Understanding Evolution

A case study: sickle cell anemia

Sickle cell anemia is a genetic disease with severe symptoms, including pain and anemia. The disease is caused by a mutated version of the gene that helps make hemoglobin — a protein that carries oxygen in red blood cells. People with two copies of the sickle cell gene have the disease. People who carry only one copy of the sickle cell gene do not have the disease, but may pass the gene on to their children.

The mutations that cause sickle cell anemia have been extensively studied and demonstrate how the effects of mutations can be traced from the DNA level up to the level of the whole organism. Consider someone carrying only one copy of the gene. She does not have the disease, but the gene that she carries still affects her, her cells, and her proteins:

  1. There are effects at the DNA level

    Even without the disease, the gene still affects cells and 

  2. There are effects at the protein level

    Normal hemoglobin in normal red blood cell and clumped hemoglobin in 
    sickle-shaped red blood cell

    Download these graphics (one and two in a series of three) from the Image library.

    Normal hemoglobin (left) and hemoglobin in sickled red blood cells (right) look different; the mutation in the DNA changes the shape of the hemoglobin molecule, allowing it to clump together.


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