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When an X chromosome inactivates, these pseudoautosomal regions are spared. This means that, unlike most X-linked genes, those in the pseudoautosomal regions don’t get switched off. Consequently, normal cells potentially express two copies of these genes in all cells. The two copies are expressed either from the two X chromosomes in a normal female or from the X and the Y in a normal male. But in Turner’s syndrome, the affected female only has one X chromosome, so she expresses only one copy of the genes in the pseudoautosomal region, half as much as normal. In Trisomy X, on the other hand, there are three copies of the genes in the pseudoautosomal regions. As a result, the cells in an affected region will produce proteins from these genes at 50 per cent above the normal level.
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Nessa Carey (The Epigenetics Revolution: How Modern Biology is Rewriting our Understanding of Genetics, Disease and Inheritance)