A colour blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour blind

Options

A. 0.5
B. 1
C. Nil
D. 0.25

Answer

D

Explanation

When a colourblind man `(X^(C) Y)` marries to a woman with normal sight (XX) who has no family history of colour blindness, all of their sons will be normal pure and all of their daughters will be carries as shown below

So in the next generation, the children of all of their son will be normal in all conditions (except the case in which the wife involved is not carrier neither colourblind). For carrier daughters.
If they many to a normal man 50% of their grandsons will becomes colourblind as

(ii) If carrier marries to a colourblind man 50% of their grandson will be colourblind along with 50% of the grand daughter while rest 50% of the grand daughter while rest 50% of the grand daughters will be carriers as

So in both the above cases the result shows 50% of grand sons will be colourblind which in terms of over all progenies (son + daughters) comes as 25% thus confirming the probability as 0.25.