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Old 02-26-2012, 04:46 PM
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Lemur866 Lemur866 is offline
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Join Date: Jul 2000
Location: The Middle of Puget Sound
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We don't have two genders because of the structure of DNA. Each gender contributes a distinct set of chromosomes, one from each parent. This is because many species are diploid--each chromosome is paired. It's possible to have haploid species where chromosomes are not paired, or polyploid species where chromosomes aren't just doubled but are tripled or quadrupled and so on.

It is simply very common for species to reproduce asexually, so the requirement to have two sexes isn't a consequence of the genetic structure of life on Earth. And species that do reproduce sexually are often hermaphroditic, able to produce both eggs and sperm, although it is also very common for hermaphroditic species to have mechanisms to make self-fertillization difficult.

There just aren't any species on Earth that require three sexes to complete the mating process, even though there are plenty of species that have more genders than "male" and "female". But there aren't any species that require three gametes for reproduction, and it's pretty easy to see why.

When you engage in sexual reproduction you're already throwing away half your genes. That's a pretty high burden, and you only do that if there's a substantial advantage to sexual reproduction. But with a three gamete mating system you'd be throwing away 2/3rds of your genes. But to what advantage? You already get all the benefits of sexual reproduction in a two gamete system, what's the benefit of a more complicated system?