Some genes are located on the X chromosome (sex-linked).

Males possess only 1 X chromosome, and exhibit a recessive sex-linked trait (such as color-blindness) if he inherits the allele from his mother.

Females exhibit the disorder only if she receives the recessive allele from both parents.

Some examples:

• color-blind female marries normal-sighted male

• father and son color-blind, mother and daughter normal-sighted