A colorblind woman marries a man who is not colorblind. All of their sons, but none of their daughters, are colorblind. Which of the following statements correctly explains these results?
A) The gene for color vision is incompletely dominant to the gene for sex determination.
B) The gene for color vision is codominant with the gene for sex determination.
C) The gene for color vision is found on the X chromosome.
D) The gene for color vision is found on the Y chromosome.

Question

Quizplus · Accepted Answer

Color blindness is X-linked recessive, meaning the gene responsible for color vision is found on the X chromosome. Females have two X chromosomes, so in order to be colorblind, they must inherit two copies of the colorblindness allele. However, males only have one X chromosome, so if they inherit the colorblindness allele, they will be colorblind. Since the mother is colorblind, she must have one X chromosome with the colorblindness allele. If she passes that allele to her sons, they will be colorblind. The daughters, on the other hand, would need to inherit two copies of the colorblindness allele, which is less likely since their father does not have the colorblindness allele.

A Colorblind Woman Marries a Man Who Is Not Colorblind