Question

Assume that the coat colour in a mammal is dictated by a genetically controlled biosynthetic pathway as depicted in the diagram given.
The pigment gene 'X' makes an enzyme 'X' that catalyses the conversion of the precursor to the grey pigment (P$1$). The dominant allele 'X' produces the enzyme necessary for this conversion. Similarly a dominant gene 'Y' produces an enzyme 'Y' that controls the conversion of the P$1$ to a black pigment (P$2$). On the other hand, the dominant allele 'Z' produces an active enzyme 'Z' that can completely inhibit the conversion of precursor to P$1$. In all cases, the recessive alleles respectively produce defective enzymes with no activity. All the three genes assort independently with no interference from other genes. State the colour of a make with the genotype XXyyZZ and a female with the genotype xxYYzz.

• A. Male is black and female is white
• B. Both make and female are white
• C. Male is white and female is black
• D. Male is grey and female is white

Answer 1

Answer: (B)

Both make and female are white

The genotype of a male is XXyyZZ. Hence, the gamete will be XyZ. The genotype of female is xxYYzz. Hence, the gamete will be xYz. The F${}_1$ generation will have genotype XxYyZz. Hence, it will produce all the three active enzymes X, Y and Z. Since, enzyme Z will block the activity of enzyme X, both male and female will be white in colour.

Thus, the correct answer is option B.