Essential Cell Biology 5th edition
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Essential Cell Biology 5th edition

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670 CHAPTER 19 Sexual Reproduction and GeneticsFigure 19−27 A dihybrid (two traits) crossdemonstrates that alleles can segregateindependently. Alleles that segregateindependently are packaged into gametesin all possible combinations. So the Y alleleis equally likely to be packaged with the Ror r allele during gamete formation; and thesame holds true for the y allele. Thus fourclasses of gametes are produced in roughlyequal numbers: YR, Yr, yR, and yr. Whenthese gametes are allowed to combine atrandom to produce the F 2 generation, theresulting pea phenotypes are yellow-round,yellow-wrinkled, green-round, and greenwrinkledin a ratio of 9:3:3:1.phenotype: yellow-roundgenotype:YYRRYRPARENTAL GENERATIONgametesphenotype: yellow-roundgenotype:Y Rrphenotype: green-wrinkledgenotype: rrrgametesCROSS-FERTILIZATIONF 1 GENERATIONSELF-FERTILIZATIONYRYRFEMALEGAMETESYrYYRRYrMALEGAMETESrRY RRYYRrYYrrYYRrY RRRrY RrY RrY RrY RrY rrRRY rrRrRrrrF 2 GENERATIONgene (Yy). During meiosis, the chromosomes bearing the Y and y alleleswill be separated, producing two types of haploid gametes—ones thatcontain a Y allele and others that contain a y. In a plant that self-fertilizes,these haploid gametes come together to produce the diploid individualsof the next generation—which may be YY, Yy, or yy. Together, the meioticmechanisms that distribute the alleles into gametes and the combiningof gametes at fertilization provide the physical foundation for Mendel’slaw of segregation.But what about independent assortment of multiple traits? Because eachECB5 e19.27/19.27pair of duplicated homologs attaches to the spindle and lines up at themetaphase plate independently during meiosis, each gamete will inherita random mixture of paternal and maternal chromosomes (see Figure19−15A). Thus the alleles of genes on different chromosomes will segregateindependently.Consider a pea plant that is heterozygous for both seed color (Yy) andseed shape (Rr). The homolog pair carrying the color alleles will attachto the meiotic spindle with a certain orientation: whether the Y-bearinghomolog or its y-bearing counterpart is captured by the microtubulesfrom one pole or the other depends on which way the bivalent happensto be facing at the moment of attachment (Figure 19−28). The same

Mendel and the Laws of Inheritance671duplicatedhomologouschromosomesrRparent plantgenotype:Y Rrdiploid germ-line cell in parent plantYFigure 19−28 The separation ofduplicated homologous chromosomesduring meiosis explains Mendel’slaws of segregation and independentassortment. Here we show independentassortment of the alleles for seed color,yellow (Y) and green (y), and for seed shape,round (R) and wrinkled (r), as an example ofhow two genes on different chromosomessegregate independently. Althoughcrossovers are not shown, they would notaffect the independent assortment of thesetraits, as the two genes lie on differentchromosomes.PAIRED DUPLICATED HOMOLOGSALIGN RANDOMLY ON SPINDLEAT METAPHASE OF MEIOSIS IORmeioticspindleMEIOSIS IMEIOSIS IIRY RY r r R R rY rYgametesis true for the homolog pair carrying the alleles for seed shape. Thus,whether the final gamete receives the YR, Yr, yR, or yr allele combinationdepends entirely on which way the two homolog pairs were facing whenthey were captured by the meiotic spindle; each outcome has the samedegree of randomness as the tossing of a coin.Genes That Lie on the Same Chromosome CanSegregate Independently by Crossing-OverECB5 e19.28/19.28Mendel studied seven traits, each of which is controlled by a separategene. It turns out that most of these genes are located on different chromosomes,which readily explains the independent segregation he observed.But the independent segregation of different traits does not necessarilyrequire that the responsible genes lie on different chromosomes. If twogenes are far enough away from each other on the same chromosome,they will also sort independently, because of the crossing-over that occursduring meiosis. As we discussed earlier, when duplicated homologs pairto form bivalents, the maternal and paternal homologs always undergocrossing-over. This genetic exchange can separate alleles that were formerlytogether on the same chromosome, causing them to segregate into

670 CHAPTER 19 Sexual Reproduction and Genetics

Figure 19−27 A dihybrid (two traits) cross

demonstrates that alleles can segregate

independently. Alleles that segregate

independently are packaged into gametes

in all possible combinations. So the Y allele

is equally likely to be packaged with the R

or r allele during gamete formation; and the

same holds true for the y allele. Thus four

classes of gametes are produced in roughly

equal numbers: YR, Yr, yR, and yr. When

these gametes are allowed to combine at

random to produce the F 2 generation, the

resulting pea phenotypes are yellow-round,

yellow-wrinkled, green-round, and greenwrinkled

in a ratio of 9:3:3:1.

phenotype: yellow-round

genotype:YYRR

YR

PARENTAL GENERATION

gametes

phenotype: yellow-round

genotype:Y Rr

phenotype: green-wrinkled

genotype: rr

r

gametes

CROSS-FERTILIZATION

F 1 GENERATION

SELF-FERTILIZATION

YR

YR

FEMALE

GAMETES

Yr

YYRR

Yr

MALE

GAMETES

r

R

Y RR

YYRr

YYrr

YYRr

Y RR

R

r

Y Rr

Y Rr

Y Rr

Y Rr

Y rr

RR

Y rr

Rr

Rr

rr

F 2 GENERATION

gene (Yy). During meiosis, the chromosomes bearing the Y and y alleles

will be separated, producing two types of haploid gametes—ones that

contain a Y allele and others that contain a y. In a plant that self-fertilizes,

these haploid gametes come together to produce the diploid individuals

of the next generation—which may be YY, Yy, or yy. Together, the meiotic

mechanisms that distribute the alleles into gametes and the combining

of gametes at fertilization provide the physical foundation for Mendel’s

law of segregation.

But what about independent assortment of multiple traits? Because each

ECB5 e19.27/19.27

pair of duplicated homologs attaches to the spindle and lines up at the

metaphase plate independently during meiosis, each gamete will inherit

a random mixture of paternal and maternal chromosomes (see Figure

19−15A). Thus the alleles of genes on different chromosomes will segregate

independently.

Consider a pea plant that is heterozygous for both seed color (Yy) and

seed shape (Rr). The homolog pair carrying the color alleles will attach

to the meiotic spindle with a certain orientation: whether the Y-bearing

homolog or its y-bearing counterpart is captured by the microtubules

from one pole or the other depends on which way the bivalent happens

to be facing at the moment of attachment (Figure 19−28). The same

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