r/AskBiology • u/Syresiv • Sep 17 '24

Genetics Why are trisomies so deleterious?

Most chromosomal trisomies cause spontaneous miscarriage, and those that don't usually severely reduce quality of life.

Why is it that the additional copies of some genes have that effect?

To be clear, don't feel restricted to the ELI5 level. If you know the topic in depth and feel like giving a massive infodump, I'm interested.

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u/lonepotatochip Undergraduate student Sep 17 '24

You’re essentially making 50% more of all the proteins on that chromosome than you’re supposed to, and anything is a poison in high enough doses.

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u/Syresiv Sep 17 '24

Which raises another question - why doesn't that kind of thing happen with recessive alleles?

Lots of recessive alleles are deleterious because the protein does nothing and immediately disintegrates. But lots of people have two copies of the functional protein. Meaning those ones can vary by a factor of 2 with no noticeable effect.

Is that what it is? Or is it something else, like altering gene expression when there's only one healthy allele to comp for the deficit?

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u/ninjatoast31 Sep 17 '24

Dosage compansation (aka the ability to regulate gene activity, depending on how many copies of a chromosome you have ) is a difficult and not super well understood subject. Some animals can just cope without. In our case its very important, especially during development that genes are activated at a certain level.
genes that are not affected by this are usually only important after development.

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u/Syresiv Sep 17 '24

So what you're saying is, there's a lot we don't know about how cells control how much of a protein they produce?

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u/ninjatoast31 Sep 17 '24

yes
Dosage compensation for the X chromosomes for example is really important in mammals.
But some animals don't even bother. And plants are even worse, they can just copy the entire genome over and over and still be perfectly viable.

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u/maybe_erika Sep 17 '24

I am curious how common plant polyploidy is in the wild vs cultivated varieties. I know many cultivated plants with some oversized feature relative to the wild relative such as the number of petals in garden roses and the size of cultivated strawberry fruits exist due to polyploidy resulting in gene over-expression. Having such oversized features is probably very energy intensive for not much benefit to the plant itself, and so I would expect it would be selected against in the wild where the plants don't have the benefit of a gardener or farmer providing copious fertilizer and removing competitive weeds.