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Originally posted by vovansim
You are correct. In women, only one of the two X-chromosomes is active and actually determines the phenotype.
You sure? Because if one of the strands carry a defective gene, the other can compensate for it. Suppose if there is a defective gene (different locus) on both strands. Then you need both X chromosomes.
(\__/) 07/07/1937 - Never forget
(='.'=) "Claims demand evidence; extraordinary claims demand extraordinary evidence." -- Carl Sagan
(")_(") "Starting the fire from within."
Human females inherit two copies of every gene on the X chromosome, whereas males inherit only one. <...> Are males at a disadvantage in the amount of gene product their cells produce? The answer is no, because females have only a single active X chromosome in each cell.
<...>
The cells of females have only one functioning copy of each X-linked gene - the same as males.
X-inactivation occurs early in embryonic development. In a given cell, which of a female's X chromosomes becomes inactivated and converted into a Barr body is a matter of chance (except in marsupials like the kangaroo, where it is always the father's X chromosome that is inactivated). After inactivation has occurred, all the descendants of that cell will have the same chromosome inactivated.
However, in some animals (and I think in humans too, but I am not sure) the different cells of a female can have different X-chromosomes deactivated. That's the way, for example, they explain tri-color cats. These are the cats that have patches of three different colours: white, brown, and black. Only females have those: males can only be unicolored or bi-colored, unless, that is, due to genetic defect they get multiple X-chromosomes. And that is because different parts of the female's skin can have different X-chromosomes inactive. Thus, some parts would be black, and others - brown. A simplified but more detailed explanation of how that works can be found here:
Originally posted by Urban Ranger
You can always go for something like XXXX or XXXY. That will work theoretically.
Yes, and that happens in real life, too. Like I explained in the above post (and in the source of that quote, you can actually find more on that multiple X chromosomes, on the bottom of the page), the extra X-chromosomes just get deactivated and turn into Barr bodies.
It appears that it is not necessary for for all the genes on the Barr body to not be expressed.
(\__/) 07/07/1937 - Never forget
(='.'=) "Claims demand evidence; extraordinary claims demand extraordinary evidence." -- Carl Sagan
(")_(") "Starting the fire from within."
Originally posted by Urban Ranger
It appears that it is not necessary for for all the genes on the Barr body to not be expressed.
Yeah, that's the odd part...
But I think you are right in that an embryo with only Y chromosomes wouldn't be functional. After all, we know that such disorders as colour-blindness, hemophilia (is that what it's called when the blood does not clog?), and some others are related to genetic disorders of the X-chromosome. Thus, it does contain genes that code for some important stuff - related to blood and vision, at least.
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