Shadowrun seems to view its PC races as burgeoning "ring species." For the most part they can all interbreed. I would assume there are some that CAN interbreed genetically speaking but it's extremely rare to do so naturally due to some physical reason, like a Dwarf and a Troll. There's no "gene mixing" with each set of sequences being exclusive in expression and no one in particular apparently being a dominant set (they express in a dominant way, but the Troll sequence isn't dominant over the Human sequence unless the trate that controls dominant expression doesn't have anything to do with the racial sequence), meaning no "half-whatevers" in existence.
Trolls and Orcs can "express" (via goblinization) or be naturally born as a Troll or an Orc. Elves and Dwarves are "born that way" and have never been said to "goblinize" later in life into an Elf or a Dwarf. That tends to suppor the "early stages of a ring species" idea in that there's three "distinct" branches of humanity, Full Humans, Natural Born Variants and Converted Variants all requiring distinctly different generic expressions at different points in their lifetimes.
As the previous post said most D&D races can be simulated with the metavariants and SURGE effects if they're not already "in the book" as desired. Outside of those, you can tinker around with mixing and mathcing existing variants and SURGE qualities and get just baout anything else.
