Turning now to heterozygous advantage, the classic example—cliché almost—is sickle-cell anaemia in humans. The sickling gene is bad, in that individuals with two copies of it (homozygotes) have damaged blood corpuscles that look like sickles, and suffer from debilitating anaemia. But it is good in that individuals with only one copy (heterozygotes) are protected against malaria. In areas where malaria is a problem, the good outweighs the bad, and the sickling gene tends to spread through the population, in spite of the adverse effects on individuals unlucky enough to be homozygotes.
