Editing Half-Elf (section)

===Infertility===
As mentioned earlier, two species can produce a child if they're in the same genus, but the child will themselves be infertile. The reason for this is a bit complex and has to do with the way genes are swapped around during the production of gametes. So you know how all your chromosomes come in pairs, one set from mom and one from dad? Well during the production of a gamete, each chromosome shuffles genes with it's partner at random (with the exception of the XY pair in men, for reasons we'll get to in a bit), which is the reason a married couple doesn't pop out the same baby every time the condom breaks. 

Now, if two organisms are genetically distinct enough to be different species, then their DNA has done enough mutation that a chromosome in one wouldn't be able to line up properly with the corresponding one from the other. So when their kid starts making gametes of their own, gene A gets swapped with a gene B instead of another gene A, resulting in the genetic equivalent of unreadably corrupted data (in the case of parents from different genre, the other chromosome doesn't even ''have'' gene A).