Sickle cell disease - keeps pro football player out of game

Ryan Clark, a football player for the Pittsburgh Steelers, was held out of a game in Denver because he's a carrier for sickle cell disease (anemia). This is a sex-linked trait that is common in certain regions of Africa and around the Mediterranean Sea.

Comments should focus on why the player was not allowed to play and whether you agree it was a good decision. Another topic you can comment on is why the sickle cell trait is more common in Africa and why many African-Americans also carry this trait.