Download Sex-linked genes, genes located on one of the sex chromosomes (X

Sex-linked genes, genes located on one of the sex
chromosomes (X or Y) but not the other. Since, typically
the X chromosome is longer, it bears a
lot of genes not found on the Y
chromosome, thus most sex-linked
genes are X-linked genes.
Typically, X-linked traits show
up more in males than females
because typical XY males only have one X chromosome, so if they
get the allele on their X chromosome, they show the trait.
If a typical XX female is a carrier, 50% of her sons will get that
X chromosome and show the trait. In order for an XX female to
exhibit one of these X-linked traits, most of which are
recessive mutations, she would have to have two copies of the
allele (X'X').
In humans, two well-known X-linked traits are hemophilia and
red-green colorblindness. Hemophilia is the failure (lack of
genetic code) to produce certain substance needed for proper
blood-clotting, so a hemophiliac’s blood doesn’t clot, and (s)he
could bleed to death from an injury that a normal person might
not even notice.
Again, colorblindness and hemophilia, while rare overall, are more
common in XY males, because they only have one X chromosome.
For an XX woman to be colorblind, for example, her mother would
have to be a carrier for the trait and her father would have to be
colorblind.