Question

Hair pigmentation in mammals follows the generic enzymatic pathway depicted below. Enzyme A synthesizes a black pigment from a precursor, and the contribution of enzyme B results in the final agouti pigment:

Precursor molecule (colorless) Black pigment Agouti pigment

Each enzyme is coded by a single allele with complete dominance. If a crossing design begins with true breeding parents for coat color with the genotypes AAbb and aaBB. What will be the expected F2 genotypic and phenotypic ratios of the eventual F2 progeny?

Answer 1

Given,

Colourless molecule is converted to black pigment by enzyme A. Black pigment is converted to agouti pigment by enzyme B.

Parent 1: AAbb = Black

Parent 2: aaBB = Colourless

AAbb X aaBB = AaBb (F1) The entire progeny is agouti coloured since dominant alleles for both the genes are present.

When AaBb X AaBb according to dihybrid cross in F2 :

A_B_ = 9 ( Agouti )

A_bb = 3 ( Black )

aaB_ = 3 ( Colourless )

aabb = 1 ( Colourless )

Hence genotypic ratio is 9 : 3 : 3 : 1

and phenotypic ratio is 9 (agouti) : 3 (black) : 4 (colourless)