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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.

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  • $\begingroup$ then why is the other x chromosome gets inactivated?... $\endgroup$ – JM97 Feb 1 '17 at 0:00
  • $\begingroup$ @JM97 I'm afraid I didn't understand your question :/ $\endgroup$ – 21joanna12 Feb 1 '17 at 16:32
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    $\begingroup$ What @JM97 appears to be saying is that in females there is a process called X-chromosome inactivation which shuts down one of the chromosomes to prevent females having a double dose of a particular gene product. This would invalidate your premise that genes that require a double-dose of expression (as it were) could be only expressed in females. $\endgroup$ – David Feb 1 '17 at 20:42
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It is not easy for an individual to figure out if it is haploid or diploid at a given sequence. To my knowledge, this is feasible only by searching for specific case of heterozygosity.

The most famous case is mating types in yeasts. In yeasts, haploid cells have mating types. Individuals are either "a" or "$\alpha$". Individuals can only fuse with individuals of a different mating type. As a result all diploid cells contain both the "a" allele on one chromosome and the "$\alpha$" allele on the other chromosome. The "a" sequence codes for proteins that can bind only the proteins coded by the "$\alpha$" sequence. If "a-$\alpha$" dimers are formed, then the cell "knows" it is diploid. Read more about it on wikipedia > mating of yeast.

Following this, a cell could figure out what sexual chromosome it has by searching for heterozygosity. This is typically NOT how gender determination works in mammals. I can't think of a specific example where this would be a thing but I would surprise if it is not.

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