Question

Pedigrees help geneticists understand how traits are inherited between generations. Red-green color blindness is a sex-linked disorder that is determined by a recessive allele on the X chromosome.

Briand is a filled-in square.

If Brian has a son who is color-blind, what is known about the genotype of the son's mother?
She must have at least one allele for color blindness.
She must have two alleles for color blindness.
She cannot be homozygous recessive for the gene for color blindness.
She cannot be a heterozygous carrier of the gene for color blindness.

Answer 1

Correct answer: She must have at least one allele for color blindness.

Explanation: A son receives his X chromosome from his mother. For a son to be color‑blind (XcY), the mother's X must carry the recessive allele — so she is either a carrier (XcX) or affected (XcXc).