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?

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Solution

Q.
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 [2015]

1 Nil

2 0.25

3 0.5

4 1

Ans.
(2)

When a colour blind man ( $X^{c} Y$ ) marries a normal woman (XX), all of their daughters are carriers and all of their sons are normal, as shown in following figure:

When the carrier daughter ( ${XX}^{c}$ ) is married to a normal man, the probability of their son being colour blind is 0.25, as shown in following figure:

From above crosses, it is clear that the probability of occurrence of colour blindness in the grandson of a colour blind man and a normal woman is 0.25.

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