Well, as Rose and MijRai both said. MijRai gave the canonical baseline probabilities quite cleanly.
Anyone got an Example of it happening?
I thought it was similar to how Rosa mentioned & they could breed through the Human "part" of them but you couldn't have 2 meta's give birth to a 3rd meta.
There is a good example in Street Legends: Home Edition of 3 brothers of 3 different metatypes. I recall a dwarf, a human, and an ork. That's probably the most blatantly obvious example in canon.
And anyone that tells me that's not canon is a blatant liar!
It is hard to convey tone in writing, so I am responding to this under the assumption you are being completely serious. If you were joking, then you can pretty much ignore this...
Street Legends: Home Edition was an April Fools product. As is often the case with Catalyst's April Fools day Shadowrun products, I would say they are probably Half and Half realistic. Usually the general premise is "joke" and the fleshed out rules are the "but what if they really did that." It falls in the same vein as Rigger 4 (with the prominent Amish War Buggy, and Battletech-style mechs), Friendship is Tragic (the adventure where the runner team is hired to fend off said Amish from killer Centaurs to the them of My Little Pony), and this years UnCONventional Warfare (which takes place at a gaming convention).
So if none of that seems at all odd to you, then yes, the three brothers of different metatypes is completely canon to the setting no "joke" implied...
In all seriousness, though, I have a question that might clarify that particular part of the discussion regarding metahuman genetics. I can't seem to find my file for Street Legends: Home Edition, and I don't quite feel like downloading it again right this minute. Are those three brothers explicitly written as being born from the same parents? I could easily see them all being half-brothers and that wouldn't break any genetic "rules."