All the X -chromosome linked genetic disease I have studied (Color-blindness, Haemophilia, Muscular dystrophy) happen due to a recessive gene.

At first, I thought it was because there could be a 'good' copy of that gene on the other X chromosome (in case of females). So that's why these genes are recessive. But this is a bogus reasoning, I soon realised.

So, why X-linked genetic diseases happen due to a recessive gene? Maybe not all. But why some are recessive?


First of all, most observed mutations are recessive (see the answer to this question; in brief: homozygous carriers are strongly selected against, heterozygous carriers are not affected only if the allele is recessive). So you wouldn't be surprised to find the same pattern on the sex chromosomes.

A good way to think about the specialty of recessive alleles on the X chromosome, is to think about why these diseases are most often visible in males (and you are on the right track there) ... Most alleles causing heavy disease - be it a mutation on the autosomes or the sex chromosomes - are rare (it is for the same reason they are recessive which is described in more detail in the answer linked above; it is due homozygous carriers being affected by purifying selection again) and therefore female carriers are most likely heterozygous and therefore healthy. In males, however, a carrier is always affected by the disease because the carrier is hemizygous.

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