Sex linkage is the
phenotypic expression of an
allele that is related to the
gender of the individual and is found on the
sex chromosomes. This mode of inheritance is in contrast to the inheritance of traits on
autosomal chromosomes, where both sexes have the same probability of expressing the trait. Since, in humans, there are many more genes on the X than there are on the Y, there are many more X-linked traits than there are Y-linked traits.
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On the X chromosome. "Linked" in genetics does not mean merely associated. An X-linked gene travels with the X chromosome and therefore is part of the X chromosome.