aaBB AAbb albino X albino
F2 9/16 A_B_ pigmented 3/16 aaB_ albino] 3/16 A_bb albino ■ 7/16 albino 1/16 aabb albino J
The 9 : 7 ratio in these snails is probably produced by a two-step pathway of pigment production ( FIGURE 5.9). Pigment (compound C) is produced only after compound A has been converted into compound B by enzyme I and after compound B has been converted into compound C by enzyme II. At least one dominant allele A at the first locus is required to produce enzyme I; similarly, at least one dominant allele B at the second locus is required to produce enzyme II. Albinism arises from the absence of compound C, which may happen in three ways. First, two recessive alleles at the first locus (genotype aaB_) may prevent the production of enzyme I, and so compound B is never produced. Second, two recessive alleles at the second locus (genotype A bb) may prevent the production of enzyme II. In this case, compound B is never converted into compound C. Third, two recessive alleles may be present at both loci (aabb), causing the absence of both enzyme I and enzyme II. In this example of gene interaction, a is epistatic to B, and b is epistatic to A; both are recessive epistatic alleles because the presence of two copies of either allele a or b is necessary to suppress pigment production. This example differs from the suppression of coat color in Labrador retrievers in that recessive alleles at either of two loci are capable of suppressing pigment production in the snails, whereas recessive alleles at a single locus suppress pigment expression in Labs. __
Epistasis is the masking of the expression of one gene by another gene at a different locus. The epistatic gene does the masking; the hypostatic gene is masked. Epistatic genes can be dominant or recessive.
A number of modified ratios that result from gene interaction are shown in Table 5.2. Each of these examples represents a modification of the basic 9:3:3:1 dihybrid ratio.
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