Compound C

Conclusion: Genotypes W_ Y_ and W_yy do not produce enzyme I; ww yy produces enzyme I but not enzyme II; ww Y_ produces both enzyme I and enzyme II.

Plants with genotype yy do not encode a functional form of enzyme I

A 5.8 Yellow pigment in summer squash is produced in a two-step pathway.

Duplicate recessive epistasis Let's consider one more detailed example of epistasis. Albinism is the absence of pigment and is a common genetic trait in many plants and animals. Pigment is almost always produced through a multistep biochemical pathway; thus, albinism may entail gene interaction. Robert T. Dillon and Amy R. Wethington found that albinism in the common freshwater snail Physa heterostroha can result from the presence of either of two recessive alleles at two different loci. Inseminated snails were collected from a natural population and placed in cups of water, where they laid eggs. Some of the eggs hatched into albino snails. When two albino snails were crossed, all of the F1 were pigmented. On intercrossing the F1, the F2 consisted of 9/16 pigmented snails and 7/16 albino snails. How did this 9: 7 ratio arise?

The 9: 7 ratio seen in the F2 snails can be understood as a modification of the 9:3:3:1 ratio obtained when two individuals heterozygous for two loci are crossed. The 9 : 7 ratio arises when dominant alleles at both loci (A_B_) produce pigmented snails; any other genotype produces albino snails:

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