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

Answer 1

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 (X${}^c$X${}^c$ and (X${}^c$Y). 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 (X${}^c$Y) will pass its affected copy of X chromosome to daughters. But females with two copies of the affected gene show the disorder (X${}^c$X${}^c$) while females heterozygous (X${}^c$X) 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.