12.5: Evolution and Human Health - Balanced Polymorphisms
- Page ID
- 62366
Sometimes diseases end up being good for a population. In biology there are two views: the individual level and the population level. While a disease may be terrible for an individual, at the population level it may end up being evolutionarily beneficial. Sometimes two diseases will actually balance each other and give a person a selective advantage. This is called a balanced polymorphism. This is only beneficial in the heterozygous form (Etheridge-Criswell, 2018).
One of the most famous examples of this is malaria and sickle-cell. Sickle-cell is a point mutation in the hemoglobin (red blood cell) that occurs during protein synthesis and creates the wrong amino acid and therefore the wrong protein. The shape of the blood cell is sickled, like a crescent moon, instead of round. This keeps the blood cell from carrying enough oxygen to the body and also makes the blood cells difficult to move through veins and capillaries. The full (homozygous recessive) form of the disease can be fatal. However, in sub-Saharan Africa nature has selected for this mutation because it balances against malaria, the top killer in the continent. If someone is homozygous dominant and has completely normal hemoglobin, he or she is not protected against malaria; if homozygous recessive, he or she may die of anemia. But, in the heterozygous form, the person has enough oxygen and is immune to malaria infection (Etheridge-Criswell, 2018).
Other types of balanced polymorphisms include that Tay Sachs disease (fatal at infancy if homozygous recessive) protects against tuberculosis and Cystic Fibrosis (which causes excess mucus in the lungs) protects against cholera; both are only beneficial in the heterozygous form (Etheridge-Criswell, 2018).
One of the most interesting examples of how a mutation can be beneficial for a population into the future is seen in HIV. Around 1% of native Europeans is completely immune to HIV and about 10% is resistant. These people either have one or both copies of a mutation called CCR5-delta32, which is a mutation of 32 bases of the CCR5 gene. This mutation means that retroviruses like hepatitis and HIV cannot dock and enter receptor cells. What is fascinating is that research shows this mutation is thought to have originated around 700 years ago in Europe, right when the Bubonic Plague was rampant. It seems that those who naturally survived this plague had this mutation, which was selected for and passed down, and which saved the lives of their descendants centuries later. Remember that mutations can be harmful, neutral or beneficial. This is something to consider with the new availability of gene therapies. Sometimes having a mutation and/or disease can end up being a good thing (Etheridge-Criswell, 2018).