Consider the hypothetical scheme of determination of coat color in mammals. Gene

Question

Consider the hypothetical scheme of determination of coat color in mammals. Gene A controls the conversion of a white pigment P(0) into a gray pigment P(1); the dominant allele A produces the enzyme without biochemical activity. Gene B controls the conversion of the gray pigment P(1) into a black pigment P(2); the dominant allele B produces the enzyme necessary for this conversion, and the recessive allele b produces an enzyme without biochemical activity. The dominant allele C of a third gene produces a polypeptide that completely inhibits the activity of the enzyme produced by gene A, that is, prevents the reaction P(0) ---> P(1). Genes A, B and C assort independently, and no other genes are involved.
a) In the F2 of a cross AA bb CC x aa BB cc, what is the expected phenotypic ratio?
b)What F2 phenotypic segregation ratio would be expected for the same cross if the dominant allele C, of the third gene produced a product that completely inhibited the activity of the enzyme produced by gene B-that is, prevented the reaction P(1) ---> P(2), rather than inhibiting the activity of the enzyme produced by gene A?

Explanation / Answer

It is given that, P(0) ----A----> P(1)-----B-----> P(2) white               gray              black If C gene is present the conversion of white to gray is blocked. So, production of black also not seen. a) AA bb CC x aa BB cc Progeny will be of AaBbCc - white coat color b) In this case the C gene inhibits the conversion of gray to black, so the progeny would be AaBbCc - gray coat color.

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