Evolution is one of life’s most powerful tools. Fueled by natural selection, it allows life to improve over generations. It makes people wonder why certain hereditary diseases are so common. However, sometime we have to take a closer look.
One classic example is Sickle Cell Anemia, or sickle cell disease (SCD), which is a hereditary disease that causes blood cells to have an irregular, sickle-like shape instead of their normal round shape. This causes complications in carrying oxygen and a risk of blood cells getting stuck and clogging blood vessels. It is common in certain parts of Africa and Asia. It would seem odd that a deadly hereditary disease would be common, but there is a secret this diseases possesses.
SCD is recessive (having 2 recessive alleles). However, if a person is heterozygous for this disease (one recessive and one dominant allele), they are actually resistant to malaria. The parasite that causes malaria has a hard time getting into sickle-shaped cells. Heterozygous individuals have some of their cells sickle shaped, but not enough to exhibit symptoms. Therefore, it is advantageous for a person to be heterozygous for SCD. Apparently, the resistance to malaria is worth the risk of having SCD.
SCD isn’t the only disease that is resistant to malaria. Hemoglobin E disease, common in Southeast Asia, is similar to sickle cell anemia, except that its symptoms are much milder. For some people, it’s a benign disease, but its effect on malaria is still the same.
However, despite being a much better alternative, it isn’t very common in Africa. This is mostly due to the fact that, even if it’s imperfect, sickle cell anemia is already there (The Power of Random). Despite being a better alternative because of its milder symptoms, the niche of malaria-prevention has already been filled and prospered for many generations. It will take a lot of time for Hemoglobin E disease to be more common in Africa, if it ever becomes more common.