If a gene is found only on the X chromosome and not the Y chromosome, it is
said to be a sex-linked trait. Because the
gene controlling the trait is located on the sex chromosome, sex linkage is
linked to the gender of the individual. Usually such genes are found on the
X chromosome. The Y chromosome is thus missing such genes (See Diagram above.).
The result is that females will have two copies of the sex-linked gene while
males will only have one copy of this gene. If the gene is recessive, then males
only need one such recessive gene to have a sex-linked trait rather than the
customary two recessive genes for traits that are not sex-linked. This is why
males exhibit some traits more frequently than females.
