There is a small wormy parasite that attacks our red blood cells (RBCs).

It turns out that people with mutant and half-mutant RBCs are less likely to be attacked by the parasite.

Also, the red blood cells of the mutant variety are more likely to be cleared by white blood cells than those that are non-mutants.

Full-blown (homozygous) mutant RBCs kinda suck at their day jobs though. Whereas half-mutant (heterzygous) RBCs are still mostly functional by comparison.

So being a little weird, but not totally weird, gives an advantage over normies (wild-type) when the RBC parasite is common.

Were there no parasite around, the advantage would go to the non-mutant RBCs because they do their job best. Their downside is being easy targets.