Sickle Cell Anemia
A carrier of sickle cell is malaria resistant, however if you are homozygous for sickle cell you have sickle cell anemia. Sickle cell anemia causes trouble with your red blood cells and your arteries. If both your parents are malaria resistant, their children have a 25% chance of getting sickle cell anemia.
The people of West Africa have developed a resistance to malaria, most cases of malaria are stopped there due to this genetic mutation that was developed. As this develops there is more of a chance that both children's parents will be carriers of this mutation. Which gives children a slightly greater chance of getting both parents genes for the mutation, which means they are homozygous for the mutation which causes sickle cell anemia.