In females one of the two X chromosomes will condensate and turn into Barr bodies. So chromosome abnormality-free women will only have a single transcriptionally-active chromosome. Then why can abnormalities like Turner's syndrome still occur in the presence of a single active X chromosome?


Pseudoautosomal region in mammals

On sexual chromosomes, we have a so-called PseudoAutosomal Region (PAR). The PAR is the region which still recombine with the Y chromosome (in males only) and is though to be important to allow separation of chromosomes during meiosis.

Pseudoautosomal region and dosage compensation

Because the PAR is diploid in males too, there is no need for a mechanism of dosage compensation and hence, in females, the PAR of the inactivated X-chromosome remain active.

Turner Syndrome

Turner syndrome is caused by an issue of dosage of functional elements found in the PAR. In people having Turner syndrome the PAR is haploid, while it remains diploid in both healthy males and healthy females.

Here is a small review paragraph from Sybert and McCauley (2004) that report the evidences suggesting that Down syndrome is caused by haploidy in the PAR. Please see the linked paper directly for further references.

Loss of interstitial or terminal long-arm material of the X chromosome (Xq) can result in short stature and primary or secondary ovarian failure.15 Deletions distal to Xq21 appear to have no effect on stature. In general, loss of the short arm (Xp) results in the full phenotype. Very distal Xp deletions are compatible with, but do not ensure, normal ovarian function. Loss of this region usually confers short stature and the typical skeletal changes, in part as a result of haploinsufficiency of the short stature–homeobox (SHOX) gene, located in the pseudoautosomal region of Y and Xp. The SHOX gene is probably not the only gene responsible for the skeletal features. Aneuploidy itself may contribute to growth failure. Loss of a region at Xp22.3 appears to be associated with the neurocognitive problems in Turner's syndrome. Loss of the testis-determining factor (SRY) gene locus on the short arm of the Y chromosome (e.g., 46,X,del(Yp)) also leads to the phenotype of Turner's syndrome, even without a 45,X cell population. A region on Xp11.4 has been proposed as critical for the development of lymphedema.

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    $\begingroup$ In addition, 35% of the genes in the inactivated X chrosomes are "escapees", which means that they remain active. The rest is mostly repressed by the expression of gene Xist. $\endgroup$ – user38945 Nov 30 '18 at 17:02
  • $\begingroup$ @Jagoe I did not know that +1 Do you happen to know what fraction of these 35% are from the PAR? If you happen to have a reference, I would happily add that to the answer. Thanks $\endgroup$ – Remi.b Dec 1 '18 at 21:28
  • $\begingroup$ I've found a reference that tells of a similar figure (12-20% for humans, 3-7% for mice) for genes escaping "XCI" (X-chromosome inactivation) [cell.com/trends/genetics/pdf/S0168-9525(16)30002-6.pdf]. So yeah, the original percentage I wrote might not be accurate. I heard it from a genetics professor at college. Regarding PAR, well, I just heard it existed. $\endgroup$ – user38945 Dec 2 '18 at 16:55
  • $\begingroup$ @Jagoe Ok. Note that your link is broken $\endgroup$ – Remi.b Dec 2 '18 at 18:16
  • $\begingroup$ cell.com/trends/genetics/pdf/S0168-9525(16)30002-6.pdf $\endgroup$ – user38945 Dec 2 '18 at 18:20

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