How is this possible genetically? The only way I can see this working is that if there is some sort of dominant Uma gene in the X chromosome, but it is extremely rare, and also there is some gene in the Y chromosome that somehow overrides it. I don't know enough about Uma lore to know if they are rarer than humans or not.
This would also mean that humans and Umas are the same species since they can produce fertile offspring. And since the Uma gene is just part of the overall human genome, they aren't even a distinct subspecies or race.
It also implies that the Uma gene is a recent adaptation, since it is uncommon but must necessarily be dominant. And in several thousand years most human females will be Umas.