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.

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

She must have at least one allele for color blindness. This is because the son inherited his Y chromosome from his father, meaning he received his X chromosome from his mother. Since the son is color-blind, his mother must have passed on the recessive allele for color blindness to him. This means that the mother must have at least one allele for color blindness.