X-linked recessive disease question?

Why would it be unlikely to see a woman who is homozygous dominant for an X-linked recessive disease?

X-linked recessive disease question?
I'll assume that you mean a woman who is homozygous for two recessive alleles - your quesion doesn't entirely make sense with the "dominant" in. Anyway, that's the most likely scenario for X-linked recessive alleles.





Let's say that a boy has this disease. Because he's male, he'll have XY sex chromosomes. Because this disease only has an allele on the X chromosome, there is no corresponding allele on the Y chromosome. So he only needs one recessive allele to have the disease. For that to happen, only the mother needs to be carrying this recessive allele, and not necessarily show any symptoms if she is heterozygous for the alleles.





Let's take a girl suffering from this disease. For the girl to have it, she MUST be homozygous double recessive, because the female sex chromosomes are XX. This means that she has inherited the recessive allele from both parents, and the father has to suffer from the disease (because he only has one X chromosome, that must be the one passed on). This is a very unlikely scenario, as the father is unlikely to either a) survive long enough to have kids or b) decide to have kids at all. Basically, it's extremely unlikely for females to have the disease according to the laws of probability.





I'm taking it for granted here that you know what alleles etc. are. Please say so if you don't!

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