Question

Colorblindness is an X-linked recessive trait. A couple has two sons: one with normal vision and the other is color-blind. Which of the following statements HAS to be TRUE?

A. The father is colorblind.
B. The mother is colorblind.
C. The father has normal color vision.
D. The sons are not true siblings.
E. None of the above.

Answer 1

The statement that must be true is that the mother is a carrier of the colorblindness trait, as she has one son who is colorblind and another with normal vision. The correct option is B.

Step-by-step explanation:

Colorblindness is an X-linked recessive trait, meaning that it is carried on the X-chromosome and a male only needs one copy of the recessive allele to express the condition. In the case presented, the correct statement that HAS to be TRUE is that the mother is a carrier of the colorblindness trait.

Since the couple has one son with normal vision and another who is color-blind, the mother must have one normal allele and one allele for colorblindness. This is because she can pass on either allele to her sons.

The colorblind son must have inherited the recessive allele from his mother, as he would have received his Y chromosome from his father, which does not carry the X-linked trait. Conversely, the son with normal vision must have inherited his mother's normal allele. Option B. is the correct one.