Supervillain or Superhero? How autosomal recessive disorders affected human evolution  

Oftentimes in movies, we root for the superhero and despise the super villain. After all, who wants Dr. Evil and Mr. Bigglesworth to dominate the world by turning the moon into a death star? His success wouldn’t benefit anyone other than himself and his cohort of miscreants.  Civilization and freedom would be compromised, effectively squashing humans as individuals. But, by rooting for the devilishly carnal Austin Powers, human civilization is maintained and evil is vanquished. Using a little creative interpretation, we can find these character tropes in evolutionary biology.

A basic belief in evolutionary biology is that a gene will only survive over generations if it can provide some form of advantage to a species (the superhero). If a gene provides a disadvantage (the super villain), such as a lethal disorder or disease, it should eventually disappear from the gene pool. Basically, how many reproductive opportunities a gene provides its carrier will determine its success. However, certain genes, act as both a superhero and a supervillain. As genes combine to form traits, they can sometimes show both positive and negative assets within the trait. An example of this are autosomal recessive disorders, where the amount of negative assets present can provide either an advantage or disadvantage to the carrier. They are the superheroes and supervillains version of our genes!

Typically, humans have 46 chromosomes on which all our genes are located. Arranged into 23 different pairs, one of each chromosomes that creates the pair is passed from your mother or father. 22 of these chromosomal pairs are known as autosomal chromosome. Autosomal chromosomes decide every trait you have aside from gender. When variation within a gene results in a negative trait, and is passed from both parents this is known as a recessive mutation. An autosomal recessive disorder develops when a recessive mutation occurs on an autosomal chromosome and both parents pass the mutation. Even then there is a 25% chance of having inherited the mutated gene and developing the disorder. If only one parent passes the mutation there is a 50% chance of inheriting one of the mutated genes and becoming a carrier.

Autosomal recessive disorders are extremely harmful and deadly to the afflicted. Those burdened with an autosomal recessive disorder find themselves with complicated health issues and shorted lifespans. However, those who are carriers can demonstrate increased resistance to certain diseases. Diving into this lottery of inheritance lets look at the pros and cons of one of these disorders-Sickle Cell Anemia.

Sickle Cell Anemia

Sickle-cell anemia occurs when there isn’t a sufficient amount of healthy red blood cells to carry oxygen throughout the body. Caused by a mutation in the gene that affects hemoglobin, the usual rounded flexible shape of the hemoglobin becomes rigid and sticky, resembling sickles. Mutated hemoglobin, known as hemoglobin S., cause a wide variety of negative complications, such as anemia, pain caused by blocked blood flow, swelling of the hands and feet, delayed growth, increased infection rate, and vision problems. However, when a carrier for sickle-cell anemia, known as sickle cell trait, there is instead an increased resistance to Malaria.

An infectious disease, caused by parasitic protozoans, malaria is transferred to humans only through the female Anopheles mosquito. when infected, the protozoans move through blood vessels to liver cells and reproduce asexually, eventually releasing back into red blood cells to continue their life-cycle. If a host has the sickle-cell trait however, the defective hemoglobin S present within their body will rupture, preventing the malaria parasite from continuing its cycle. Unfortunately, anyone afflicted with sickle-cell anemia who contracts malaria become more vulnerable, since all of their red blood cells will sickle, causing a lack of oxygen throughout their body.

What Does This All Mean?

Its clear that autosomal recessive disorders are devastating and deadly and being a carrier provides protection against some pretty terrible diseases. But, most interestingly is who these particular disorders and diseases affect and why. Populations most affected by Malaria show higher rates of Sickle Cell. However, it is important to note that a higher occurrence of a specific disorder is not directly related to a particular race, nor does it indicate a certain population is at a disadvantage to others. Rather, it is directly related to the geographic region, and environmental pressures, an individual’s ancestry is adapted to. If a population group lives in an environment where potentially dangerous diseases are prevalent, the fitness of that population will select for traits that provide a genetic advantage. Over generations, that population will be more resistant and at an advantage for living in that particular environment.

When looking at sickle-cell anemia and sickle cell trait, higher percentages are found among peoples in Sub-Saharan Africa (up to 30% in Nigeria alone), the Chalkidiki district of Greece (18-32%), and on the Eastern Province of the Arabian Peninsula (20-30%) Within these geographical locations, and consequently among their populations, higher rates of malaria are seen. However, these are not the only locations in which we see sickle cell. High rates of sickle cell can also be seen among African American populations in the United States. This is attributed to the dispersal of population groups specifically associated with higher rates of sickle cell. Reviewing historical documents has enabled researchers to track these movements of large populations groups. Looking at records documenting the United States Atlantic slave trade many slaves came from the coasts of the Sub-Saharan African region. Movement of sickle cell has also been documented during the 1950’s large-scale immigration from the Arabian Peninsula into Germany of Turkish nationals.

Spread of Malaria Worldwide
Spread of Sickle Cell Trait Worldwide

While genes that result in autosomal recessive disorders negatively affect the individual who carries them, the more the common carriers of the disorders are protected from other diseases. This advantage allows the carrier to reside in environments with strong selective pressures without hindering their fitness, enabling them to effectively pass on their genes to future generations.

Though the majority of individuals will never experience the negative effects of having an autosomal recessive disorder, we must all strive to help resolve the calamity of these supervillain genes. 

  • To learn more about autosomal recessive disorders click here.
  • To learn more on how sickle cell kicks malaria’s metaphorical tooshie click here.
  • To learn more about what malaria is click here.
  • To learn more about the evolutionary link of sickle cell and malaria click here.
  • To donate to fight against autosomal recessive disorders click here.

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