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CHAPTER 13 Beyond Mendel continued Modifications of Dominance Relationships • Incomplete dominance • Codominance Gene Interactions and Modified Mendelian Ratios • Phenotypes result from complex interactions of molecules under genetic control. Using genetic analysis one can often detect the patterns of these interactions. For example: • a. In the dihybrid cross AaBb´ x AaBb, nine genotypes will result. If each allelic pair controls a distinct trait and exhibits complete dominance, a 9;3;3;1 phenotypic ratio results. • b. Deviation from this ratio indicates that interaction of two or more genes is involved in producing the phenotype. • Two types of interactions occur: • a. Different genes control the same general trait, collectively producing a new phenotype. • b. One gene masks the expression of others (epistasis) and alters the phenotype. • The examples that we will study here are dihybrid, but in the “real world” larger numbers of genes are often involved in forming traits. Gene Interactions That Produce New Phenotypes • Nonallelic genes that affect the same characteristic may interact to give novel phenotypes, and often modified phenotypic ratios. • For example: Comb shape in chickens, influenced by two gene loci to produce four different comb types. Each will breed true if parents are homozygous (Figure 13.8). RRpp shows Rose comb. RRPP has walnut comb. rrPP has pea comb rrpp has single comb • In a cross between a homozygous rose-combed RRpp bird and a single-combed rrpp bird: • The F1 Rrpp will all have rose combs. • The F2 will be 3 rose R-pp;1 single rrpp • Crossing true-breeding rose RRpp and pea rrPP results in (Figure 13.9): • An F1 with all walnut combs RrPp. • An F2 showing a ratio of 9 walnut R-P-;3 rose R-pp;3 pea rrP-;1 single rrpp. • These interactions fit the expected ratios for a Mendelian dihybrid cross. The molecular basis for each phenotype is unknown, but it appears that the dominant alleles R and P each produce a factor that modifies comb shape from single to a more complex form. • The molecular basis for each phenotype is unknown, but it appears that the dominant alleles R and P each produce a factor that modifies comb shape from single to a more complex form. • Fruit shape in summer squash shows a 9:6:1 ratio. In some varieties of summer squash, two genes are involved in the expression of the shape. Each gene is completely dominant. Interaction between the two loci produces a new phenotype (Figure 13.10). • Long fruit aabb are always true-breeding. Sphere-shaped fruit A-bb or aaB- are not always truebreeding, and sometimes produce long aabb or disk-shaped A-B- fruit. • A cross between true-breeding spherical strains AAbb x aaBB produces a disk-shaped F1. The F2 will be 9⁄16 disk-shaped A-B-, 6⁄16 spherical A- bb or aaB-, and 1⁄16 long aabb. This modification of the Mendelian ratio indicates that two loci are involved. • The precise molecular basis of these phenotypes is unknown. Epistasis •In epistasis, one gene masks the expression of another, but no new phenotype is produced. A gene that masks another is epistatic. A gene that gets masked is hypostatic. Several possibilities for interaction exist and all of them will modify the 9;3;3;1 dihybrid ratio: Epistasis may be caused by recessive alleles, so that aa masks the effect of B (recessive epistasis). Epistasis may be caused by a dominant allele, so that A masks the effect of B. Epistasis may occur in both directions between genes, requiring both A and B to produce a particular phenotype (duplicate recessive epistasis). Recessive epistasis. • Coat color determination in labrador retriever dogs (Figure 13.13) • Gene B- makes black pigment, while bb makes brown. • Another gene, E- allows expression of the B gene, while ee does not. • Genotypes and their corresponding phenotypes: • E-B- is black • E-bb is brown (chocolate) • ee-- produces yellow with nose and lips either dark eeBor pale eebb What would be the F2 result of a Cross between a black dog and a golden dog ? Recessive Epistasis E Allele B_ allele Black Pigment Pigment Precursor No Pigment bb allele ee alleles 9/16 C_B_ = black 3/16 C_bb = brown 3/16 ccB_ = white 1/16 ccbb = white Brown Pigment 9 black: 3 brown: 4 white • Summary: In recessive epistasis, eeBand eebb have the same phenotype, producing an F2 ratio of 9;3;4. • Does this biochemical explanation ring any bells? What other gene did you study today that shows similar behavior? Dominant epistasis • In dominant epistasis A-B- and A-bb have the same phenotype, producing an F2 ratio of 12:3:1. • For example, in summer squash fruit have 3 common colors, white, yellow, and green (Figure 13.14). • Yellow is recessive to white but dominant to green. • Gene pairs are W/w and Y/y. • W- are white no matter the genotype of the other locus. • ww are yellow in Y- and green in yy. • What are all the genotypes and phenotypes? Lets suppose a cross between two dihybrids. What is their color? Solve the cross. And determine the proportions for dominant epistasis W_Y_= 9/16 W_yy = 3/16 12/16 white phenotype ww wwY_ = 3/16 yellow phenotype Dominant Epistasis wwyy = 1/16 green phenotype 12 white: 3 yellow: 1 green