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suvlub t1_j50la00 wrote

Reply to comment by Claycrusher1 in Given that reproduction is difficult or impossible when both animals have different numbers of chromosomes, how did so many species evolve to have so many different numbers of them? by MercurioLeCher

A sex cell has 1 copy of each chromosome. They are created by meiosis, i.e. a classical cell with 2 copies of each splits into 2 sex cells. During this process, each chromosome finds its buddy, so they split nicely and you end up with 1 copy of each, not random half. That would be bad.

In the person with 45 chromosomes (assuming this specific kind of mutation where 1 chromosome is fusion of 2), the combined chromosome pairs up with random one of the smaller ones, and the other is left without buddy. That's bad. If you are lucky, it ends up in the same cell as the other small chromosome. If you are not, it ends up in the other cell.

The article I linked in the first comment has nice pictures illustrating this.

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yerfukkinbaws t1_j513zuo wrote

The two unfused chromosomes from the parent with 46 will usually both pair with the fused chromosome that came from the parent with 44. This is called a trivalent, instead of the usual bivalent that forms in meiosis. The pairing actually usually goes just fine since the genetic content is not changed and these chhromosome fusions usually involve chromosomes that only had one arm before (acrosomes). What this means is that it's not as random as all that. There is still a chance that separating the chromosomes can go wrong, but the offspring of people who've had a fusion of this type are usually not infertile, just reduced fertility sometimes. Often not even by much and many, many cases are believed to be undiagnosed since there's no "symptoms."

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