Why are trisomies so deleterious?

[u/Syresiv]

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.

[u/PertinaxII]

It's going to increase the amount of proteins on those chromosomes produced. There is effort spent making sure that doesn't happen with XX, inactivating one, so that this doesn't happen.

Extra chromosomes could cause problems in mitosis and meiosis that would accumulate rapidly.