> Probably only beneficial if you have one copy, not two. One copy of the gene partially protects you from malaria; two copies of the gene make you very sick.

> Probably only beneficial if you have one copy, not two. One copy of the gene partially protects you from malaria; two copies of the gene make you very sick. That's why even in Africa only a small portion of the population has the gene. The gene is an evolutionary advantage as long as the prevalence in the population is low. As soon as the frequency of the gene in the population goes up, it becomes a disadvantage because if both you and your partner have the gene your kid has a 25% chance of getting two copies.