The genes for sickle cell anemia and thalassemia are common among Africans and people of African descent because inheriting a single gene reduces the risk of getting malaria significantly. Carriers with two sickle cell genes have a debilitating and eventually fatal blood disease, while thalassemia is much less serious.
Dr. Tom Williams of the Kenya Medical Research Institute told the BBC that researchers began looking at the genes assuming that an individual carrying both would be doubly protected.
"But it turns out that when you start combining the two, you can lose the effect of both," Williams said. "Our study shows that it can be very complicated to turn up genetic associations and properly understand them."