And now for the exciting conclusion to these two posts.  

Disclaimer: I’m not a doctor – just a bioengineer who happens to find pathophysiology fascinating.  If you find an error, please let me know!  Also, NONE of my drawings are to scale.

In this post, we looked at how one tiny change in a person’s DNA caused her to suffer from sickle cell anemia.  We learned that, untreated, about 50% of children with sickle cell anemia die before their fifth birthday.  If you understand natural selection, you’ll recognize this as a puzzle: if sickle cell anemia is so deadly, why are there so many people still affected by it?  Let me explain the puzzle a little further.

For almost all traits, a person has two copies of instructions: one from her father and one from her mother.  Sickle cell disease is autosomal recessive – which just means that both parents must pass on the sickle cell trait for the child to be sick.  People who only received the trait from one parent aren’t sick, but can pass the disease on to their children.

In this case, Mom and Dad are both carriers of a genetic disease.  Statistically, 1/4 of their children will be completely healthy, 1/2 of their children will be healthy carriers of the disease, and 1/4 of their children will be sick.

In this case, Mom and Dad are both carriers of a genetic disease. Statistically, 1/4 of their children will be completely healthy, 1/2 of their children will be healthy carriers of the disease, and 1/4 of their children will be sick.

In most cases where an autosomal recessive trait is deadly early in childhood, the disease dies out.  A child affected by the disease won’t live to have children of his own, and thus won’t pass down the bad information.  With no one to pass it on, the disease stops.

But, sickle cell disease hasn’t followed that pattern.