The sickle cell allele is an example of a co-dominant gene found on chromosome 11. Those who are homozygous for sickled cell disease has a mutation in their DNA causing the third amino acid in the B-globin go from Glu to Val. This blood disorder causes all their hemoglobin to take on a sickle shape. This leads to anemia, infection, pain attacks, stroke, spleen damage, liver damage, slowed growth, delayed puberty and early death. Having sickle cells also makes the blood plasma carry more CO (carbon monoxide) which kills malaria infection (plasmodium). Malaria is caused by protozoan of the genus Plasmodium. Normally infection begins with a bite from an infected mosquito. The parasite travels from the mosquito to your liver, where the parasite begins to reproduce. The parasite leaves the liver and travels to the bloodstream, where it infects red blood cells eventually destroying them which can result in a terrible death. *side note/hint: sickle cell has co-dominant expression*
1. Describe why a harmful trait, such as sickle cell disease, can be preserved in a population.

2. Explain how natural selection causes changes in allele frequencies in a population.
3. Describe how disease can act as a selective pressure in evolution.