Why are X-linked genes expressed more frequently in males than in females?

X-linked genes are expressed more frequently in males than in females primarily due to the presence of two X chromosomes in females. Males have only one X chromosome, paired with a Y chromosome, which does not carry alleles for most traits present on the X chromosome. As a result, any gene that is located on the X chromosome is expressed in males because they lack a second X chromosome that could mask the expression of that allele.

In contrast, females, having two X chromosomes, may have one normal allele that can mask a recessive allele present on the other X chromosome. This means that even if a female inherits a recessive X-linked trait, she may not express the trait if the other X chromosome carries a dominant allele that masks its effects. Therefore, the expression of X-linked genes tends to be more prevalent in males, where the singular X chromosome ensures that the expression of X-linked traits occurs without the presence of a counterpart that could provide a dominant allele.