Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 21
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For example, plants from a true-breeding variety with round and yellowseeds were crossed with plants from a variety with wrinkled and green seeds. The F1 progeny were hybrid for bothcharacteristics, or dihybrid, and the phenotype of the seeds was round and yellow.
The F1 phenotype was roundand yellow because round is dominant to wrinkled and yellow is dominant to green (Figure 2.2).Then Mendel self-fertilized the F1 progeny to obtain seeds in the F2 generation. He observed four types of seedphenotypes inFigure 2.7The 3 : 1 ratio of round : wrinkled, when combined atrandom with the 3 : 1 ratio of yellow : green, yields the9 : 3 : 3 : 1 ratio that Mendel observed in the F2 progenyof the dihybrid cross.the progeny and, in counting the seeds, obtained the following numbers:round, yellow315round, green108wrinkled, yellow101wrinkled, green32Total556In these data, Mendel noted the presence of the expected monohybrid 3 : 1 ratio for each trait separately.
Withrespect to each trait, the progeny wereFurthermore, in the F2 progeny of the dihybrid cross, the separate 3 : 1 ratios for the two traits were combined atrandom, as shown in Figure 2.7. When the phenotypes of two traits are combined at random, then, among the 3/4of the progeny that are round, 3/4 will be yellow and 1/4 green; similarly, among the1/4 of the progeny that arewrinkled,3/4 will be yellow and 1/4 green.
The overall proportions of round yellow to round green to wrinkledyellow to wrinkled green are therefore expected to be 3/4 × 3/4 to 3/4 × 1/4 to 1/4 × 3/4 to 1/4 × 1/4 οr9/16 : 3/16 : 3/16 : 1/16The observed ratio of 315 : 108 : 101 : 32 equals 9.84 : 3.38 : 3.16 : 1, which is reasonably close to the 9 : 3 : 3 : 1ratio expected from the cross-multiplication of the separate 3 : 1 ratios in Figure 2.7.The Principle of Independent AssortmentMendel carried out similar experiments with other combinations of traits and, for each pair of traits he examined,consistently observed the 9 : 3 : 3 : 1 ratio.
He also deduced the biological reason for this observation. To illustratehis explanation using the dihybrid round × wrinkled cross, we can represent the dominant and reces-Page 43sive alleles of the pair that affect seed shape as W and w, respectively, and the allelic pair that affect seed color as Gand g. Mendel proposed that the underlying reason for the 9 : 3 : 3 : 1 ratio in the F2 generation is that thesegregation of the alleles W and w for round or wrinkled seeds has no effect on the segregation of the alleles G andg for yellow or green seeds. Each pair of alleles undergoes segregation into the gametes independently of thesegregation of the other pair of alleles.
In the P1 generation, the parental genotypes are WW GG (round, yellowseeds) and ww gg (wrinkled, green seeds). Then, the genotype of the F1 is the double heterozygote Ww Gg. Notethat this genotype can also be designated using the symbolismin which the slash (also called a virgule) is replaced with a short horizontal line.The result of independent assortment in the F1 plants is that the W allele is just as likely to be included in a gametewith G as with g, and the w allele is just as likely to be included in a gamete with G as with g. The independentsegregation is illustrated in Figure 2.8.
When two pairs of alleles undergo independent assortment, the gametesproduced by the double heterozygote are1/ 4 WG 1/ 4 Wg 1/ 4 wG 1/ 4 wgWhen the four types of gametes combine at random to form the zygotes of the next generation, the result ofindependent assortment is shown in Figure 2.9. The cross-multiplication-like format, which is used to show howthe F1 female and male gametes may combine to produce the F2 genotypes, is called a Punnett square. In thePunnett square, the possible phenotypes of the F2 progeny are indicated.
Note that the ratio of phenotypes is 9 : 3 :3 : 1.The Punnett square in Figure 2.9 also shows that the ratio of genotypes in the F2 generation is not 9 : 3 : 3 : 1. Withindependent assortment, the ratio of genotypes in the F2 generation is1:2:1:2:4:2:1:2:1The reason for this ratio is shown in Figure 2.10. Among seeds with the WW genotype,Figure 2.8Independent segregation of the Ww and Gg allelepairs means that, among each of the W and wclasses, the ratio of G : g is 1 : 1. Likewise, amongeach of the G and g classes, the ratio of W : w is 1 : 1.the ratio of GG : Gg : gg equals 1 : 2 : 1.
Among seeds with the Ww genotype, the ratio is 2 : 4 : 2 (the 1 : 2 : 1 ismultiplied by 2 because there are twice as many Ww genotypes as either WW or ww). And among seeds with theww genotype, the ratio of GG : Gg : gg equals 1 : 2 : 1. The phenotypes of the seeds are shown beneath thegenotypes. The combined ratio of phenotypes is 9 : 3 : 3 : 1.
From Figure 2.10, one can also see that among seedsthat are GG, the ratio of WW : Ww : ww equals 1 : 2 : 1; among seeds that are Gg, it is 2 : 4 : 2; and among seedsthat are gg, it is 1 : 2 : 1. Therefore, the independent segregation means that, among each of the possible genotypesformed by one allele pair, the ratio of homozygous dominant to heterozygous to homozygous recessive for theother allele pair is 1 : 2 : 1.Mendel tested the hypothesis of independent segregation by ascertaining whether the predicted genotypes wereactually present in the expected proportions. He did the tests by growing plants from the F2 seeds and obtaining F3progeny by self-pollination.
To illustrate the tests, consider one series of crosses in which he grew plants from F2seeds that were round, green. Note in Figures 2.9 and 2.10 that round, green F2 seeds are expected to have thegenotypes Ww gg and WW gg in the ratio 2 : 1. Mendel grew 102 plants from such seeds and found that 67 of themproduced both round, green and wrinkled, green seeds (indicating that the parental plants must have been Ww gg)and 35 of them produced only round, green seeds (indicating that the parental genotype wasPage 44Figure 2.9Diagram showing the basis for the 9 : 3 : 3 : 1 ratio of F2 phenotypesresulting from a cross in which the parents differ in two traits determinedby genes that undergo independent assortment.Page 45Figure 2.10The F2 progeny of the dihybrid cross for seed shape and seed color.
In each of the genotypesfor one of the allelic pairs, the ratio of homozygous dominant, heterozygous, and homozygousrecessive genotypes for the other allelic pair is 1 : 2 : 1.WW gg). The ratio 67 : 35 is in good agreement with the expected 2 : 1 ratio of genotypes. Similar good agreementwith the predicted relative frequencies of the different genotypes was found when plants were grown from round,yellow or from wrinkled, yellow F2 seeds.
(As expected, plants grown from the wrinkled, green seeds, which havethe predicted homozygous recessive genotype ww gg, produced only wrinkled, green seeds.)Mendel's observation of independent segregation of two pairs of alleles has come to be known as the principle ofindependent assortment, or sometimes as Mendel's second law:The Principle of Independent Assortment: Segregation of the members of any pair of alleles isindependent of the segregation of other pairs in the formation of reproductive cells.Although the principle of independent assortment is of fundamental importance in Mendelian genetics, in laterchapters we will see that there are important exceptions.Dihybrid TestcrossesA second way in which Mendel tested the hypothesis of independent assortment was by carrying out a testcrosswith the F1 genotypes that were heterozygous for both genes (Ww Gg).
In a testcross, one parental genotype isalways multiple homozygous recessive, in this case ww gg. As shown in Figure 2.11 the double heterozygotesproduce four types of gametes—WG, Wg, wG, and wg—in equal frequencies, whereas the ww gg plants produceonly wg gametes. Thus the progeny phenotypes are expected to consist of round yellow, round green, wrinkledyellow, and wrinkled green in a ratio of 1 : 1 : 1 : 1; the ratio of phenotypes is a direct demonstration of the ratio ofgametes produced by the double heterozygote because no dominant alleles are contributed by the ww gg parent toobscure the results. In the actual cross, Mendel obtained 55 round yellow, 51 round green, 49 wrinkled yellow, and53 wrinkled green, which is in good agreement with the predicted 1 : 1 : 1 : 1 ratio. The results were the same inthe reciprocal cross—that is, with the double heterozygote as the female parent and the homozygous recessive asthe male parent.
This observation confirmed Mendel's assumption that the gametes of both sexes included eachpossible genotype in approximately equal proportions.The Big ExperimentTaking his hypothesis a step further, Mendel also carried out crosses between varieties that differed in three traits:seed shape (round or wrinkled, alleles W and w), seed color (yellow or green, alleles G and g), and flower color(purple or white, alleles P and p).
The phenotype of the trihybrid F1 seeds was round and yellow, and the plantsgrown from these seeds had purple flowers. By analogy with the dihybridPage 46Figure 2.11Genotypes and phenotypes resulting from a testcross of a Ww Ggdouble heterozygote.cross, if the alleles of all three genes undergo independent assortment, then self-fertilization of the F1 flowersshould result in combinations of phenotypes given by successive terms in the multiplication of [(3 4)D + (1 4)R]3,which yields the ratio 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. For Mendel's cross, the multiplication is carried out in Figure 2.12.The most frequent phenotype (27/64) has the dominant form of all three traits, the next most frequent (9/64) has thedominant form of two of the traits, the next most frequent (3/64) has the dominant form of only one trait, and theleast frequent (1/64) is the triple recessive.
Observe that if you consider any one of the traits and ignore the othertwo, then the ratio of phenotypes is 3 : 1; and if you consider any two of the traits, then the ratio of phenotypes is9 : 3 : 3 : 1. This means that all of the possible one- and two-gene independent segregations are present in theoverall three-gene segregation.The observed and expected numbers in Figure 2.12 indicate that agreement with the hypothesis of independentassortment is very good. This, however, did not satisfy Mendel.
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