Sex-linked traits are a kind of non-Mendelian inheritance pattern that has traits that are passed on via the sex chromosomes of an organism. Most traits, in humans, are passed down in the somatic cells of the body via the DNA that is condensed into chromosomes during mitosis or cell replication. However, there are a few traits that are passed down on the X or Y chromosomes that are known as sex-linked traits since they are found on the sex chromosomes and often appear in only one sex as opposed to the other.
Part of the reason that sex-linked traits only show up in one sex over the other has a lot to do with the differences between the X and Y chromosome. The Y chromosome is much smaller than the X chromosome, even though they are considered a matching pair. A human female has a genotype of XX, meaning they have two X chromosomes. Males are XY and have only one X chromosome and a Y chromosome. If a female has a recessive trait on one of the X chromosomes, then most likely the corresponding part on the other X chromosome will mask that recessive trait and it will not be seen. However, the males do not have another X chromosome and the Y chromosome is too small to have a corresponding part to the X chromosome that can mask the recessive trait. Therefore, if the male has the recessive trait on the X chromosome, it will be visible in the phenotype because there is only one allele that controls that trait.
Most sex-linked traits are found on the X chromosome, but only show up in human males due to the mismatched XY chromosome pair. However, there are a few sex-linked traits found on the Y chromosome only in humans that again, only males show since females do not have a Y chromosome. This does not mean that females cannot show sex-linked traits. It is much more rare in females because they must have two recessive alleles for that trait on their X chromosomes rather than just one that males need to show the same trait.
One example of a sex-linked trait is hemophilia. Hemophilia is a recessive disorder that causes the blood not to clot due to a missing blood clotting factor. The alleles for hemophilia are found on the X chromosome. Most humans that have the disorder of hemophilia are males, although there have been recorded cases of a few females with the disease.