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Prae_ t1_ix2ninh wrote

Reply to comment by MagicSquare8-9 in Is it possible to have 4 copies of a single chromosome? by Temporary_turbulance

Excellent question. That's because the inactivation is not complete, and up to 25% of the genes on the inactivated X can be expressed.

Some of these are in the pseudoautosomal regions (PAR). Autosomal is the big word for "not sex chromosome", the PAR are regions on the X and Y which behave like autosomes and not sex chromosome, because they are shared between X and Y.

And also in the rest of X sometimes genes can get reactivated because no rules can ever be simple in biology.

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