Introduction
An individual’s characteristics are determined by the copies of their parents’ genes. Typically, one copy is inherited from the mother and one from the father. When the genes from both parents are the same, the individual is said to be "homozygous"; when the genes from the parents are different, the individual is "heterozygous". Sometimes, a homozygous pair of genes can cause a genetic disorder that may lead to decreased quality of life or even death. If only one copy of the diseased gene is present, the heterozygous individual may either have a less severe form of the disorder or not have it at all, but they can still pass the diseased gene to their offspring. Heterozygous individuals for genetic disorders are called carriers. Interestingly, it has been found that carriers for certain genetic disorders have a higher reproductive fitness, meaning they are more likely to produce offspring.
Heterozygote Advantage
It has also been found that genetic carriers for certain disorders have increased immunity to specific infectious diseases compared to those who are completely unaffected (homozygous individuals without a copy of the diseased allele). Regions with a high incidence of infectious disease also tend to have a high incidence of genetic disease. While homozygous individuals for the genetic disorder have low fitness and homozygous unaffected individuals are more susceptible to infectious disease, heterozygotes are both unaffected by the genetic disorder and less susceptible to infectious disease.
Malaria and Sickle Cell Anemia
A classic example of heterozygote advantage is the higher fitness carriers of sickle cell anemia have over unaffected individuals in regions particularly susceptible to malaria. Heterozygotes possess one normal copy of the hemoglobin gene (HbA) and one diseased copy of the hemoglobin gene (HbS), which alters the shape of red blood cells from their usual disc shape to a crescent or sickle shape. These sickled cells cannot sustain Plasmodium falciparum, the parasite that causes malaria; the parasite cannot replicate, and it is easier for the immune system to fight against it. This is evidenced by the high incidence of both malaria and sickle cell anemia in Sub-Saharan Africa and Southeast Asia. Heterozygotes of sickle cell anemia are therefore less susceptible to malaria than those without the HbS gene and remain unencumbered by the severe symptoms of sickle cell anemia, resulting in higher fitness and increased quality of life.
Other Examples
Similar to the heterozygote advantage of sickle cell anemia carriers against malaria, it is suggested that carriers for other genetic disorders may also have a heterozygote advantage against other infectious diseases, though more research is required before definitive conclusions can be drawn. Examples include the presumed protection carriers of cystic fibrosis have against cholera, typhoid fever, and tuberculosis; and the protection carriers of hemochromatosis have against certain infectious pathogens, although it is not yet clear which specific diseases. With more research, it will become easier to understand why certain genes that code for harmful disorders are passed on, and to emphasize the importance of genetic testing while battling the stigma against being a carrier for a genetic disease.
Works Cited
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