A colourblind man marries a woman with bormal sight who has no histroyh of colour blindess in her family. What is the probability of their grandson becoming blind

Options

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

Answer

D

Explanation

When a coloublind man `(X^@Y)` marries 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 carriers as shown below
So, in the next generation, the children of all of their sons will be normal in all conditions (except the case in which the wife involved is not carrier neither colourblind ) for carrier daughters. (i) If they marry to a normal man, 50% of their grandsons will become colourblind as
It carrier daughter marries to a colourblind man, 50% of their grandson will be colourblind along with 50% of the granddaughter while rest 50% of the granddaughters will be carriers as
So, it both the above cases, the result shows that 50% of grand sons will be colorblind which in terms of over all progenies (son + daughters) comes as 25% thus, confirming the probablity as 0.25.