A person with sickle cell anemia has an autosomal recessive genetic mutation that results in red blood cells with abnormal, rigid, sickle shape. That is because there is a single nucleotide polymorphism in the gene to produce the protein hemoglobin. In position 6, on the polypeptide chain, the amino acid glutamic acid is subsituted by valine. When it folds into its shape, it is sickle shaped, rather than the normal round shape. It cannot transport oxygen as well as normal hemoglobin leading to anemia and these abnormal red blood cells sometimes get stuck in the capillaries. This can lead to pain and what is known as a crisis. Under a microscope a person without sickle cell has red blood cells that resemble a donut shape and a person with sickle cell anemia has cells that look like a sickle shape.