[Astromancer]

One thing I would expect if people had only a one in four chance of breeding true would be stressed families. Assume the possibilities are Human, Elf, Dwarf, Halfling, Gnome, Troll, or Ork. And that these beings fit typical RPG treatments of these beings. A Gnome or Halfling couple raising a Troll child, that would tower above them and have a tendency toward a short temper and attention span, might have to neglect their other children, if they can have the time and energy to have more children, to care for the Troll.

Picture an Elf couple with an Elf child and a Human child. Not only is the Human kid likely to feel neglected, but he'll die of old age before his parents even seem to age. On the Elf side, the family will be aware of the emotional wounds of their Human child. Feel the misery of never fully conecting with their child slash sibling. Then the child will die in what for an Elf would be early adolescence.