Abstract
Carries oxygen in red blood cells. A single base change here causes sickle-cell disease.
A single A→T substitution at the sixth codon (GAG → GTG) swaps glutamate for valine. That one change makes hemoglobin molecules clump and red cells crescent into rigid sickles — but it also confers protection against malaria, which is why the mutation persists in regions where malaria is endemic.