Simulating Heterozygous
Advantage
A simulation to demonstrate how selection influences evolution
over time. Background
Hardy Weinberg Conditions for Genetic Equilibrium 1. Large population 4. No selection 2. No migration 5. Random mating 3. No mutations Hardy Weinberg Mathematical Equations 1. p + q = 1
2. p2 + 2pq + q2 = 1
Background
Blood, Genes and Malaria by Jared Diamond
ü  Theme/Main Idea: •
Alleles that offer a selective disadvantage in certain
environmental conditions can offer a selective advantage
in others.
•
Populations in which allele frequencies change over time
experience evolution due to natural selection (or genetic
drift). •
The prevalence of the sickle cell allele in regions where
malaria is high is a real world example of a
heterozygous advantage .
1
Blood, Genes and Malaria by Jared Diamond
Sickle Cell Anemia
•
•
Description: Disorder of the red blood cells, which
originated due to a single genetic mutation.
Symptoms: Red blood cells takes on a sickle shape.
ü  Homozygotes (aa): experience blocked circulation,
organ degeneration, increased bacterial infections.
Death often occurs in individuals before the age of
20. ü  Heterozygotes (Aa): experience mild or no symptoms
of sickle cell anemia. ü  Homozygotes (AA): experience no symptoms of
sickle cell anemia.
Blood, Genes and Malaria by Jared Diamond
Sickle Cell Anemia and Malaria
•
Malaria is a potentially fatal disease caused by a parasite
transmitted by mosquitoes.
•
Malaria occurs in over 100 countries and territories
including Africa. •
Sickle Cell Heterozygotes (Aa) offer resistance to
Malaria. •
Malaria turns normal red blood cells into sickle-shaped
cells, these cells are destroyed by the body preventing
malaria from spreading.
Blood, Genes and Malaria by Jared Diamond
Sickle Cell Anemia and Natural Selection
•
Where malaria is common, the recessive allele (q) is
maintained in the population because heterozygotes (Aa)
for sickle cell anemia have a selective advantage (like
Africa). •
Where malaria is not common (like North America), the
recessive allele (q) will become less common in the
population because heterozygotes (Aa) for sickle cell
anemia do not have a selective advantage. 2
Blood, Genes and Malaria by Jared Diamond
Sickle Cell Anemia and Hardy-Weinberg
1. Large population Condition is met 2. No migration Condition is violated 3. No mutations Condition is violated 4. No selection Condition is violated 5. Random mating Condition is violated
Blood, Genes and Malaria by Jared Diamond
Conclusions: •
The sickle cell allele is the best known genetic defense
against malaria. It protects against one disease, while
causing another (sickle-cell anemia). •
Natural Selection operates differently in different
environmental conditions, causing allele frequencies to
change.
•
One or more Hardy-Weinberg conditions for genetic
equilibrium are violated with respect to the sickle cell
condition. Therefore, evolution is occurring. 3