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Case study: sickle cell anemia (2 of 2)

  1. red blood cells
    sickle cells
    Normal red blood cells (top) and sickle cells (bottom). Download this graphic (third in a series of three) from the Image library.
    There are effects at the cellular level
    When red blood cells carrying mutant hemoglobin are deprived of oxygen, they become "sickle-shaped" instead of the usual round shape (see picture). This shape can sometimes interrupt blood flow.

  2. There are negative effects at the whole organism level
    Under conditions such as high elevation and intense exercise, a carrier of the sickle cell allele may occasionally show symptoms such as pain and fatigue.

  3. There are positive effects at the whole organism level
    Carriers of the sickle cell allele are resistant to malaria, because the parasites that cause this disease are killed inside sickle-shaped blood cells.

This is a chain of causation. What happens at the DNA level propagates up to the level of the complete organism. This example illustrates how a single mutation can have a large effect, in this case, both a positive and a negative one. But in many cases, evolutionary change is based on the accumulation of many mutations, each having a small effect. Whether the mutations are large or small, however, the same chain of causation applies: changes at the DNA level propagate up to the phenotype.