Download Sickle Cell Anemia

The Puzzle of Sickle Cell Anemia
Directions: Answer each question in the space provided under each paragraph.
_______________________________________________________________________
1. How is the health of homozygous (aa) people with the sickle cell recessive allele different from
heterozygous (Aa) people with the sickle cell recessive allele?
Sickle cell anemia is a hereditary disease that affects the red blood cells in your body that carry oxygen. People
that are homozygous (aa) for the recessive sickle cell allele have sickle cell anemia and often die at an early
age. The red blood cells of people homozygous (aa) for the sickle cell allele collapse into sickle shapes which
can block blood flow cause severe pain, organ failure and eventually death. Heterozygous (Aa) individuals,
who have both a defective and a normal allele, will only have a few red blood cells become sickle shaped
allowing the individual to stay healthy. The disease is now known to have originated in central Africa, where 1
in 100 people are homozygous (aa) for the defective allele meaning they have sickle cell anemia. In the United
States, however, only 1 African-American out of every 500 has sickle cell anemia.
_______________________________________________________________________
2. Why has natural selection NOT acted against the sickle cell allele in Africa by reduced its
frequency in the African population? (In other words, why is this fatal allele so common in Africa?)
The defective allele is common in central Africa because people who are heterozygous (Aa) for the sickle cell
allele are much less susceptible to malaria. Over one million people die of malaria each year, most of them in
central Africa and tropical Asia. In central Africa, the number of deaths from sickle cell anemia is far lower
than would occur from malaria if heterozygous (Aa) individuals were not malaria-resistant. One in five
individuals is heterozygous (Aa) for the sickle cell allele and survives malaria. But only 1 person in 100 is
homozygous (aa) for the allele and dies of anemia. As you can see, natural selection is acting on the sickle cell
allele in opposite directions. Stated simply, natural selection has favored the sickle cell allele in central Africa
because the payoff of survival while being heterozygous (Aa) and immune to malaria is greater than the price
in death of being homozygous (aa) for sickle cell anemia.
_______________________________________________________________________
3. Why is the frequency of individuals with the sickle cell allele so much lower among AfricanAmericans?
Malaria is only found in tropical environments, so the disease does not exist in the United States. It is also
important to understand that most African-Americans descended from Africans who arrived in this country
hundreds of years ago. Therefore the protection against malaria that is associated with being heterozygous
(Aa) for the sickle cell allele offers no real advantage in the United States. As a result, the allele for sickle cell
anemia has become far less common in the United States than it is in central Africa.