There's some recent thought that CF heterozygotes are more resisitant to
diseases that cause severe diarrhea and dehydration, since the molecular
defect interferes with water loss. During times of cholera epidemic, for
instance, heterozygotes would theoretically be les affected by the symptoms
and therefore have an increased likelihood of survival.
For HH, presumably the fact that this condition increases the amount of
iron stored in the system would protect those whose diets didn't contain
For all these "common" recessive disorders -- CF, HH, sickle-cell -- the
advantages that are confered to carriers are no longer needed in areas and
during times where malaria, cholera, and iron-poor diets aren't a problem.
Genetic diversity is important for the survival of species, since
catastrophic factors can wipe out whole populations except for those who
are resistant to the effects. Antibiotic resisitant bacteria are a good
Edward M. Kloza
ekloza at fbr.org