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 parent would be normal
d. Both the parent would be colour blind

Answer 1

Hint: A weakened ability to discriminate between such colours. The disorder is also hereditary and a genetic fault passed down to a child by their parents causes colour vision loss.

Complete answer:
> Option A is incorrect. If mother is colour blind and father is normal, mother transmits her X chromosome either to her sons or her daughter in this case. Therefore, it cannot be said confidently that the colour blind is the only one son and the rest of the siblings are natural.

> Option B is incorrect. The father transfers to the daughters his X chromosome, so the colour-blind father inherits the affected X chromosome of the daughter. The genotype of the female carrier will therefore be (XcX) and that of the average man will be (XY). The female carrier for colour blindness would only inherit 50% of her son from the disease. The daughters have obtained their regular X-chromosome copy from their regular father and will be normal phenotypically.

> Option C is correct. Color blindness is a recessive condition which is X-linked. In order to develop the disorder, men need only one set of faulty genes. Females, however, need to be colour-blind with two sets of faulty genes. One son is natural (XY) out of the two sons and one son is colour-blind (XcY). They inherited the mother's X gene. Therefore, the mother is phenotypically normal but the carrier of the disorder and genotype XcX. Since both daughters are regular, the normal X gene was inherited from the father. Hence, the father genotype is XY.

> Option D is incorrect. In the event that both parents are colour blind, their sons and daughters are 100% likely to be colour blind, and their daughters are 100% likely to bear the gene. So, this choice is incorrect.

Hence, the correct answer is option (C).

Note: Physical or chemical damage to the retina, the optic nerve, or parts of the brain may also cause colour blindness. Typically, diagnosis is with the Ishihara colour test and genetic testing is used in other approaches.