Bonus Science Fact of the Week
(Because I am procrastinating on studying for my Ph.D...)
Humans have 46 chromosomes, or 23 pairs. Two of those are sex chromosomes (x and y).
Sooooo..... I would like to point out that the reason why humans can create kids is because one parent provides 23 chromosomes, and the other parent provides the other 23 (all packages up nicely in these cells called “gametes”). When you have a human mate with something like an alien, you’re not going to get a viable offspring.
(I mean, that’s even if they parents can mechanically get together! Most likely, with the speciation due to evolving on seperate planets for who-knows-how-long, a human and alien will not have “compatible parts,” so to speak.)
Back to my non-viable kid comment earlier though: what do I mean by that? Well, there are two options if the egg can be fertilized:
1) The egg is fertilized but does not form a viable fetus for a variety of different reasons (or doesn’t even start growing after fertilization).
2) The fertilized egg does make a living fetus (yay!) that can grow, but the child is now sterile.
Number 2 is the more interesting option here, and we need to know why it happens. To be fertile, one must be able to make gametes (the sperm or the egg/oocyte). You can only make gametes through a process called meiosis, and that doesn’t work for you if you don’t have the same chromosome count from each parent.
This is actually the problem with the mule, which is a cross between a donkey and a horse. The mule can grow, but all mule are sterile (they can’t even mate between other miles). That’s because the horse and the donkey have two different chromosome counts. To form your gametes during meiosis, there’s a step called Metaphase I where your chromosomes in pairs line up. Buuuuuuuut, if you got half of your genes from mom (23 human chromosomes) and the other from dad (28 alien chromosomes), the chromosomes don’t line up correctly, causing meiosis to come to a halt.
So, this puts Spock from Star Trek in a new light: maybe he wasn’t shunned just because he was half human, maybe it also had something to do with him being sterile. I mean, that’s an interesting take on a character!
In summary: the likelyhood of naturally occuring human-alien hybrid kids are extremely unlikely, BUT if it does happen, consider making that character sterile. See how they handle it, and how society treats them for it.