Question

If a colour-blind man marries a woman who is homozygous for normal colour vision, the probability of their son being colour-blind is
A) $0.75$
B) $1$
C) $0$
D) $0.5$

Answer 1

Colour blindness is a genetic defect in which a person is not able to distinguish between red-green colour. It is a X-linked recessive trait which is inherited by criss- cross inheritance. It is caused due to the defect in the cones. The disorder which is caused due to hereditary illness cannot be cured.

Complete answer:
The colour vision in the eye is possible due to the colour pigment containing cells called the cones present in the retina of the eye. The defect is generally not treatable. It is an inheritable disorder which is a X-linked recessive trait.

In the given question, A colour blind man (XcY) marries a woman who is homozygous for normal colour vision (XX).

The offspring produced by the cross is represented as:

| Xc| Y
---|---|---
X| XcX (carrier daughter)| XY (normal son)
X| XcX (carrier daughter)| XY (normal son)

The result obtained from such cross is
1)50% ($0.5$) of the daughters are carriers. Option D is incorrect.
2)All the sons born are not colour blind. Option C is correct.

Hence, the correct answer is option (C)

Note: The son inherits the X genes from the mother. The colour blind or the carrier women can produce colour blind son. Colour blind daughters can only be born if the father is colour blind and the mother is either colour-blind or carrier. So, men are most likely to have colour blindness.