Ryan Clark will be sitting out this week's playoff game between his Pittsburgh Steelers and the Denver Broncos (there's something to say here about Tim Tebow and God, but I'll let it pass).
Clark suffers from the sickle cell trait, not sickle cell disease. This disorder arises from a mutated hemoglobin gene (see my Carbon Monoxide Poisoning post on Amedeo) that leads to half moon-shaped red blood cells (sickles) that are sticky and tend to die.
Humans carry two copies of each gene. Sickle cell disease is when both hemoglobin copies are mutated, which means the majority of red blood cells are sickled. Sickle cell trait is when only one hemoglobin copy is mutated and only some of the red blood cells are sickled.
Interestingly, while sickle cell trait comes with its own set of problems (including trouble exerting yourself at high altitudes), it does protect the person against malaria.
In Africa, upwards of 40% of the population could have sickle cell trait. Of course, this genetic advantage is a double edged sword. Should two parents who have the trait have a child, that child has a 25% chance of inheriting sickle cell disease and dying, but a 50% change of being immune to malaria.
Pittsburgh Steelers LINKY: http://news.blogs.cnn.com/2012/01/04/sickle-cell-trait-keeps-steelers-safety-out-of-playoff-game/?hpt=hp_t3
Malaria and Sickle Cell Trait LINKY: http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html
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