I don't think we know enough in concrete terms to start speculating on Pony genetics. All that could just be a result of the (very) local population in what appears to be a rural area. But it's an interesting question, no doubt, albeit not one I can see somehow divining an answer for unless Lauren Faust and her crew plan on showing us a mated pair of two different races along with their child.
Remember that the only real sample we have to judge is restricted to Ponyville. It doesn't at all have to be indicative of the rest of Equestrian society. The most we can say is that based on what inferences we can make and what we can derive from the evidence, things seem to work like they do ONLY here in Ponyville. It's about the size of a small town by the looks of it, it's not hard to think that things would work differently from say a big city like Canterlot in terms ranging from sociology to economics to demographics.
Come to think of it, Ponyville is an Earth pony town is it not? I think this is mentioned in Winter Wrap-up where the whole ritual to bring the Spring in is historically an exclusively Earth pony event. It's part of why Twilight didn't want to use her Magic to help fix things up quicker. This all makes sense considering the majority of the population appear to be Earth ponies and if the town was founded originally by Earth pony settlers, because they would not have had Magic of their own.

Also, isn't Celestia the only Unicorn/Pegasus?


Also again, the Cutie marks interest me immensely but for quite a different reason than their genetics.



Also again again (goddamnit) I too think that Fluttershy, Twilight, Dash do government work. Fluttershy seems to be in the position of a groundskeeper and animal behaviorist, working to keep the flora, fauna and general ecology in hand. She was in charge of organizing the animals in Winter Wrap-up and that was an official town events with (dis)organized teams that had their own colored uniforms and armbands and shit.
Rainbow Dash, when she's not lazing about, looks to do aerial work clearing away the clouds. I'd also imagine she'd be good at running messages and other errands but as far as I know that's speculation.
Twilight is an interesting case. At the beginning of the series we see she works in the capitol, likely a book-keeper or what passes in Equestria for a lower-level bureaucrat. Her obsessiveness with rules and procedure bear this theory out, but she's seen as willing to get her hands dirty so I suppose she's quite an effective bureaucrat. The Equestrian government seems quite efficient if Twilight is anything to judge by. She also clearly has experience organizing ponies as seen at the end of Winter Wrap-up and is a natural leader and a mediator. Perhaps she was a diplomatic functionary. an envoy? She's also a rationalist and a skeptic, qualities that make sense for someone coming from the big city in to some podunk town in the middle of the Equestrian countryside (that has it's share of superstitions and xenophobia, Zacora etc). Perhaps a scientist then? A scholar? Or would that be just a hobby?
Yeah, you can tell I've got a shit-ton on this show. I think it's a testament to its quality that it can keep me interested and get me to write so much on it.
edited 17th Mar '11 4:29:38 AM by Gault