Color blindness is a sex-linked trait. if we could see the pedigree chart for the several more generations of the family illustrated here, we would expect

a. more males to be colorblind
b. more females to be colorblind
c. no females to ever be color blind
d. an equal numbers of male and females that are color blind

The right answer is a. more males to be colorblind

Color blindness has different causes, but in the majority of cases, the disease is hereditary and is transmitted genetically by the mother on the 23rd chromosome. This is the chromosome that determines the sex of the child. For a boy to suffer from red-green daltonism, his X chromosome must carry the defective colorblind gene. For girls, the defective gene must be present on both X chromosomes, hence the highest prevalence of red-green daltonism in boys.

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.