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The major gene of the Y-chromosome is SRY. Would it be possible to get the X-chromosome and add SRY to create a "fuller" Y-chromosome?

What advantage does the skinny Y-chromosome give an individual anyway? Females only use one X-chromosome (via the Barr body) so they don't seem to gain as much from having the pair.

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Females do not always use only one X chromosome - eg Drosophila. And why would an X with SRY be better than the current Y? It is not junk - it is now being found to affect many genes through epigenetics. –  GriffinEvo Dec 31 '12 at 10:54

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SRY translocation to the X chromosome clinically exists and characterizes the De La Chapelle Syndrome. The phenotype associated to this syndrome demonstrates the necessity of other components on the Y chromosome to develop full masculine characteristics.

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Would it be possible to get the X-chromosome and add SRY to create a "fuller" Y-chromosome?

Possible? Sure. You could sequence both chromosomes and then create some artificial DNA that included the combined sequences.

Would it be viable? Probably not.

What advantage does the skinny Y-chromosome give an individual anyway?

If there is a genetic condition in the father's X-chromosome, the male child is spared from it.
If there is a genetic condition in the mother's X-chromosome, the male child has a 50/50 shot at being spared from it.
If the genetic condition requires two homogenous X-chromosomes, the male child is spared.
If the genetic condition requires two heterozygous X-chromosomes, the male child is spared.

These are all advantages of simply requiring a Y-Chromosome, and there are certainly disadvantages to having a Y-Chromosome as well; X-linked genetic disorders will definitely affect any male heirs, unlike...

Females only use one X-chromosome (via the Barr body) so they don't seem to gain as much from having the pair.

...Female offspring. Because women have two X-chromosomes they have redundancy. If there is a bad gene on the extra X-chromosome that is deactivated, then it doesn't matter - where for a man with the same X-chromosome it would definitely affect them.

Also, because of the redundancy, what X-linked conditions do exist usually affect women less-so. If both X-chromosomes are utilized for a specific gene, then one bad protein product is partially made-up for with the one good protein product.

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If the genetic condition requires two homogenous X-chromosomes, the male child is spared..... hmmm can you point to an example of that? If the condition is recessive the male child is definitely not spared. –  nico Dec 29 '12 at 8:53
    
Nope, as I'm not sure if one exists. I was being hypothetical. –  MCM Dec 29 '12 at 13:15
    
Further points> Y chromosomes, contrary to popular belief, are not junk - they contain very few genes but these can have epigenetic effects (for example via regulation of other genes). This gives a huge benefit to even "skinny" Y chromosomes because it offers the opportunity to resolve intralocus sexual conflict as it is male limited. Another point is that females do not always use just one of the X chromosomes (X inactivation). Some species use other methods of dosage compensation - drosophila males double the transcription of their X to match the output of both in the females. –  GriffinEvo Dec 30 '12 at 14:28

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