Question

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.

Explanation:

Color - blindness is an X - linked recessive trait. A son gets his X chromosome from his mother. For a son to be color - blind (XᵇY, where Xᵇ is the allele for color - blindness), his mother must provide the Xᵇ allele. So she must have at least one Xᵇ allele (either be a carrier XᴮXᵇ or color - blind XᵇXᵇ). The other options are incorrect: she doesn't need two alleles (a carrier has one), she can be homozygous recessive, and she can be a heterozygous carrier.

Answer:

She must have at least one allele for color blindness.