Question

ii brian has a son who is color - blind, what is known about the genotype of the sons 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 and Y from his father. If Brian's son is color - blind, the son's X chromosome (with the color - blindness allele) must come from his mother. So the mother must have at least one allele for color blindness (she could be a carrier - heterozygous or homozygous recessive). The second option is wrong because she doesn't need two alleles (being a carrier - one allele is enough). The third option is wrong as she can be homozygous recessive. The fourth option is wrong as she can be a heterozygous carrier.

Answer:

She must have at least one allele for color blindness.