I read that X-inactivation doesn't tend to happen in males, but then when someone is XXY, they are a male because of the Y. However these individuals tend to live. So does that mean that X-inactivation happens in males or else these individuals would die?

  • $\begingroup$ Trisomy 21 (of the 21st chromosome) and being an XXY individual are two quite different things. Isn't your question about the XXY sexual chromosome pattern (which is BTW not trisomy, but Klinefelter's syndrome)? Or is it about a special case of Klinefelter AND trisomic 21 individual (I don't even know if that exists...)? $\endgroup$ – Raoul Sep 9 '14 at 23:51
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    $\begingroup$ Do you confound the Down syndrome (Trisomy 21) and the Klinefelter Syndrome (XXY)? $\endgroup$ – Remi.b Sep 10 '14 at 0:28
  • $\begingroup$ So basically, the question is: "Do XXY (Klinefelter Syndrome) individuals inactivate (dosage compensation) one of their X-chromsome?". Why is it written Trisomy 21 in the title? $\endgroup$ – Remi.b Sep 10 '14 at 0:32

X-inactivation does occur in XXY individuals. 

In phenotypically normal XX females, one X chromosome is inactivated.  Which one of the two X chromosomes is inactivated in any given cell is random. Therefore, about one half of the cells should have the maternal X active and the other half should have the paternal X active.

XXY individuals show a wide range of clinical symptoms and intellectual ability.  Iitsuka et al. (2000) suggested that this variation may be explained by a bias in X-inactivation.   They found that about 31% (5 of 16) individuals (either XXY or XXYY) shows a very strong bias of X inactivation, which they defined as at least 80% of the cells inactivating one particular X chromosome. Two individuals has biased inactivation of the paternal X and the other three had biased inactivation of one of the maternal X chromosomes. The sample size is rather small (not surprising) but X inactivation is occuring.  Samango-Sprouse (2001) argues that incomplete inactivation of one X chromosome may also explain some of the more severe clinical mental disorders.

For Trisomy 21 (Down's Syndrome), the individual as three copies of chromosome 21, so X inactivation is not an issue. However, scientists are studying whether Xist can shut down the third copy of chromosome 21.  The Xist gene is transcribed to produce a large molecule of RNA. That RNA is what usually causes X inactivation.  Jiang et al. (2013) were able to show that Xist can coat one of the chromosome 21 copies, performing essentially the same inactivation function that it does for the X chromosome. So far, though, this work has been entirely in the lab (in vitro).

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