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.
Examples of Sex-linked Traits:
Male Pattern Baldness
Duchenne Muscular Dystrophy
Sample Sex-linked Trait Problems
In humans, red-green colorblindness is a recessive sex-linked trait. It is found on the X chromosome, not the Y. Because, males only have one X chromosome, they have a much greater chance of having red-green colorblindness. Females would have to be homozygous recessive in order to have red-green colorblindness.