When a baby is first born, one of the first comments out of people’s mouths tends to involve appearance. Maybe the baby has her mother’s eyes, or her father’s hair. Parents often want the best genes for their child, with no genetic ailments or illness. In reality, genetics is a bit more complicated than receiving different traits from each parent. So, are the genes that have been determined to be the “best” always what’s “best”.
People have often heard of “survival of the fittest,” thinking of those that are the fastest or the strongest. However, the “fittest” are actually the individuals with the most reproductive success. A trait that may be harmful in one area may be beneficial in another, like snakes being green in a forest but yellow in a desert for better camouflage. This trend is also seen in a genetics concept known as overdominance. Before discussing this further, let’s rewind and discuss some of the history of genetics.
One aspect of genetics involves dominant and recessive traits. One famous scientist, Gregor Mendel, first experimented with dominant and recessive traits with pea plants in the 1800s. One of the traits he examined was flower color, and he was able to examine patterns involving the flower color. Below is an image of Gregor Mendel himself. If you’re interested in more of Mendel’s story, click here!
Gregor Johann Mendel
Source: National Library of Medicine
When talking about genetics, the terms homozygous and heterozygous are often used to describe the alleles an individual has. An allele is an alternative form of a gene. When an individual is homozygous for a trait, this means the individual has two of the same alleles for a gene. When an individual is heterozygous for a trait, this means that they have two different alleles for the same gene. Different alleles can cause different phenotypes, changing the appearance of the individual.
Mendel looked at the different alleles in pea plant flowers, he noticed that most of the flowers were purple. He determined that this was the most prominent phenotype, or appearance of the flower. The other less common flower color phenotype was white. He stated that this gene for the purple flower color was dominant. When an allele is dominant, it determines the phenotype of heterozygous individuals. Therefore, he determined that the allele that causes the purple flower color is dominant to the allele that causes white flower color. Below is an image of a white flowering pea plant. Because this flower is white, that means it is recessive for flower color.
A Flowering Pea Plant
Source: np-e07/iStock/Getty Images
It’s no surprise that any trait that followed this pattern has been known to be called Mendelian Genetics. However, not all patterns of inheritance follow Mendelian inheritance, and the way individuals get all of their traits can be a little bit more complicated. This is where overdominance comes into play. It unfortunately does not follow the typical Mendelian inheritance patterns. Overdominance occurs when the heterozygous individual is more reproductively successful than either homozygous individual. If you are interested in other genetic dominance mechanisms other than overdominance, click here!
It can be a common misconception that an individual with two dominant genes should always be more “fit” than an individual that has one recessive allele, or an individual that is homozygous recessive. One example of overdominance in humans is a disease known as sickle cell anemia. This is a disease that causes the red blood cells to be sickle shaped, ultimately allowing less oxygen transportation to the body’s muscles and organs, which can be deadly. The images below show what a normal red blood cell looks like compared to a red blood cell that is sickle shaped. For more on sickle cell anemia, click here!
Images of a Normal Red Blood Cell (Left) and a Sickled Red Blood Cell (Right)
Source: Mary Martin/ Science Source
So this then raises the question: Why would anyone want sickled cells if it could kill you? That surely doesn’t sound like reproductive success. As mentioned previously, certain traits can be beneficial in different environments. With sickle cell anemia, the trait for sickled cells can actually be beneficial in areas where malaria is also common. Malaria is often caused when humans are bitten by a mosquito that is infected with malaria, and the mosquito passes it to the affected human, and it can be fatal.
It may be difficult to see how malaria and sickle cell anemia relate to each other, but it is important to note that plasmodium that causes malaria reproduces in the bloodstream, specifically using human red blood cells. Red blood cells that are sickled are not able to be used by the malaria plasmodium, so the malaria cannot affect sickled cells. While being homozygous recessive for sickle cell anemia can kill someone, being homozygous dominant with no sickle cells can put the individual at risk for contracting malaria. When an individual is heterozygous, and they can make sickle cells and normal cells, they can still get enough oxygen flow necessary to live, but also do not have enough normal red blood cells to support the malaria plasmodium, ultimately making them immune to malaria while not being fully affected by sickle cell anemia.
Being heterozygous for a trait can also have some other advantages, such as allowing for a variation in function. When an individual has two different alleles, they can sometimes make two different enzymes. These different enzymes can allow an individual to survive in more conditions than those that can only make one type of enzyme. For example, if one allele codes for activation in lower temperature ranges and another allele codes for activation at higher temperature ranges, that individual can survive at a wider range of temperatures, compared to their homozygous counterparts. The infographic below displays a nice, cohesive summary of overdominance.
Overdominance Infographic
Designer: Jerica Walters
Other human diseases are regulated by overdominance mechanisms such as cystic fibrosis. Cystic fibrosis often causes many undesirable effects such as coughing or shortness of breath. However, some benefits to having cystic fibrosis include higher bone density, heart health, and greater blood sugar control. So, it may have been beneficial for some individuals to have cystic fibrosis in order to benefit from these positives of cystic fibrosis, even if it could make some aspects of life a little bit more difficult. For more examples of overdominance in other organisms, click here!
Tay-Sachs disease is another human disease that is believed to be controlled by an overdominance mechanism. Tay-Sachs causes neurological issues and currently has no cure. It can often show expression when the affected individual is an infant, but in some other cases, people can be unaffected until their early twenties. Studies have shown that individuals that carry the Tay-Sachs gene but are not affected phenotypically, may be at a lower risk of contracting tuberculosis. While having two copies of the Tay-Sachs gene could kill the affected individual, having only one copy could help prevent other deadly diseases. If you are interested in reading the full study conducted with Tay-Sachs, click here!
Overall, overdominance is one of many genetic trends that does not follow typical inheritance patterns. This pattern of inheritance helps to showcase how the success of an individual not only relies on their genes, but also the location in where the individual lives. While being completely dominant for a beneficial trait typically seems to be the best option for reproductive success, overdominance helps to showcase that maybe being recessive isn’t so bad after all.
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Sources:
Brooker, R. J. (2022). Chapter 3/ Key Terms, & Chapter 5/ Overdominance Occurs When Heterozygotes Have Greater Reproductive Success. In Concepts of Genetics (4th ed., pp. 65, 96–98). essay, McGraw Hill.
Mayo Foundation for Medical Education and Research. (2021, November 23). Cystic fibrosis. Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/cystic-fibrosis/symptoms-causes/syc-20353700
Professional, C. C. medical. (n.d.). Tay-Sachs Disease: What is it?. Cleveland Clinic. https://my.clevelandclinic.org/health/diseases/14348-tay-sachs-disease
Spyropoulos, B., Moens, P. B., Davidson, J., & Lowden, J. A. (1981, May). Heterozygote advantage in Tay-Sachs carriers?. American journal of human genetics. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1685035/
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