Question

4) In this problem we will explore some of the many ways that mutations in two different genes can interact to produce different Mendelian ratios. Consider a hypothetical snail species that has shells that may be fully banded, ventrally banded, or completely unbanded (no bands). Imagine mutations in two different unlinked genes that can, in certain combinations, block the formation of banding, either fully or on most of the dorsal portion of the shell. In principle, there are two different possible arrangements for two biochemical steps responsible for the formation of banding in snail shells: The two genes might act such that a pigment precursor modified into a pigment by a dominant, wild-type allele of Gene A requires a recessive, wild-type expression of Gene B to deposit the pigment over the whole shell. Individuals that lack the genetic capability to deposit (functional) pigment have just ventrally banded (i.e., dorsally unbanded) shells. Alternatively, the two genes could act such that a pigment precursor modified into a pigment by a recessive, wild-type allele of Gene A requires a dominant, wild-type expression of Gene B to deposit the pigment over the whole shell. Individuals that lack the genetic capability to deposit (functional) pigment have just ventrally banded (i.e., dorsally unbanded) shells. Gene A Gene B Pigment Pigment precursor Fully banded shells You cross two individuals that are heterozygous for both Gene A and Gene B- A) Give the possible genotypes, their corresponding phenotypes, and the expected ratio of numbers of individuals in each phenotypic class if the genes interact as described by the first scenario (Scenario I) (10 pts.).