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2.5: Summary Example: Holism in Anthropology, Sickle Cell Anemia and Malaria

  • Page ID
    5010
  • [ "article:topic", "Enculturation", "authorname:aschoenberg" ]

    Exercise \(\PageIndex{1}\)

    Read the section called "Selection Against Both Homozygotes" from Dennis O'Neil Natural Selection

    Anthropology is holistic because it covers many branches of knowledge. To understand sickle cell anemia we need look at the smallest change in a base pair, and at the global migration of alleles. We need to look two thousand years back in time to a transition from hunter-gatherers to horticulturalists, to the racial discrimination of the 20th Century. We apply the knowledge to the most deadly disease on the planet, and to mixology.

    Mutation starts the process. In the sperm or egg on the 11th chromosome, at the 17th nucleotide of the gene for the beta chain of hemoglobin, there is a point mutation where an A is replaced by a T, which changes the codon GAG (for glutamic acid) to GTG (which encodes valine). Thus the 6th amino acid in the chain becomes valine instead of glutamic acid. The beautiful architecture of the hemoglobin molecule collapses, as if you took the capstone off of an arch, and the red blood cell takes on sickle shape. The sickled cells get caught in blood vessels and don't carry oxygen as well.

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    Figure \(\PageIndex{1}\)

    Exercise \(\PageIndex{2}\)

    Skim the medical literature on sickle cell anemia and click on the location to get a sense of what a gene is

    This new allele is called the S allele, and as a Mendelian trait, you get one from each parent.

    AA=normal hemoglobin
    AS=sickle cell trait, sickle cell carrier
    SS=sickle cell anemia

    The allele frequency of any Single Neucleotide Polymorphism (SNP) is about 1 in 100,000, so you might expect the allele frequency of the S allele to stay at that rate: S=0.00001 But in some places the frequency of the S allele gets as high as 1 in 5, or S=0.2 When population geneticists see changes in allele frequencies they know that evolution is occurring, and the connection to malaria makes it clear that this is a case of natural selection.

    The sickled cells are bad for blood flow and carrying oxygen, but they are good because they protect you from the parasites which cause malaria.

    Malaria is an infectious disease caused by the parasite, Plasmodium falciparum, and is carried by mosquitoes, and easily spread. One little parasite gets into one of your red blood cells, reproduces 30,000 times, pops the cell, and then go on to infect thousands of other cells, which then go on to infect thousands more, until you are really sick.

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    Figure \(\PageIndex{2}\)

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    Figure \(\PageIndex{3}\)

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    Figure \(\PageIndex{4}\)

    But, when the parasite infects a sickle shaped cell, there is less room to reproduce, and it doesn't pop the cell, so it's not spread. Sickle Cell Anemia is bad, but it gives you immunity to Malaria. 

    Natural selection acts on the mutation to change its allele frequency.

    Fetuses produce a special kind of hemoglobin (HbF) that helps pull mom's oxygen across the placenta. Once born, the fetuses normally stop producing HbF, but some adults inherit a gene that tells their body to persist in producing fetal hemoglobin all their lives. The correlation between people with blood diseases like Sickle Cell anemia and hereditary persistence of fetal hemoglobin suggests that natural selection may have selected for the persistence of fetal blood to mitigate the effects of sickle cell anemia. Malaria makes it advantageous to have Sickle Cell, and then Sickle Cell makes it advantageous to have hereditary persistence of fetal hemoglobin.

    Sickle Cell Anemia is an example of biocultural evolution because human cultural activity was the cause for people's genetic change. People in West Africa developed a new subsistence practice that produced more food by clearing land and planting crops. But it also created open spaces for mosquitoes to breed, and higher population densities that made it easier for malaria to spread. As malaria became endemic it became more advantageous to have the S allele. 

    Other cultural factors range from racism to mixology. Because of racism, and the misconception of Sickle-Cell Anemia as a racial disease, the US military initially prohibited African-Americans from flying planes fearing that all African-Americans would suffer sickling events at high altitudes. British Colonialists lacking malaria resistance turned to the bark of a tree from South America called quinine, and they preferred to drink this bitter tonic with gin for good measure. Unfortunately, because of natural selection, most malaria parasites are now resistant to quinine, and drinking gin & tonics in the tropics is more likely to cause dehydration than prevent malaria.

    The holistic approach of anthropology allows us to understand sickle cell anemia through a wide range of disciplines including archaeological research on sites in West Africa, the genetics of humans, plasmodium parasites, and mosquitoes, racism in the US, and even mixology.

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    Figure \(\PageIndex{5}\)Bill Gates notes

    Notes

    Imagination Question

    • People with Sickle Cell Trait may suffer during heavy exertion, and have been discriminated against because of their condition, and there is debate in sports and different branches of the US military as to whether to keep their condition confidential to protect them from discrimination, or publicize their condition to * protect them from the health risks.

    Imagination Actions

    Vocabulary

    • malaria
    • sickle cell anemia
    • Plasmodium falciparum