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Mende ian Genetcs

MAIN ~ Mendel explained how a dominant allele can mask thepresence of a recessive allele.

Real-World Reading Link There are many different breeds of dogs, such asLabrador retrievers, dachshunds, German shepherds, and poodles. You might likea certain breed of dog because of its height, coat color, and general appearance.These traits are passed from generation to generation.

How Genetics BeganIn 1866, Gregor Mendel, an Austrian monk and a plant breeder, publishedhis findings on the method of inheritance in garden pea plants. The passing of traits to the next generation is called inheritance, or heredity.Mendel, shown in Figure 7, was successful in sorting out the mystery ofinheritance because of the organism he chose for his study—the pea plant.Pea plants are true-breeding, meaning that they consistently produce offspring with only one form of a trait.

Pea plants usually reproduce by self-fertilization. A common occurrence in many flowering plants, self-fertilization occurs when a malegamete within a flower combines with a female gamete in the sameflower. Mendel also discovered that pea plants could easily be cross-pollinated by hand. Mendel performed cross-pollination by transferring a male gamete from the flower of one pea plant to the femalereproductive organ in a flower of another pea plant.

Connection History Mendel rigorously followed various traits in thepea plants he .red. He analyzed the results of his experiments andformed hypotheses concerning how the traits were inherited. The studyof genetics, which is the science of heredity, began with Mendel, who isregarded as the father of genetics.

Reading Check Infer why it is important that Mendel’s experimentsused a true-breeding plant.

The Inheritance of TraitsMendel noticed that certain varieties of garden pea plants producedspecific forms of a trait, generation after generation. For instance, henoticed that some varieties always produced green seeds and othersalways produced yellow seeds. In order to understand how these traits areinherited, Mendel performed cross-pollination by transferring male gametes from the flower of a true-breeding green-seed plant to the femaleorgan of a flower from a true-breeding yellow-seed plant. To prevent selffertilization, Mendel removed the male organs from the flower of theyellow seed plant. Mendel called the green seed plant and the yellow-seedplant the parent generation also known as the P generation.

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Essent~ questionsp what is the significance of Mendel’s

experim~ts to thestudy of genetics?what is the law ofrsegregation andthe law of independent assortment?

i whatarethepo55~e offspring froma cross usi • a Punnett square? -

Review vocabulary5~gregati0~ the separation of allelicgenes that typically occurs during meiosis

New vocabularygeneticSalleledominantrecessivehomozygOusheterozygousgenotypephenotypelaw of segregationhybridlaw of independent assortment

Mu lingual eGlossary

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Figure 7 Gregor Mendel is known as thefather of genetics.

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Section 2 • Mendelian Genetics 277

Self-fertilization

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CAREERS IN BIOLOGY

Genetics Laboratory TechnicianA technician in a genetics laboratoryassists a researcher by conductingexperiments and helping to maintainthe lab.

F1 and F2 generations When Mendel grew the seeds from thecross between the green-seed and yellow-seed plants, all of the resoffspring had yellow seeds. The offspring of this P cross are called tfirst filial (F1) generation. The green-seed trait seemed to have disap.peared in the F1 generation~ and Mendel decided to investigate whethe trait was no longer present or whether it was hidden, or masked.

Mendel planted the F1 generation of yellow seeds, allowed the planto grow and self-fertilize, and then examined the seeds from thisThe results of the second filial (F2) generation—the offspring fromcross—are shown in Figure 8. Of the seeds Mendel collected, 6022~syellow and 2001 were green~ which almost is a perfect 3:1 ratio of yelto green seeds.

Mendel studied seven different traits—seed or pea color, flowercolor, seed pod color, seed shape or texture, seed pod shape, stemlength, and flower position—and found that the F2 generation planfrom these crosses also showed a 3:1 ratio.

Genes in pairs Mendel concluded that there must be two formsthe seed trait in the pea plants_yellow-seed and green~seed_ah1dlteach was controlled by a factor, which now is called an allele. Anis defined as an alternative form of a single gene passed from gene~tion to generation. Therefore, the gene for yellow seeds and thegreen seeds are each different forms of a single gene.

Mendel concluded that the 3:1 ratio observed during his expCtl

could be explained if the alleles were paired in each of the plans Hcalled the form of the trait that appeared in the F, generation doand the form of the trait that was masked in the F1 generationIn the cross between yellow-seed plants and green-seed plant5~ the)low seed was the dominant form of the trait and the green seed W~5recessive form of the trait.

Figure 8 The results of Mendel’s crossinvolving true-breeding pea plants withyellow seeds and green seeds are shown here.Explain why the seeds in the F,generation were all yellow

Concepts in Motion

Animation

Yellow peas(male)

x

Jr

Green peas(female)

Generation

Parental (P)(pure-breeding)

First filialgeneration (F1)

Second filialgeneration (F2)

All yellow

6022 yellow :20013:1

278 chapter 10 • Sexual Reproduction and Genetics

onlinaIl When he allowed the F1 generation to self-fertilize,jendel showed that the recessive allele for green seeds had not disapared but was masked. Mendel concluded that the green-seed form of

trait did not show up in the F1 generation because the yellow-seedform of the trait is dominant and masks the allele for the green seedform of the trait.

When modeling inheritance, the dominant allele is represented by

a capital letter, and the recessive allele is represented by a lowercase letter. An organism with two of the same alleles for a particular trait ishomOZTh°’~ (ho muh ZI gus) for that trait. Homozygous, yellow seedlards are YY and green-seed plants are yy. An organism with two differ

~nt alleles for a particular trait is heterozygous (heh tuh roh ZY gus) forthat trait, in this case Yy. When alleles are present in the heterozygousstate, the dominant trait will be observed.

GenotYPe and phenotype A yellow-seed plant could be homozygous or heteroZygous for the trait form. The outward appearance of anorganism dbes not always indicate which pair of alleles is present. Theorganism’s~de pairs are called its genotype. In the case of plants with

gre~ yellow seeds,their genotypes could be YY or Yy. The observable charac

teristic or ~htward expression of an allele pair is called the phenotype.The phenotype of pea plants with the genotype yy will be green seeds.

Mendel’~!aW of segregation Mendel used homozygousyellow-see4~hd green-seed plants in his P cross. In Figure 9(A), the topdrawingshcws that each gamete from the yellow seed plant contains one

king Recall that~thë chromosome number is divided in half during meiosis. Thehe resulting gai~hetes contain only one of the pair of seed-color alleles.

The bo~~pm.drawing in Figure 9(A) shows that each gamete from the

er green-seedi~lant contains one y allele. Mendel’s law of segregation statesthat the two alleles for each trait separate during meiosis. During fertilization, two~alleles for that trait unite.

The third drawing in Figure 9(B) shows the alleles uniting to produce

the genotype Yy during fertilization. All resulting F1 generation plants willtie have the genotype Yy and will have yellow seeds because yellow is domi

nant to green These heterozygous organisms are called hybrids.

VOCABULARYWORD ORIGIN

Homozygous and Heterozygouscome from the Greek words homos,meaning the same; hetero, meaningother or different; and zygon, meaningyoke

Figure 9 During gamete formationin the YYor yy plant, the two alleles separate,resulting in Yoryin the gametes. Gametesfrom each parent unite during fertilization.

Gametes(pollen or eggs)

0Gamete

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Gameteformation

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0

Gametes(one pollen grain

and one egg)

0Fertilization

Y= yellow-determining alleleFertilization y= green-determining allele

Yy= yellowpea showing

dominant trait

Grows into plant0

YYyellow pe

)Ygreen peaeGamete formation

rows into plant

Seed F1 Hybriddevelopment

Zygote

0Section 2 • Mendelian Genetics 279

Figure 11 The law of independent assortment isdemonstrated in the dihybrid crass by the equal chance thateach pair of alleles (Yy and Rr) can randomly combine witheach other.Predict how many possible gamete types areproduced.

Monohybrid cross The diagram in Figure 10 shows howMendel continued his experiments by allowing the Yy~self-fertilize. A cross such as this one that involves hybri~ forsingle trait is called a monohybrid cross. The Yy plants Prodtwo types of gametes—male and female each with either theor y allele. The combining of these gametes is a ran omeveniThis random fertilization of male and female gametes resnJ~.the following genotypes—YY, Yy, Yy, or yy, as shown in Figur~1Notice that the dominant Y allele is written first, whether itcame from the male or female gamete. In Mendel’s F1 cross,there are three possible genotypes: YY, Yy, and yy; and thetypic ratio is 1:2:1. The phenotypic ratio is 3:1 yello seethtogreen seeds.

Dijiybrid cross Once Mendel established inheritance pat.terns of a single trait, he began to examine simultaneousinheritance of two or more traits in the same plant. In gardpeas, round seeds (R) are dominant to wrinkled seeds (r),yellow seeds (Y) are dominant to green seeds (y). If Mendelcrossed homozygous yellow, round-seed pea plants withhomozygous green, wrinkle-seed pea plants, the P cross cobe represented by YYRR x yyrr. The F1 generation genotypewould be YyRr—yellow, round-seed plants. These F1-genera.tion plants are called dihybrids because they are heterozygofor both traits.

Law of independent assortment Mendel allowed Fpea plants with the genotype YyRr to self-fertilize in a dihybcross. Mendel calculated the genotypic and phenotypic ratof the offspring in both the F1 and F2 generations. Fromresults, he developed the law of independent assortmentwhich states that a random distribution of alleles occurs diming gamete formation. Genes on separate chromosomes sortindependently during meiosis.

As shown in Figure 11, the random assortment of allelesresults in four possible gametes: YR, Yr, yR or yr, each of wh’equally likely to occur. When a plant self-fertilizes, any of thefour allele combinations could be present in the male gamete,and any of the four combinations could be present in the ferngamete. The results of Mendel’s dihybrid cross included ninedifferent genotypes: YYRR, YYRr, YYrr, YyRR, YyRr, Yyrr~ yyyyRr, and yyrr. He counted and recorded four different phenotypes: 315 yellow round, 108 green round, 101 yellow wriand 32 green wrinkled. These results represent a phenotyP”ratio of approximately 9:3:3:1.

In the early 1900s, Dr. Reginald Punnett developed what isknown as a Punnett square to predict the possible 0ffspiinIgcross between two known genotypes. Punnett squares maketeasier to keep track of the possible genotypes involved in a

P Yellow pea w Green pea

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Male gamete N — “Female gamete

V

F1 Yellow pea

Male . . . FemaleSelf-fertilization

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F2

Yellow peas Green pea

Figure 10 During the F1 generation self-fertilization,the male gametes randomly fertilize the female gametes.

Alleles in — Gamete — Possible alleleparental formation combinations

cell in gametes

@1 Reading Check Evaluate How can the random distribof alleles result in a predictable ratio?

Punnett SquaresyR+

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200 Chapter 10 • Sexual Reproduction and Genetics

punnett uare—monohybrid cross Can you roll your tonguelike the person pictured in Figure 12? Tongue-rolling ability is a dominanttrait, which can be represented by T Suppose both parents can roll theirtongues and are heterozygous (Tt) for the trait. What possible phenotypescould their children have?

Examine the Punnett square in Figure 12. The number of squaresis determined by the number of different types of alleles—T or t—produced by each parent. In this case, the square is 2 squares x 2 squaresbecause each parent produces two different types of gametes. Noticethat the male gametes are written across the horizontal side and the

rid female gametes are written on the vertical side of the Punnett square.The possible combinations of each male and female gamete are writtenon the inside of each corresponding square.

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Predict Probability in Genetics

T = Ability to roll tonguet = Inability to roll tongue

x Tt O”’ Gamete typesGamete

types

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Figure 12 The ability to roll one’s tongueis a dominant trait. The Punnett square is avisual summary of the possible combinations ofthe alleles for the tongue-rolling trait.

Review Personal Tutor

Inquiry Virtual Lab

nquiry Minilab

How can an offspring’s traits be predicted? A Punnett square can help predict ratios of dominanttraits to recessive traits in the genotype of offspring. This lab involves two parents who are both heterozygous for free earlobes (E), which is a dominant trait. The recessive trait is attached earlobes (e).

Procedure1. Read and complete the lab safety form.2. Determine the gamete genotype(s) for this trait that each parent contributes.3. Draw a Punnett square that has the same number of columns and the same number of rows as the

number of alleles contributed for this trait by the gametes of each parent.4. Write the alphabetical letter for each allele from one parent just above each column, and write the

alphabetical letter for each allele from the other parent just to the left of each row.5. In the boxes within the table, write the genotype of the offspring resulting from each combination

of male and female alleles.Analysis1. Summarize the possible offspring phenotypes that could occur.2. Evaluate the phenotypic ratio of the possible offspring. What is the genotypic ratio of the possible

offspring?

Section 2 • Mendelian Genetics 281

YYRR yyrrI

Gametes YR yr

F1 (all identical)

YR

Yr

yr flflflflType Genotype Phenotype Number Phenotypic

Ratioyellowroundgreen

Recombinant yyR round

yellowRecombinant V rr wrinkled 101

greenflIT wrinkled 32

Figure 13 The dihybrid Punnett square visually presents the possiblecombinations of the possible alleles from each parent.

Section SummaryI The study of genetics began with Gregor

Mendel, whose experiments with gardenpea plants gave insight into the inheritaof traits.

I Mendel developed the law of segregationand the law of independent assortment.

I Punnett squares help predict the offspringof a cross.

How many different genotypes are found i~Punnett square? One square has TT two squaresTt, and one square has tt. Therefore, the genoty.piratio of the possible offspring is 1:2:1. The phenoratio of tongue rollers to non tongue rollers is 3:1.

Punnett square—dihybrid cross Nowexamine the Punnett square in Figure 13. Notithat in the P cross, only two types of alleles areduced. However, in the dihybrid cross—whenF1 generation is crossed four types of alleles ftthe male gametes and four types of alleles fromfemale gametes can be produced. The resultingphenotypic ratio is 9:3:3:1—9 yellow round to 3round to 3 yellow wrinkled to 1 green wrinkjedMendel ‘s data closely matched the outcome predby the Punnett square.

Pro abilityThe inheritance of genes can be compared to thprobability of flipping a coin. The ocoin landing on heads is 1 out of 2, or 1/2. Ifthe same coin is flipped twice, the olanding on heads is 1/2 each time or 1/2 x 1/2,01/4 both times.

Actual data might not perfectly match the pdicted ratios. You know that if you flip a coin yomight not get heads 1 out of 2 times. Mendel’sresults were not exactly a 9:3:3:1 ratio. However,larger the number of offspring involved in a crthe more likely it will match the results predictby the Punnett square.

Understand Main Ideas1. ~ Diagram Use a Punnett square to explain how a dominan

allele masks the presence of a recessive allele.

2. Apply the law of segregation and the law of independent assortmentgiving an example of each.

3. Use a Punnett square In fruit flies, red eyes (R) are dominant to pinkeyes (r). What is the phenotypic ratio of a cross between a heterozygousmale and a pink-eyed female?

Think Critically4. Evaluate the significance of Mendel’s work to the field of genetics.

MATH in Biology5. What is the probability of rolling a 2 on a six-sided die? What is the p

bility of rolling two 2s on two six-sided die? How is probability used instudy of genetics?

P

Female YyRr Male YyRr

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Parental Y R 315 9:16

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Parental

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Section 2 Assessment

282 Chapter 10 • Sexual Reproduction and Genetics Assessment Online

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Essential Questions

p How does the process of melosisproduce genetic recombination?How can gene linkage beused tocreate chromosome maps?

e ~ Why is polyploidy important-tothefield of agriculture?

Review Vocabulary

ed protein: large, complex polymeressential to all life that providesstructure for tissues and organs andhelps carry out cell metabolism

New Vocabularygenetic recombinationpolyploidy

eG:.it 0 Multilin

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Figure 14 Genes that are linked togetherthe same chromosome usually travel together

in the gamete.Calculate the number of possible corn

3nt binations if two or three of these gametes were to combine.

Replicatedhomoiogousthromosom~

Gene Linkage and Polyploidy

MAIN ~ The crossing over of linked genes is a source ofgenetic variation.

Real-World Reading Link You might have seen many varieties of roses at agarden center that range in color from red to pink to white. Plant breeders usescientists’ knowledge of genes to vary certain characteristics in an effort to maketheir roses unique.

Genetic RecombinationConnection The new combination of genes produced by

crossing over and independent assortment is called genetic recombination. The possible combinations of genes due to independent assortment can be calculated using the formula 2’~, where n is the number ofchromosome pairs. For example, pea plants have seven pairs of chromosomes. For seven pairs of chromosomes, the possible combinationsare 2~, or 128 combinations. Because any possible male gamete can fertilize any possible female gamete, the number of possible combinationsafter fertilization is 16,384 (128 x 128). In humans, the possible numberof combinations after fertilization would be 223 x 223, or more than70 trillion. This number does not include the amount of genetic recombination produced by crossing over.

Gene LinkageChromosomes contain multiple genes that code for proteins. Genes thatare located close to each other on the same chromosome are said to belinked and usually travel together during gamete formation. Followclosely related genes A and B in Figure 14 through the process of meiosis. The linkage of genes on a chromosome results in an exception toMendel’s law of independent assortment because linked genes usuallydo not segregate independently.

Meiosis I Meiosis II

A Parental

A Parental

a Parental

a Parental

Homologsseparate

Centromeresseparate andgametes form

Section 3 • Gene Linkage and Polyploidy 283

Figure 15 This chromosome map of theX chromosome of the fruit fly Drosophilamelanogaster was created in 1913.

Gene linkage was first studied using the fruit fly Drosophilamelanogaster. Thousands of crosses confirmed that linked genes usu.ally traveled together during meiosis. However, some results revealetithat linked genes do not always travel together during meiosis. Scien.tists concluded that linked genes can separate during crossing over.

Chromosome maps Crossing over occurs more frequentlygenes that are far apart than those that are close together. A drawinga chromosome map shows the sequence of genes on a chromosomecan be created by using crossover data. The very first chromosome mapswere published in 1913 using data from thousands of fruit fly crosses.Chromosome map percentages are not actual chromosome distances, bthey represent relative positions of the genes. Figure 15 shows the firstchromosome map created using fruit fly data. Notice that the higher th~crossover frequency, the farther apart the two genes are.

Lab~Map Ch osomeS Inquiry Mini

Where are genes located on a chromosome? The distance between two genes on a chromosomerelated to the crossover frequency between them. By comparing data for several gene pairs, a gene’srelative location can be determined.

procedure1. Read and complete the lab safety form.2. obtain a table of the gene-pair crossover frequencies from your teacher.3. Draw a line on a piece of paper and make marks every 1 cm. Each mark will represent a crossover

frequency of 1 percent.4. Label one mark near the middle of the line A. Find the crossover frequency between Genes A and

on the table, and use this data to label B the correct distance from A.5. Use the crossover frequency between genes A and C and genes B and C to infer the position of gene6. Repeat steps 4—S for each gene, marking their positions on the line.Analysis1. Evaluate whether it is possible to know the location of a gene on a chromosome if only one other gene

is used.2. Consider why using more crossover frequencies would result in a more accurate chromosome map

284 Chapter 10 • Sexual Reproduction and Genetics

1

strawberries (8n)

in a cross, the exchange of genes is directly related to the crossover frequency between them. This frequency correlates with therelative distance between the two genes. One map unit between twogenes is equivalent to 1 percent of the crossing over occurringbetween th Genes that are farther apart would have a greaterfrequency 0 crossing over. Coffee (4,,)

P Figure 16 Various commercial plants,0 ~P 01 y such as strawberries and coffee, are polyploids.Most species have diploid cells, but some have polyploid cells. Polyploidyis the occurrence of one or more extra sets of all chromosomes in anorganism. A triploid organism, for instance, would be designated 3n,which means that it has three complete sets of chromosomes. Polyploidyrarely occurs in animals. In humans, polyploidy is always lethal.

Roughly one in three species of known flowering plants are polyploid.l’olyploid plants are selected by plant growers for their desirable characteristics. Commercially grown bread wheat (6n), oats (6n), and sugarcane (8n) are polyploid crop plants. Polyploid plants, such as the onesshown in Fi re 16, often have increased vigor and size.

ection 3 Assessme tSection Summary Understand Main IdeasI Genetic recombination involves both 1. Analyze how crossing over is related to variation.

crossing over and independent assortment. 2. Draw Suppose genes C and Dare linked on one chromosome and genesI Early chromosome maps were created c and dare linked on another chromosome. Assuming that crossing over

based on the linkage of genes on the does not take place, sketch the daughter cells resulting from meiosis, showchromosome. ing the chromosomes and position of the genes.

I Polyploid organisms have one or more extra 3. Describe how polyploidy is used in the field of agriculture.sets of all chromosomes. Think Critically

4. Construct a chromosome map for genes A, B, C and 0 using the followingcrossing over data: A to 0=25 percent A to 8=30 percent Cto D=1 5 percent B to 0=5 percent; B to C=20 percent.

5. Evaluate what advantage polyploidy would give to a plant breeder.

Biology6. Write a short story describing a society with no genetic variation in humans.

Assessment Online Quiz Section 3 • Gene Linkage and Polyploidy 285