To go full Muh World Building, in my campaign any Half-* races are long-established, usually attributed to a mythical event where a god created the unholy abominations. So when a human and an elf get down, any resulting children are Human or Elf, an almost never half elf. When two half elves boink, the resulting children are all half-elf.
If the RAW allows races to pick their stat bonuses, I almost always assign a "manditory" racial bonus, with the other option being more background based (Dwarves always have +1 CON, they can choose +1 in CON, STR, WIS, etc) if character is of 'mixed blood' I might expand the racial choices.