sickle cell is a recessive trait.
that means that you have to inherit the gene from both your parents.
eg, if each of your parents has one good copy, and one defective copy of the gene, there will be a 25% chance you'll get 2 copies of the defective gene.
when that happens, before modern medicine, you'd probably die.
however, if you had 1 good copy, and 1 defective copy, then you'd not get malaria.
in a part of the world where most folks get malaria repeatedly, the benefits outweigh the problems.
thalassemia is a related genetic condition that also offers some protection against malaria, however, just as with sickle cell, if you get too much of the gene, your chances for survival are not good.
both cases are interesting results of mutation and natural selection.
