The condition known as Sickle Cell Anemia (SCA) demonstrates many aspects concerning genes, mutations, natural selection and evolution in humans. Here are the most important points:

1) Hemoglobin is the protein that carries oxygen in the red blood cell. In the case of SCA, a mutant form of hemoglobin is present. This mutation most likely occurred in a human population in Africa or India several thousand years ago. It is a "point" mutation -- changing a single amino acid in the long hemoglobin protein chain. The change is chemically small, but very important in how hemoglobin functions.

2) SCA is inherited as a co-dominant trait, in the following manner:

HbA - Symbol of the gene for "normal" hemoglobin

HbS - Symbol of the gene for "sickle" hemoglobin

3) Under low oxygen levels, the sickle hemoglobin binds to itself and causes the red blood cell to change shape. Normal red blood cells are disk-shaped and depressed in the middle, but with lots of HbS hemoglobin, the cell shape is changed to a "sickle." In the following photograph, some cells have "sickled" and others have not.

4) In the homozygote, people with the severe form called "Sickle Cell Anemia", this sickling of the blood cells causes alot of problems. Untreated, the condition often shortens the life span considerably, with many people dying as children or young adults. Even the heterozygote form, called "Sickle Cell Trait" can be a serious condition, especially under low oxygen conditions, and oftens shortens the life span of the individual.

5) The gene is quite prevalent in many African groups, and in fact, is widespread in the Old World. Examine this distribution of the Sickle-cell gene. The question arises, why would such a "bad" gene be found in so many places and at such a high frequency. In some parts of Africa, as many as 40% of the people are affected by one form of the condition or the other. We would expect natural selection to remove the mutant gene from the population.

6) However, if the gene improves fitness, then it would explain its widespread distribution. This is exactly what has been found. Examine this map of the distribution of Falciparum malaria, a debilitating disease spread by mosquitoes. It can be seen that the distribution of HbS fits inside this distribution.

7) It has now been shown that having one copy of the HbS gene (ie. being heterozygous) is actually a type of "resistance" to malaria. People can still get bit by malaria-carrying mosquitoes and get infected, but the changes in the red blood cell (where the Falciparum parasite reproduces) makes it difficult for the parasite to grow. Heterozygotes do not suffer nearly as much from malaria as do homozygous "normal". Unfortunately, the cost of such protection is the tremendous toll taken in the form of homozygous sickle-cell and a much reduced life expectancy.

8) Thus, Sickle-cell Anemia is a genetic "buffer" against malaria. It arose as a random mutation and, by chance, provided some resistance to malaria --- a devastating disease in the tropics.

9) But, as Africans were "transported" to the Americas during the days of slavery--any advantage of the sickle cell gene was removed as there is no malaria in North America. What do you think has happened to the frequency of the gene in African-Americans compared to the groups from which they have derived?

Some references:

National Institute of Health - Sickle Cell

PBS - Statement on the Evolution of Sickle Cell