Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 27
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This is the ratioobserved in the segregation of the C, c and P, p alleles in Figure 2.23, and the 9 : 7 ratio is the ratio of purpleflowers to white flowers. Taking all the possible modified ratios in Figure 2.24B together, there are nine possibledihybrid ratios when both genes show complete dominance. Examples are known of each of the modified ratios.However, the most frequently encountered modified ratios are 9 : 7, 12 : 3 : 1, 13 : 3 , 9:4:3, and 9:6:1. The types ofepistasis that result in these modified ratios are illustrated in the following examples, taken from a variety oforganisms. Other examples can be found in the problems at the end of the chapter.9 : 7 This is the ratio observed when a homozygous recessive mutation in either or both of two differentgenes results in the same mutant phenotype.
It is exemplified by the segregation of purple and whiteflowers in Figure 2.23. Genotypes that are C— for the C genePage 63and P— for the P gene have purple flowers; all other genotypes have white flowers. In this notation, thedash in C— means that the unspecified allele could be either C or c, and so C— refers collectively to CCand Cc. Similarly, the dash in P— means that the unspecified allele could be either P or p.12 : 3 : 1 A modified dihybrid ratio of the 12:3:1 variety results when the presence of a dominant allele ofone gene masks the genotype of a different gene. For example, if the A— genotype renders the B— and bbgenotypes indistinguishable, then the dihybrid ratio is 12:3:1 because the A— B— and A— bb genotypes areexpressed as the same phenotype.In a genetic study of the color of the hull in oat seeds, a variety having white hulls was crossed with a varietyhaving black hulls.
The F1 hybrid seeds had black hulls. Among 560 progeny in the F2 generation produced byselffertilization of the F1, the following seed phenotypes were observed in the indicated numbers:418black hulls106gray hulls36white hullsNote that the observed ratio of phenotypes is 11.6 : 2.9 : 1, or very nearly 12:3:1. These results can be explained bya genetic hypothesis in which the black-hull phenotype results from theFigure 2.24Modified F2 dihybrid ratios. (A) The F2 genotypes of two independently assorting genes with completedominance result in a 9 : 3 : 3 : 1 ratio of phenotypes if there is no interaction between the genes (epistasis).(B) If there is epistasis that renders two or more of the phenotypes indistinguishable, indicated by the colors,then the F2 ratio is modified.
The most frequently encountered modified ratios are 9 : 7, 12 : 3 : 1, 13 : 3, 9 : 4 : 3,and 9 : 6 : 1.Page 64presence of a dominant allele (say, A) and the gray-hull phenotype results from another dominant allele (say, B)whose effect is apparent only in the aa homozygotes. On the basis of this hypothesis, the original true-breedingvarieties must have had genotypes aa bb (white) and AA BB (black). The F1 has genotype Aa Bb (black). If the A,aallele pair and the B,b allele pair undergo independent assortment, then the F2 generation is expected to have thefollowing composition of genotypes:9/16A—B—(black hull)3/16A—bb(black hull)3/16aaB—(gray hull)1/16aabb(white hull)This type of epistasis accounts for the 12:3:1 ratio.13 : 3 This type of epistasis is illustrated by the difference between White Leghorn chickens (genotype CC II) andWhite Wyandotte chickens (genotype cc ii.
Both breeds have white feathers because the C allele is necessary forcolored feathers, but the I allele in White Leghorns is a dominant inhibitor of feather coloration. The F1 generationof a dihybrid cross between these breeds has the genotype Cc Ii, which is expressed as white feathers because ofthe inhibitory effects of the I allele. In the F2 generation, only the C- ii genotype has colored feathers, so there is a13:3 ratio of white : colored.9:4:3 This dihybrid ratio (often stated as 9:3:4) is observed when homozygosity for a recessive allele with respectto one gene masks the expression of the genotype of a different gene. For example, if the aa genotype has the samephenotype regardless of whether the genotype is B— or bb, then the 9:4:3 ratio results.In the mouse, the grayish coat color called "agouti" is produced by the presence of a horizontal band of yellowpigment just beneath the tip of each hair. The agouti pattern results from the presence of a dominant allele A, and inaa animals the coat color is black.
A second dominant allele, C, is necessary for the formation of hair pigments ofany kind, and cc animals are albino (white fur). In a cross of AA CC (agouti) × aa cc (albino), the F1 progeny areAa Cc and agouti. Crosses between F1 males and females produce F2 progeny in the following proportions:9/16A—C—(agouti)3/16A—cc(albino)3/16aaC—(black)1/16aacc(albino)The dihybrid ratio is therefore 9 agouti: 4 albino : 3 black.9:6:1 This dihybrid ratio is observed when homozygosity for a recessive allele of either of two genes results in thesame phenotype, but the phenotype of the double homozygote is distinct. For example, red coat color in DurocJersey pigs requires the presence of two dominant alleles R and S.
Pigs of genotype R—ss and rr S— havesandycolored coats, and rr ss pigs are white. The F2 dihybrid ratio is therefore9/16R—S—(red)3/16R—ss(sandy)3/16rrS—(sandy)1/16rrss(white)The 9:6:1 ratio results from the fact that both single recessives have the same phenotype.2.7—Complications in the Concept of DominanceIn Mendel's experiments, all traits had clear dominant-recessive patterns. This was fortunate, because otherwise hemight not have made his discoveries.
Departures from strict dominance are also frequently observed. In fact, evenfor such a classical trait as round versus wrinkled seeds in peas, it is an oversimplification to say that round isdominant. At the level of whether a seed is round or wrinkled, round is dominant in the sense that the genotypesWW and Ww cannot be distinguished by thePage 65outward appearance of the seeds. However, as emphasized in Chapter 1, every gene potentially affects many traits.It often happens that the same pair of alleles shows complete dominance for one trait but not complete dominancefor another trait. For example, in the case of round versus wrinkled seeds, the genetic defect in wrinkled seeds isthe absence of an active form of an enzyme called starch-branching enzyme I (SBEI), which is needed for thesynthesis of a branched-chain form of starch known as amylopectin. Compared with homozygous WW, seeds thatare heterozygous Ww have only half as much SBEI, and seeds that are homozygous ww have virtually none (Figure2.25A).
Homozygous WW peas contain large, well-rounded starch grains, with the result that the seeds retain waterand shrink uniformly as they ripen, so they do not become wrinkled. In homozygous ww seeds, the starch grainslack amylopectin; they are irregular in shape, and when these seeds ripen, they lose water too rapidly and shrinkunevenly, resulting in the wrinkled phenotype observed (Figure 2.25B and C).The w allele also affects the shape of the starch grains in Ww heterozygotes. In heterozygous seeds, the starchgrains are intermediate in shape (Figure 2.25B). Nevertheless, their amylopectin content is high enough to result inuniform shrinking of the seeds and no wrinkling (Figure 2.25C). Thus there is an apparent paradox of dominance.If we consider only the overall shape of the seeds, round is dominant over wrinkled.
There are only twophenotypes. If we consider only the overall shape of the seeds, round is dominant over wrinkled. There are onlytwo phenotypes. If we examine the shape of the starch grains with a microscope, all three genotypes can bedistinguished from each other: large, rounded starch grains in WW; large, irregular grains in Ww; and small,irregular grains in ww. If we consider the amount of the SBEI enzyme, the Ww genotype has an amount abouthalfway between the amounts in WW and ww.The round, wrinkled pea example in Figure 2.25 makes it clear that "dominance" is not simply a property of aparticular pair of alleles no matter how the resulting phenotypes are observed. When a gene affects multiple traits(as most genes do), then a particular pair of alleles mightFigure 2.25Phenotypic expression of three traits affected by Mendel's alleles W and w determininground versus wrinkled seeds.
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