Sex chromosomes determine whether an organism is male or female. In humans and many other species, an XX organism is female and an XY organism is male. X-linked alleles are located on the X chromosome. A female will express the phenotype of an X-linked allele coding for a recessive trait only if she inherits two copies of it, one from each parent. On the other hand, a male will express the phenotype of an X-linked allele coding for a recessive trait if he inherits it from his mother, since he has no second X chromosome to mask the recessive trait. Because sex-linked genes are expressed differently, depending on gender, sex-linked traits do not follow Mendelian inheritance patterns.