conuly

My understanding is before Type 1 was treatable with insulin injections, people who developed it died in fairly short order. If you got it, it was a death sentence. Maybe there are milder forms, and if that is the case, it might be a believable hypothesis; cystic fibrosis and sickle cell anemia are "good" and confer benefit when an individual is heterozygous. Not when someone has the disease.

Well, okay, if you're homozygous for sickle cell (for example), you don't have to worry about malaria. But you've got other big problems instead. If having it was beneficial, we'd see a different distribution of alleles in the affected-by-malaria populations.