sickle-cell disease
Healthy red blood cells are soft, round discs that bend through the tiniest blood vessels like water balloons squeezing through a doorway. In sickle-cell disease a single change in the hemoglobin protein makes it clump into stiff rods when oxygen is low, bending the cells into hard crescent (sickle) shapes that snag and block small vessels. The result can be sudden episodes of pain, organ damage and anemia, because sickled cells are also fragile and die early.
The disease is autosomal recessive and traces to one specific point mutation in the beta-globin gene (HBB): an A-to-T change swaps a single amino acid (glutamate for valine) at position 6 of the protein. A person with two mutant copies has the disease; a person with one mutant and one normal copy has sickle-cell trait and is usually healthy.
Strikingly, carriers of one sickle allele are partly protected against severe malaria, which is why the allele reached high frequency in regions where malaria is common. This is a textbook example of balancing selection, where a heterozygote advantage keeps a harmful allele in the population. Modern care includes the drug hydroxyurea and, increasingly, curative approaches such as gene therapy; this entry is educational, not medical advice.
Under a microscope, a blood smear from a patient in crisis shows many crescent-shaped red cells among the normal round ones; sequencing of HBB reveals the single A-to-T change on both copies.
One point mutation reshapes the cell; the crescent shape gives the disease its name.
Sickle-cell disease is the classic illustration that a single base change can have profound effects, and that an allele can be both harmful (in homozygotes) and protective (in heterozygotes), depending on the environment.