Question

A colour-blind man marries a daughter of a colour blind father, then in the off-springs
A)All sons are colour blind.
B)All daughters are colour blind
C)Half of the sons are colour blind
D)No daughter is colour blind.

Answer 1

Colour blindness is an inheritable disease. It is a X-linked recessive gene defect in which a person is unable to distinguish between blue-green colours. As it is X-linked recessive disease, it mostly affects males.

Complete answer:
The cones present in the retina of the eye is responsible for the colour vision. The defect in the cones causes colour blindness. The defect in the cones which is inheritable cannot be cured.
According to the question, A colour-blind man marries a daughter of a colour blind father, then the genes are represented as-
Colour blind man- (xcY).
Daughter of colourblind man- (Xxc) (Daughter of a colourblind man will be a carrier as it is a X-linked recessive trait so the phenotypic effect of the colourblind x gene of the father will be masked by the dominant X gene.)
The offspring obtained will be determined as

| xc| Y
---|---|---
X| Xxc (carrier daughter)| XY (normal son)
xc| Xcxc (colour blind daughter)| xcY (colour blind son)

The offspring are 2 daughters out of which half is colour blind and half is the carrier. (Option B and D are incorrect)
2 sons out of which one is normal and the other half is colourblind. (option A is incorrect)
Hence, the correct answer is option (C)

Note: The carriers do not show any phenotypic expression of the gene. The chances of females to be colour blind is very rare. It only happens when the carrier female when marries a colour blind male then the probability of the daughter to be colourblind increases.