A disease found almost exclusively in people of African ethnic descent which is characterized by the presence of defective hemoglobin molecules that cause red blood cells to be malformed, often taking on a sickle shape. The protein defect is caused by a homozygous allele that codes for the Beta chain of hemoglobin (the genetic defect causes a valine to be inserted at position 6 instead of a glutamine; in low oxygen conditions, the malformed hemoglobins link into polymers that damage the cell's membrane and cause it to take on the sickle shape). The malformed cells can't carry oxygen properly, and can tend to clog capillaries and small veins. Symptoms include joint pain, acute abdominal pain, and ulcerations of the lower extremities.