There's no reason that medically relevant phenotypes would be that much less complicated (and lets remember that mental health, is, in fact, medically relevant.

There's no reason that medically relevant phenotypes would be that much less complicated (and lets remember that mental health, is, in fact, medically relevant.) Yes, there are a select few, rare, single nucleotide polymorphisms that are responsible for congenital genetic defects. But plenty of disease involves the interactions of thousands of genes, like cancer, and for those it's really just as meaningless.