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Girl of normal vision whose father was colour blind marries a man of normal vision whose father was also colour blind. The sons of this marriage would be
A. All normal
B. All colour blind
C. 50% colour blind
D. 25% colour blind

Answer
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Hint: Colour blindness is a disease in which a person cannot differentiate red-green colour. It is an X-linked recessive gene disorder. It is inheritable.

Complete Answer:
Colour blindness is a disability to differentiate between colours. It can cause difficulty in distinguishing ripe fruit, reading signals of traffic lights etc. It is an X- linked recessive disorder.
According to the question, the father who is colour blind(xY) will have the daughters who are the carriers and normal sons.
The gene composition would be represented as follows-
XX
xxX (Carrier daughter)xX (Carrier daughter)
YXY(normal son)XY(normal son)

The off-springs obtained are a carrier daughter and normal sons.
The daughters obtained will be the carrier of colour blindness(xX) marries a man whose father is colour blind (xY) is represented as
xX
xxx (colourblind daughter)xX (Carrier daughter)
YxY(colourblind son)XY(normal son)

Out of this marriage the off-springs obtained would be 50% daughter would be a carrier, 50% son would be normal and 50% would be colour blind.

Thus, the correct answer is (C). 50% of sons would be colour blind.

Note: The males are more prone to get colour blindness than the females. The gene responsible for the same is on the X-chromosome. Females have two copies of X. As it is a recessive linked gene, the defect is dominated by the other gene. Non colour blind females can carry the colour blind genes and pass it to their children.