Question

A normal-visioned man whose father was colourblind, marries a woman whose father was also colourblind. They have their first child as a daughter. What are the chances that this child would be colourblind?

• A. 100%
• B. 0%
• C. 25%
• D. 50%

Answer 1

The correct option is B 0%

Colour blindness is a recessive genetic disorder in which the individuals are unable to distinguish between specific colours. If the man’s father was colourblind, he would have a genotype of X${}^c$Y. Since, the man would inherit the Y chromosome from his father, there is no risk of inheriting the mutant allele from the father. Since he has normal vision, his genotype would be XY.
If the woman’s father was colour blind, his genotype would be X${}^c$Y. Since the woman would inherit one X chromosome from her father, she would have a genotype of $X^c{X}^{-}$. The blank beside the second X chromosome denotes the unknown nature of the allele present on the second X chromosome. In such a condition, 2 types of crosses are possible which can be shown below.

Case I:- Mother is carrier (X${}^c$X)


In this cross, no daughter is found to be colour blind. They are either normal, or carriers for the trait.

Case II:- Mother is colourblind (X${}^c$X${}^c$)


In this cross, daughters are carriers only. Normal or colour blind daughters are not found. Hence it can be concluded that in any of the cases, colourblind daughters are not born.