Question

A woman with normal vision, but whose father was colour blind, marries a colour blind man. Suppose that the fourth child of this couple was a boy. This boy

• A. may be colour blind or may be of normal vision
• B. must be colour blind
• C. must have normal colour vision
• D. will not be colour blind but will be a carrier of the mutant allele

Answer 1

The correct option is A may be colour blind or may be of normal vision

In the given question, the woman has a normal vision but her father was colour blind (genotype X${}^c$Y). This means the woman is a carrier with genotype X${}^c$X. She marries a colour blind man. Genotype of the man would be X${}^c$Y. From the information, the following cross can be performed


From the above cross, it can be seen that half of the sons are colour blind whereas the other half are normal. Hence, option a is correct.
Not all the male progeny are colour blind or normal. Also, males don’t have a carrier stage i.e they don’t have a heterozygous state. So, options b, c, and d are incorrect.