-Over

Crossing-Over

What happens next? The homologous chromosomes separate, and two new cells areformed. Although each cell now has 4 chromatids (as it would after mitosis),something is different. Because each pair of homologous chromosomes was separated,neither of the daughter cells has the two complete sets of chromosomes that it wouldhave in a diploid cell. Those two sets have been shuffled and sorted almost like a deckof cards. The two cells produced by meiosis I have sets of chromosomes and allelesthat are different from each other and from the diploid cell that entered meiosis I.

Animal Cell Meiosis II

Meiosis II The two cells produced by meiosis I now enter a second meiotic division.Unlike the first division, neither cell goes through a round of chromosome replicationbefore entering meiosis II. Each of the cell's chromosomes has 2 chromatids. Duringmetaphase II of meiosis, chromosomes line up in the center of each cell. In anaphaseII, the paired chromatids separate. In this example, each of the four daughter cellsproduced in meiosis II receives 2 chromatids. Those four daughter cells now containthe haploid number (N)just 2 chromosomes each.

Segregation ofChromosomes

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114 Meiosis (continued)Gamete Formation

In male animals, the haploid gametes produced by meiosis are called sperm. In someplants, pollen grains contain haploid sperm cells. In female animals, generally onlyone of the cells produced by meiosis is involved in reproduction. This female gameteis called an egg in animals and an egg cell in some plants.

In many female animals, the cell divisions at the end of meiosis I and meiosis II areuneven, so that a single cell, which becomes an egg, receives most of the cytoplasm,as shown in the figure at right. The other three cells produced in the female duringmeiosis are known as polar bodies and usually do not participate in reproduction.

Gamete Formation

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114 Meiosis (continued)Comparing Mitosis and Meiosis

In a way, it's too bad that the words mitosis and meiosis sound so much like eachother, because the two processes are very different. Mitosis results in theproduction of two genetically identical diploid cells, whereas meiosis producesfour genetically different haploid cells.

A diploid cell that divides by mitosis gives rise to two diploid (2N) daughter cells. Thedaughter cells have sets of chromosomes and alleles that are identical to each otherand to the original parent cell. Mitosis allows an organism's body to grow and replacecells. In asexual reproduction, a new organism is produced by mitosis of the cell orcells of the parent organism.

Meiosis, on the other hand, begins with a diploid cell but produces four haploid (N)cells. These cells are genetically different from the diploid cell and from one another.Meiosis is how sexually reproducing organisms produce gametes. In contrast, asexualreproduction involves only mitosis.




114 Meiosis

1. Key Concept Describe the main results of meiosis. 2. Key Concept What are the principal differences between mitosis and meiosis? 3. What do the terms diploid and haploid mean? 4. What is crossing-over? 5. Critical Thinking Applying Concepts In human cells, 2N = 46. How many chromosomes would you expect

to find in a sperm cell? In an egg cell? In a white blood cell? Explain.

Sexual and Asexual ReproductionIn asexual reproduction, mitosis occurs, but not meiosis. Which type of reproductionsexual or asexualresultsin offspring with greater genetic variation? Explain your answer.




Key Concept

What structures actuallyassort independently?

Vocabulary gene mapReading Strategy:Predicting Before you read,preview the figure GeneMap. Predict how a diagramlike this one can be used todetermine how likely genesare to assort independently.As you read, note whether ornot your prediction wascorrect.

115 Linkage and Gene Maps

If you thought carefully about Mendel's principle of independent assortment asyou analyzed meiosis, one question might have been bothering you. It's easy tosee how genes located on different chromosomes assort independently, but whatabout genes located on the same chromosome? Wouldn't they generally beinherited together?

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115 Linkage and Gene Maps (continued)Gene Linkage

The answer to these questions, as Thomas Hunt Morgan first realized in 1910, is yes.Morgan's research on fruit flies led him to the principle of linkage. After identifyingmore than 50 Drosophila genes, Morgan discovered that many of them appeared to belinked together in ways that, at first glance, seemed to violate the principle ofindependent assortment. For example, a fly with reddish-orange eyes and miniaturewings was used in a series of crosses. The results showed that the genes for those traitswere almost always inherited together and only rarely became separated from eachother.

Morgan and his associates observed so many genes that were inherited together thatbefore long they could group all of the fly's genes into four linkage groups. Thelinkage groups assorted independently, but all of the genes in one group wereinherited together. Drosophila has four linkage groups. It also has four pairs ofchromosomes, which led to two remarkable conclusions. First, each chromosome isactually a group of linked genes. Second, Mendel's principle of independentassortment still holds true. It is the chromosomes, however, that assortindependently, not individual genes.

How did Mendel manage to miss gene linkage? By luck, or by design, six of the sevengenes he studied are on different chromosomes. The two genes that are found on thesame chromosome are so far apart that they also assort independently.




115 Linkage and Gene Maps (continued)Gene Maps

If two genes are found on the same chromosome, does this mean that they are linkedforever? Not at all. Crossing-over during meiosis sometimes separates genes that hadbeen on the same chromosome onto homologous chromosomes. Crossover eventsoccasionally separate and exchange linked genes and produce new combinations ofalleles. This is important because it helps to generate genetic diversity.

In 1911, a Columbia University student was working part time in Morgan's lab. Thisstudent, Alfred Sturtevant, hypothesized that the rate at which crossing-over separatedlinked genes could be the key to an important discovery. Sturtevant reasoned that thefarther apart two genes were, the more likely they were to be separated by a crossoverin meiosis. The rate at which linked genes were separated and recombined could thenbe used to produce a map of distances between genes. Sturtevant gathered upseveral notebooks of lab data and took them back to his room. The next morning, hepresented Morgan with a gene map showing the relative locations of each knowngene on one of the Drosophila chromosomes, as shown in the figure at right.Sturtevant's method of using recombination rates, which measure the frequencies ofcrossing-over between genes, has been used to construct genetic maps, including mapsof the human genome, ever since.

Gene Map

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115 Linkage and Gene Maps

1. Key Concept How does the principle of independent assortment apply to chromosomes? 2. What are gene maps, and how are they produced? 3. How does crossing-over make gene mapping possible? 4. Critical Thinking Inferring If two genes are on the same chromosome but usually assort independently, what

does that tell you about how close together they are?

Cause-Effect ParagraphIn your own words, explain why the alleles for reddish-orange eyes and miniature wings in Drosophila areusually inherited together. Include the idea of gene linkage. Hint: To organize your ideas, draw a cause-effectdiagram that shows what happens to the two alleles during meiosis.

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Click on a Key Concept to link to the page where the concept is explained.

111 The Work of Gregor Mendel Key Concepts

The principle of dominance states that some allelesare dominant and others are recessive.When each F1 plant flowers, the two allelessegregate from each other so that each gametecarries only a single copy of each gene. Therefore,each F1 plant produces two types of gametesthose with the allele for tallness and those with theallele for shortness.

Vocabulary

genetics fertilization true-breeding trait hybrid gene allele segregation gamete

112 Probability and Punnett Squares Key Concepts

The principles of probability can be used to predictthe outcomes of genetic crosses.Punnett squares can be used to predict andcompare the genetic variations that will result from across.

Vocabulary

probability Punnett square homozygous heterozygous phenotype genotype

113 Exploring Mendelian Genetics Key Concepts

The principle of independent assortment states thatgenes for different traits can segregate

Vocabulary

independent assortment incomplete dominance

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Thinking Visually Meiosis: Flowchart

independently during the formation of gametes.Independent assortment helps account for the manygenetic variations observed in plants, animals, andother organisms.Some alleles are neither dominant nor recessive,and many traits are controlled by multiple alleles ormultiple genes.

codominance multiple alleles polygenic trait

114 Meiosis Key Concepts

Meiosis is a process of reduction division in whichthe number of chromosomes per cell is cut in halfthrough the separation of homologouschromosomes in a diploid cell.Mitosis results in the production of two geneticallyidentical diploid cells, whereas meiosis producesfour genetically different haploid cells.

Vocabulary

homologous diploid haploid meiosis tetrad crossing-over

115 Linkage and Gene Maps Key Concept

Chromosomes assort independently; individualgenes do not.

Vocabulary

gene map

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Modeling MeiosisMeiosis results in 4 new cells, each containing half the number of chromosomes in the original cells. Using theprocedures below, you will build a model to demonstrate the process of meiosis and explore how it can lead togenetic changes.

Problem What happens to the chromosomes in cells during meiosis?

Materials 4 colors of yarn (2 shades of red and 2 shades of green)scissorstransparent tapeindex cardsfelt-tip marker

Skills Using Models, Communicating Results

Procedure

1. You will use yarn and index cards to model each stage of meiosis. Use two shades of red yarn to representone homologous pair of chromosomes and two shades of green yarn to represent another pair. Use anindex card to represent a cell.

2. Cut two pieces of yarn about 5 cm long from each color of yarn. Each piece of yarn will represent achromatid.

3. Tape pieces of red and green yarn to an index card to show the appearance of two tetrads in a cell at thebeginning of meiosis.

4. Tape pieces of yarn to additional index cards to model the numbers and positions of the chromosomes andcells at each stage of meiosis. Be sure to include an example of crossing-over at the correct stage. Use afelt-tip marker to label each card with the name of the stage it represents.

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5. Arrange the finished cards to show the complete process of meiosis. Label the stages at which geneticsegregation and crossing-over occur and chromosome number changes.

6. Use your cards to explain the process of meiosis to a classmate. Then, trade roles and have your classmateuse his or her models to explain the process of meiosis to you.

Analyze and Conclude 1. Using Models What is the result of the first meiotic division (meiosis I)?2. Using Models What is the result of the second meiotic division (meiosis II)?3. Drawing Conclusions How does meiosis lead to increased genetic variation?4. Predicting How would the gametes be affected if a pair of chromatids failed to separate in the second

meiotic division?5. Using Models What parts of the cell did the yarn represent?6. Evaluating How well do you think this investigation modeled the process of meiosis? Explain your

answer.

Using Models Make a second set of models that shows the differences between the formation of sperm andthe formation of eggs.

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Coat Color in Rabbits Coat color in rabbits is determined by a single gene that has at least four different alleles. Differentcombinations of alleles result in the four colors you see here. Interpreting Graphics What allele combinations can a chinchilla rabbithave?


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Cross-Pollination To cross-pollinate pea plants, Mendel cut off the male parts of one flower and then dusted it with pollen fromanother flower. Applying Concepts How did this procedure prevent self-pollination?




Mendel's F1 Crosses When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only oneof the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive.




Mendel's F2 Generation When Mendel allowed the F1 plants to reproduce by self-pollination, the traits controlled by recessivealleles reappeared in about one fourth of the F2 plants in each cross. Calculating What proportion of the F2 plants had a trait controlledby a dominant allele?




Incomplete Dominance Some alleles are neither dominant nor recessive. In four o'clock plants, for example, the alleles forred and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowersa mix of red and white coloring.




Meiosis During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes.The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.




Crossing-Over Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over oneanother. (3) The crossed sections of the chromatids are exchanged. Interpreting Graphics How does crossing-over affect the alleleson a chromatid?




Gamete Formation Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sizedgametes called sperm. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are notinvolved in reproduction.




Gene Map This gene map shows the location of a variety of genes on chromosome 2 of the fruit fly. The genes are named after theproblems abnormal alleles cause, not the normal structure. Interpreting Graphics Where on the chromosome is the purple eye genelocated?

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Documents

Chapter 11 Introduction to Genetics · Applying Concepts In guinea pigs, the allele for a rough coat (R) is dominant over the allele for a smooth coat (r). A heterozygous guinea pig