The Origin of Sickle Cell Trait

Tens of millions of people have a genetic mutation in a single allele (one of a pair of genes) that by itself has no outward effect on the person's health, called sickle cell trait. Unfortunately, persons who inherit two copies of the allele (one from each parent) suffers from sickle cell disease, a debilitating condition in which red blood cells cab change shape, or sickle, impeding blood flow and damaging organs.

Typically, a genetic mutation that leads to health problems would be selected against over time by normal evolutionary processes. However, that's not the case for sickle cell trait, because it turns out that inheriting just one copy of the abnormal gene is protective against another disease, malaria. As a result the abnormal gene is selected for, not against, by evolutionary processes, especially in places such as Africa where malaria is common.

But where and when did the mutation arise? The genetic mutation comes in five different haplotypes, or forms, leading some scientists to speculate that the mutation arose independently perhaps as many as five different times. Others have suggested that it arose just once.

In attempt to resolve the dispute, researchers recently studied the genomes of nearly 3,000 people. 156 of them had the mutated gene for sickle cell trait. On close examination of their genomes (the details are beyond the scope of this blog), the researchers concluded that all 156 people had inherited the mutated allele from a single person who lived around 7,300 years ago. That person probably lived in western or central Africa.  Case closed, it seems.