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  4. 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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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 ?

AIPMTAIPMT 2015Principles of Inheritance and Variation

Solution:

Colorblindness is X-linked recessive disorder. A man requires only one copy of defective gene to be colorblind. Whereas a woman requires two copies of defective genes to be colorblind. When a colorblind ( $\left.X ^{ c } Y \right)$ man marries a normal woman (XX), all of their sons will be normal (XY) and all of their daughters will be carrier $\left( X ^{\circ} X \right)$. Hence, in the next generation, all the children of all of their son will be normal. There will be nil probability of being colorblind. In the next generation of their daughter, all the daughters will be either normal or carrier. Whereas $50 \%$ of the sons will be colorblind acquiring defective gene $\left( X ^{ c }\right)$ from mother. Hence, considering the grandsons of both son and daughter, there are $25 \%$ chances or 0.25 probability of grandson being colorblind.