A man and his wife both have normal color vision, however their daughter is positive for red-green color blindness. The man sues his wife for divorce on the grounds of infidelity. What are the possible genotypes of the man, his wife, and their daughter? Does genetics justify the man’s case?

Man's genotype: XᴮY

Woman's genotype: XᴮXᵇ

Daughter's genotype: XᵇXᵇ

The daughter is not the man's child.

Explanation:

Color blindness is a sex-linked trait caused by a recessive allele located in the X chromosome (Xᴮ=normal vision; Xᵇ-color blind).

Women have two X chromosomes, while men have an X and a Y chromosomes. For that reason, women need to have two recessive alleles to be color blind, while men only need one Xᵇ to be colorblind.

Since the man has normal color vision, his genotype would be XᴮY.

If the daughter is her father's, she would have inherited his dominant Xᴮ allele, so she would have normal vision. However, she is colorblind, so her genotype is XᵇXᵇ. She is not the man's daughter.

The mother also has normal color vision, but her daughter inherited a recessive allele from her, so her genotype is heterozygous XᴮXᵇ.

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.

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