Lecture on Sickle Cell Anemia and Evolution

Background on Sickle Cell Anemia

• Devon and Sky Cooper: Siblings with sickle cell anemia.
• Historical Context: Before modern medicine, sickle cell anemia was often fatal before adulthood.
• Symptoms and Complications:
  • Stroke and organ failure.
  • Devon: Acute chest syndrome, spleen removal.
  • Sky: Experiences bone pain but leads a relatively normal life.
• Genetic Basis:
  • Genetic disease; both parents may carry the sickle cell trait without having the disease themselves.

Prevalence and Genetic Aspects

• Prevalence:
  • In the US, 70,000 - 125,000 cases, with higher rates in African-Americans (1 in 500).
  • Questions why sickle cell is prevalent and specifically in African descent.
• Genetics:
  • Red blood cells become crescent-shaped, causing poor oxygen delivery.
  • Variability in symptoms not completely understood.

Tony Allison's Contribution

• Background:
  • Grew up in East Africa, career as a medical doctor and molecular biologist.
  • Interested in blood and evolutionary history.
  • Made a groundbreaking discovery linking sickle cell anemia with malaria.
• Observations and Hypothesis:
  • Noticed high sickle cell trait frequencies in coastal areas and low in highlands.
  • Connected high frequency areas to high malaria incidence.

Scientific Method and Discovery

• Testing the Hypothesis:
  • Conducted a massive study testing 5,000 blood samples.
  • Correlation between sickle cell trait and malaria was strong.
  • Mapped regions of high sickle cell incidence overlapping with regions of high malaria incidence.
• Genetic Mechanics:
  • Explained homozygous (two sickle cell genes) and heterozygous (one sickle cell gene, one normal gene).
  • Heterozygotes have a selective advantage in malarial environments.

Evolutionary Trade-offs

• Natural Selection:
  • Sickle cell trait provides malaria resistance; trade-off is increased sickle cell disease.
  • Gene mutation is not the optimal solution, but the most available at the time.
• Fitness:
  • Fitness measures a gene's likelihood of being passed to the next generation.
  • Sickle cell trait carriers survive and reproduce better in malarial zones.

Implications and Legacy

• Global Context:
  • High incidence of sickle cell in Southern Europe, India, Africa correlates with malaria zones.
  • In low-malaria areas like Boston, sickle cell trait is now more of a genetic burden.
• Tony Allison's Legacy:
  • Demonstrated real-time human evolution through natural selection.
  • His work illuminated the connection between genetics and environmental pressures.