Mom was human and daddy was a sorcerous blend of Orc and Elf...Doubly outcast...but strong and somehow...charismatic.
(Hey, if mad sorcerers can cross goats, dragons and lions....)
Of course, if you just use simple Mendelian genetics and draw the little box...
Have Parent 1 be Half-Elf & Parent 2 be Half-Orc, the 4 results for their offspring are Human (25%), Half-Elf (25%), Half-Orc (25%), and Orc-Elf (25%)
Half-Orc + Half-Elf parentage:
__H__ E
H HH HE
O HO OE
This post, of course, begs the further question...What the hell does an Orc-Elf look like (visually and mechanically)?
The relevant stuff:
Orc
+4 Str, -2 Int, -2 Wis, -2 Cha
spd30'
Darkvision 60'
Light Sensitivity
FC: Barbarian
Elf
+2Dex, -2Con
spd30'
Sleep Immunity
Low light vision
+2 Listen, Search, Spot
FC: Wizard
Possiblity:
Orc-Elf
+3Str, -2 Wis, -1 Cha or -1 Int
spd30
Darkvision and Low Light Vision as parent races
Light sensitivity, minor- bright light gives -1 modifier to rolls requiring vision for 1d2 rds.
Sleep Immunity
+1 Listen, Search, Spot
FC: Each (N)PC chooses a class to be its favored class
Has neither race's weapon familiarity, but can use weapons and equipment attuned to either race