Trisomy Abnormalities. Why?

[xvrickh]

I can understand why monosomy is harmful, but why trisomy? If we have two copies of a chromosome, we are fine. Why would adding one more chromosome mess our body up? My logic is telling me everything should still be fine because we have a set of genes to work from. If one gene is recessive and harmful, then there is a potential to have 2 genes that will mask it instead of one. I've looked through multiple biology books and online and have not found anything. If anyone does know the mechanism, could you please include a source with your answer? Thanks!

[kolean]

Everything is balanced in the body when dealing with genetics. You do know that a female can have only one X chromosome that is actively expressing, while the other one is heterochromatized into a Barr body, because it would be disasterous for two X chromosomes to be expressing. (http://en.wikipedia.org/wiki/Dosage_compensation)

You can have too much of a product from a gene, and during the expression of developmental genes, this can cause alot of havoc. Trisonomy 21, down syndrome, has facial and other abnormalities associated with genes being turned on too long and too much. This one is a viable trisonomy for humans. (http://en.wikipedia.org/wiki/Down_syndrome)

Now your comment of a gene being recessive and harmful (have to look at why it is harmful) and you have 2 genes that will mask it (the simplicity of one gene cancelling the other gene out does not work most of the time. you have to look at what the gene product does for the organism) is valid, but it is only a gene. You were talking about chromosomes that contain thousands to one hundred thousand genes (that could contain not only the protein product encoding genes, but the regulatory ncRNAs, other structural chromatin modeling regions, and other genetic structures that we haven't uncovered yet). You have to take that into account.

[JackBean]

IMHO, the question is not about metabolic enzymes, but about regulatory genes (e.g. transcription factors or all kinds of small RNAs), which, when missexpressed, can really mix things up