Question

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 through daughter being colour blind?
A) $0.25$
B) $0.5$
C) 1
D) Nil

Answer 1

Colour blindness is reduced ability to differentiate between certain colours. It is often inherited. So when a colour blind man marries a normal sight woman all of their sons will be normal but their daughters will be carriers.

Complete answer:
It is a X linked recessive disorder.
A man needs only one copy of a defective gene to be colour blind.
A woman needs two copies of defective genes to be colour blind.
So when a colour blind man marries a normal sight woman all of their sons will be normal but their daughters will be carriers.
Now if these daughters will marry a normal man. Among sons 50% will be normal and 50% will be colour blind. Among daughters 50 percent will be normal and 50 percent will be carriers.
F1 generation $XX \times {X^c}Y$
Gametes: $X,{X^c},Y$
F1 generation ${X^c}X$ $XY$ (Carrier daughter and normal son)
If the carrier daughter will marry a normal man then the progeny will be ${X^c}X \times XY$
F2 GENERATION: $XX$,$XY$, ${X^c}X$,${X^c}Y$So the probability of grandsons through daughter to be colour blind is 50 percent. Correct answer is $0.5$.

Additional information: The X linked disorder means the gene causing the disorder is located on X chromosome. Haemophilia is also X linked recessive disorder.

Notes: X linked recessive inheritance is the type of inheritance in which there is a mutation in s gene on X chromosome which causes the phenotype to be always observed in man because they have one X chromosome and one Y chromosome. However women will always be carriers for that because they have only one copy of a mutated gene.