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AlphaBetaGammaDonut t1_ix0ibyp wrote

Yes, but only the sex chromosomes - X or Y.
In other chromosomes, the genes effectively 'compete' for which one is encoded - that's where dominant vs recessive characteristics come from (eg your earlobes - in some people, the 'head side' is fully attached, but in most, it curves away and the lobe is a little dangly. 'Attached' gene is recessive to 'dangly').

The X chromosome is different. If it senses another X chromosome, one entire chromosome will shut down. Tortoiseshell cats are the go-to example here. Orange and black fur are X chromosome linked, so they have patches of orange or black fur depending on which chromosome has shut off. Extremely rarely, we see a male torti, but that's because they have XXY. I can't remember the exact deal of the Y chromosome, but I think it's that its genes 'enhance' existing programming rather than directly affect, so it doesn't matter if there's extra. Men who are XYY tend to be taller.

That said, most people who have trisomy X or Y only learn they do when they get help for reproduction issues, as it only really affects fertility. But past that, especially if it's extra X chromosomes, there tends to be developmental disorders.

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Prae_ t1_ix0phn0 wrote

> the genes effectively 'compete' for which one is encoded

Either you have in mind something very specific, or this is generally false.

Monoallelic expression is more of the exception than the norm, and is generally not responsible for dominance of traits. It happens for the ~200 imprinted genes, the X-linked genes (because of the X-chromosome inactivation you describe), and as far as I'm aware a handful of other genes.

I think you're missing the key point of dosage compensation. Having 4 copies of a chromosome means 4 times (a priori) the amount of RNA/protein, most of the time bringing you into toxic territory. X is an exception as you say because X-chromosome inactivation ensures there's always only one X getting expressed.

I'd chalk up the survivability of Y duplications more to the fact that it has so few genes than the fact that those genes are transcription factors (i.e. genes that go activate other genes elsewhere).

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MagicSquare8-9 t1_ix2bn89 wrote

Just wondering, if only 1 X is ever activated, why does 48, XXXX individuals suffer from many serious health conditions?

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Prae_ t1_ix2ninh wrote

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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