Genes

People with type O blood gene on chromosome 9 are much more susceptible to cholera (people with AB genotype are the most resistant), but seem to be slightly more resistant to malaria and slightly less likely to get various cancers. “This enhanced survival was probably enough to keep the O version of the gene from disappearing, despite its association with susceptibility to cholera” (Ridley, 141). Finally, native Americas predominantly had O version of the gene that made them maximally susceptible to cholera; but cholera was rare in Africa, and this same gene rendered them less susceptible to syphilis (144). In 1940s, Anthony Allison discovered that people with the sickle-cell mutation in Kenya (one copy of the gene, not two) were largely resistant to malaria (141).