Essentially Humans and Metahumans (or two different races of Metahumans can’t interbreed) But Humans had Metahuman children via UGE back at the start of the 6th World and some people claim that there are halfbreeds that can be created in the lab, but it’s only a rumor and—even if true—certainly not something out in the public sphere
So one, ironically, can have human or other metahuman relatives, but one can’t have a child personally with another race
It’s something that geneticists can’t explain, since the DNA is very close and and the mechanical act of sex is much easier than between any other species, but the breeding just doesn’t work. Most people blame magic. It’s the de facto answer when science fails to explain something these days
That said, a lot of people have interracial partners. Humans and Elves are the most common pairing, seeing as both races find one another reasonably attractive (elves are, for whatever reason especially attractive to humans, less so vice versa). But in the ghettoes of Pullyap Barrens and Redmond, Humans and Orks occasionally form relationships. But many people see such people as race traitors
Being both monstrous in appearance to all other races and mentally deficient to boot, Trolls rarely form relationships with other races. Though in the rare instances when they do, it is usually with orks, since both feel a camraderie in their oppression (or at least Orks feel that way, Trolls are often unaware of being oppressed).
Dwarves are clannish and underpopulated and are the least likely to intermarry or have relationships with the outside world.