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.

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

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.

Answer 2

The correct answer would be the first option.

Let R and r be the alleles for red-green color blindness. R is the dominant allele which leads to normal eyesight. r is the recessive allele which leads to the color blind phenotype when present in homozyous recessive condition.

It is given that Brian's son is colorblind. Thus, he must contain r (recessive allele). In addition, the genotype of male is XY. Thus, the genotype of Brian's son should be
`X^(r)Y`.

Now, father always pass his Y chromosome to his son and X chromosome to his daughters.

However, the mother only pass X chromosome to her son as well as daughter.

Thus, if the Brian's son has recessive copy of the allele on his X chromosome, then he must get it from his mother.

Thus, his mother should have at least one copy or allele for the color blindness.