Sex chromosomes contain genes specifying sex and other genes for characteristics unrelated to sex.
These genes are said to be sex-linked. In humans, most sex-linked characteristics result from genes on the X chromosome. Sex-linked disorders affect mostly males. Because the male has only one X chromosome, his recessive X-linked characteristic will always be exhibited. When these traits are recessive, males express them because they have only one X. Females who have the allele are carriers and will exhibit the condition only if they are homozygous.