Chickens have a ZW system, in which females have two different sex chromosomes (

Question

Chickens have a ZW system, in which females have two different sex chromosomes (ZW) and males have two of the same (ZZ- much like mammalian females). In chickens, the allele for red spots (r) is a sex-linked recessive; individuals with the dominant allele have no spots. The allele for white feathers (h) is recessive to the allele for colored feathers, and the allele for yellow feet is (r) is dominant to the allele for orange feet. A spot-free hen heterozygous for feather color and foot color is mated to a spotted rooster heterozygous for feather color and foot color. Predict the phenotypic ratios expected in the offspring.

Explanation / Answer

Red spot Yellow feet(dominant) - r - sexlinked recessive; R - dominant allele no spots; Yellow feet.

White feathers - h - recessive - H- colored feathers

Let us use X+as the symbol for the dominant red spot, and Xyfor the recessive red spot color. Male progeny will always inherit the Y chromosome from male parent and either of the two X chromosomes from the female parent. Female progeny always inherit the X chromosome from the male parent and either of the two X chromosomes from the female parent.

            F1 males inherit the Y chromosome from their father, and X+from their mother; hence their genotype is X+Y and they have No spot and yellow feet.

            F1 females inherit Xyfrom their father and X+from their mother; hence they are X+Xyand also have No spot and yellow feet.

In summary:

            P            X+X+(No spot; yellow feet) × XyY (Spotted rooster; orange feet )

            F1           ½ X+Y (No spot; yellow feet males)

                           ½ X+Xy(No spot; yellow feet females)

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