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Question

What is the phenotype of the offspring born to a woman with normal vision (homozygous) and a colour blind man?

A
All the sons and daughters arc with normal vision.
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B
All the sons and daughters are colour blind.
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C
All the sons are colour blind and the daughters are with normal vision.
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D
All the sons are with normal vision and the daughters, are colour blind.
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Solution

The correct option is A All the sons and daughters arc with normal vision.
Colour blindness is an X-linked recessive disorder. Option B is incorrect as to have all colorblind sons and daughters, both parents should be colorblind (XcXc and (XcY). Option C is incorrect as sons receive their X chromosome from mother, who is homozygous normal (XX) here; hence sons can’t have the disease. Since daughters receive one copy of X-chromosome from father, a colorblind man (XcY) will pass its affected copy of X chromosome to daughters. But females with two copies of the affected gene show the disorder (XcXc) while females heterozygous (XcX) for this trait will be normal but serve as a carrier of the disease. This makes option D incorrect. Option A is correct as the normal mother (XX) would transmit the normal X chromosome to all of her daughters and son; hence all sons and daughters will be normal but daughter would then serve as carriers. Correct answer is A.

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