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

Hint: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.