Catfish and Cod
Sunday, August 03, 2003
Hemophilia and sickle-cell disease.
(Link path: the Beeb, Lancet 632(9381):351-354)
Hemophilia is a terrible disease. Those who suffer from it must watch their actions very carefully, every day of their life, lest a scratch or a bruise become a major injury. Before the advent of synthetic blood clotting factors, it was close to a death sentence for everyone who couldn't afford to be watched every day of their life, i.e., everyone except descendents of Queen Victoria.
Sickle-cell disease is similarly debilitating. When sickle-cell crisis hits, the patient is helpless. Sickle-cell anemia is caused by a mutation in the beta hemoglobin gene that allows the hemoglobin molecule to polymerize within the red blood cell, deforming the cell and reducing its oxygen-porting efficiency. Even at sea level, a sickle-cell patient has significant anemia and hypoxia -- low red cell counts (because the sickled cells don't fit into capillaries and burst) and low blood oxygen levels (because of the lack of cells plus the loss of functional hemoglobin).
So if sickle-cell disease is so debilitating, why hasn't evolution rid us of the trait long ago? Why is it a common genetic disease, instead of a freak occurence that only comes up in Case Studies of the Massachussetts General Hospital?
Sickle-cell disease only occurs when an unfortunate individual bears two copies of the debilitating gene. If the patient bears only one copy, they are said to bear sickle-cell trait. And these individuals are not only healthy, they have an advantage over the average human. Their abnormal hemoglobin gives their red cells the ability to resist many times of malaria infection. This is not an insignificant trait, especially in the jungles of Africa where humanity evolved.
So while sickle-cell disease is a negative evolutionary factor, the far larger number of sickle trait bearers have such a positive advantage that sickle trait persists in the human population.
Researchers report in this week's Lancet that hemophilia trait has a similar evolutionary advantage. Hemophilia carriers have a lower chance of heart disease than the average population.
Are the hemophiliacs numerous for the same reason that sickle-cell disease sufferers are numerous? There's no data to suggest it yet. Maybe someone should go gather some.