Red-green colour blindness is a sex-linked condition. Which of the following always shows normal vision?

A. A homozygous male
B. A homozygous female
C. A heterozygous male
D. A heterozygous female

D

Although some teachers complained about the wording of this question, it turned out to be a very good discriminator. Some candidates thought a homozygous female always showed normal vision, but a female homozygous for the colour blindness allele would not have normal vision.