I am learning about genetics and sex-linked traits. All of the X-linked traits we have considered (eye colour in Drosophila, Duchene Muscular Dystrophy to name a couple)follow the rule that, a female will only express the phenotype if she has both of the alleles and a male will necessarily express the phenotype if his X chromosome contains the allele.
However this got me thinking, is there a gene that requires two copies to be expressed (and not just due to dominance considerations)? I.e., are there any x-linked genes which, even if the male has this gene, it will not be expressed as there is one copy of it only (despite the fact that this gene is on a non-homologous region of the X chromosome and therefore there is no second allele to this gene present in the male). In that case only the females could express this trait if they are homozygous for the allele, and men would never exhibit the trait.