It depends on the campaign, really.
In core D&D, you may say that "human genes" are recessive (not sure if that's the real proper term in english, my genetic courses havn't been in english), while elf or orc ones are dominants.
Hence the "elf blood" and "orc blood" properties.
In 2e Realms, they said somewhere that
half-elf + human = human
half-elf + half-elf = half-elf
half-elf + elf = half-elf
But I find more logical to have
half-elf + human = human
half-elf + half-elf = half-elf
half-elf + elf = elf
YMMV. The simplest way to solve the problem would of course be:
elf + human = nothing, sterile union
orc + human = nothing, sterile union
But you would lose some classical archetypes of fantasy.