Question

A couple has two sons and two daughters. Only one son is colour blind and the rest of the siblings are normal. Assuming colour blindness is sex-linked, which one of the following would be the phenotype of the parents?

• A. Mother would be colour blind, father would be normal.
• B. Father would be colour blind, mother would be normal.
• C. Both the parents would be normal.
• D. Both the parents would be colour blind.

Answer 1

The correct option is C Both the parents would be normal.

Colour blindness is a X-linked recessive disorder. Male requires only one set of defective gene to acquire the disease. However, female requires two sets of defective genes to be colourblind. Out of the two sons, one son is normal (XY) and one son is colourblind (X${}_c$Y). They acquired X gene from the mother. Hence, mother is phenotypically normal but carrier of the disease and having genotype X${}_c$X. Since, both daughters are normal, they have acquired normal X gene from father. Hence, the genotype of father is XY.

Thus, the correct answer is option C.