Genma's maternal grandparents were elves, a light elf and a dark elf to be precise.  His father is Yosho, and he was cast out of the clan for taking more after the darker nature of the dark elf. Genma's sister Achika took more after the light elf side.
Nodoka's mother was a dragon, and her father was a saiyajin.
Ryoga's mother was an Angel (literally), and his father was a lower demon, no wonder he gets lost and confused so easily, his whole being is confused as to whether it should follow his darker nature or his lighter nature.
What is a half-angel, half-demon to do? For that matter, what is a 1/4 dragon, 1/4 saiyajin, 1/8 Jurian, 1/8 human, 1/8 light elf, 1/8 dark elf supposed to do? (At least most of Ranma's ancestry is of a benevolent kind)
Remember that Yosho's mother is human, so Genma would be 1/4 light elf, 1/4 dark elf, 1/4 Jurian, and 1/4 human.

Of course none of them know their ancestry untill after the story starts, not even Ranma's parents, and Ryoga hadn't seen his parents in a long time and they had yet to tell him about his heritage.