In some parts of Africa, where malaria commonly occurs, there are higher frequencies of the allele that causes sickle-cell anaemia.

Individuals with the recessive sickle-cell anaemia allele have a lower chance of developing severe malaria. Therefore, they are more likely to survive and reproduce than those without the allele. The allele is then more likely to be passed down to offspring, which increases its frequency in the next generation. As individuals who do not have the sickle-cell allele (in other words, those who have two of the normal, dominant haemoglobin alleles) are more likely to get severe malaria and not survive and reproduce, the frequency of the normal haemoglobin allele also decreases in the population.