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Perhaps there is an obvious reason, and I know that most organisms that are either exclusively sexual or at least capable of sexual reproduction are diploid, but....

Is there a specific reason triploidy seems to be less common than tetraploidy?

It seems like a weird coincidence for a normally diploid organism to produce two rare gametes with extra chromosome(s) at the same time, or to do so at the same time a neighbor does so....

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Sexual reproduction means taking chromosomes from two parents. Most commonly, that means that each parent gives just half their chromosomes to their gametes, such that the offspring will have a full complement.

As such, at some point you have to multiply h*2, where h is the number of chromosomes that one parent donates, the "haploid", and 2 is the number of parents. It's possible that 'h' could be some odd number, like if each parent contributes 3 copies in the haploid case, but multiplying by 2 parents always gives you an even number in the offspring.

From there, it's straightforward to either get an extra or lose a chromosome somewhere, but improbable for it to happen for all chromosomes at once.

The Wikipedia page on ploidy is a good place to read about these sorts of things.

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