Red-green color blindness is a sex-linked recessive trait in humans. Two people with normal color vision have a color-blind son. What are the genotypes of the parents?

Genotype of Mother: XX°

Genotype of Father: XY

Explanation:

Since the Red-green colour blindness is a recessive trait, it will not be visible in a heterozygous female because dominant allele will mask the recessive allele. Father has only one X chromosome, thus if the trait was present in that X chromosome he must have shown the trait because all the alleles in Y chromosome are recessive. Since the son has inherited the trait it is obvious that he had recieved the trait from his mother who must have been the carrier.