What is the name of the hereditary blood disease that occurs chiefly among black people?

This genetic disorder affects hemoglobin, the protein within red blood cells responsible for carrying oxygen throughout the body. A specific gene mutation causes these normally round, flexible cells to become rigid and C-shaped, resembling a farm tool known as a sickle. These misshapen cells can get stuck in small blood vessels, blocking blood flow and leading to episodes of intense pain, organ damage, and a chronic shortage of healthy red blood cells, a condition called anemia.

The high prevalence of this disease in people with ancestry from sub-Saharan Africa, the Mediterranean, and parts of the Middle East is a fascinating example of natural selection. Individuals who inherit only one copy of the sickle cell gene do not have the disease but possess what is called the "sickle cell trait." This trait provides significant protection against severe malaria, a deadly parasite (Review) common in those regions. Over centuries, people with the trait were more likely to survive malaria outbreaks and pass the gene to their children.

While carrying one copy of the gene offered a powerful survival advantage, inheriting two copies—one from each parent—results in the full-blown disease. This evolutionary trade-off explains why the gene, and consequently the disease, remains most common in populations whose ancestors originated in malaria-endemic parts of the world.