of the more common explanations is that the sickle hemoglobin inhibits the plasmodium parasite from infecting the red blood cells which reduces the number of malaria parasites to infect the host. Another factor is the production of heme oxygenase-1 (HO-1) enzyme, which is highly present in the sickle hemoglobin. This enzyme produces carbon monoxide which has been proven to protect against cerebral malaria. Established associations Hematuria Hyposthenuria Renal medullary carcinoma, a cancer affecting the kidney, is a very rare complication seen in patients with sickle cell trait. Renal papillary necrosis (only considered "possible" by some sources) Splenic infarcts at high altitude. Surgery may not always be necessary. Sudden deaths during physical exertion in African-American US army recruits, and athletes Urinary tract infection Suggested Probable: complicated hyphema, venous thromboembolic events, fetal loss, neonatal deaths, and preeclampsia Possible: acute chest syndrome, asymptomatic bacteriuria, and anemia in pregnancy Insufficient evidence: retinopathy, cholelithiasis, priapism, leg ulcers, liver necrosis, avascular necrosis of the femoral head, and stroke. An association with complicated migraines has been suggested. There have been reports of pulmonary venous thromboembolism in pregnant women with sickle cell trait, or men during prolonged airflight, and mild strokes and abnormalities on PET scans in children with the trait. Sickle cell trait appears to worsen the complications seen in diabetes mellitus type 2 (retinopathy, nephropathy and proteinuria) and provoke hyperosmolar diabetic coma nephropathy, especially in male patients. Genetics Normally, a person inherits two copies of the gene that produces beta-globin, a protein needed to produce normal hemoglobin (hemoglobin A, genotype AA). A person with sickle cell trait inherits one normal allele and one abnormal allele encoding hemoglobin S (hemoglobin genotype AS). The sickle cell trait can be used to demonstrate the concepts of co-dominance and incomplete dominance. An individual with the sickle cell trait shows incomplete dominance when the shape of the red blood cell is considered. This is because the sickling happens only at low oxygen concentrations. With regards to the actual concentration of hemoglobin in the circulating cells, the alleles demonstrate co-dominance as both 'normal' and mutant forms co-exist in the bloodstream. Thus it is an ambiguous condition showing both incomplete dominance and co-dominance. Unlike the sickle-cell trait, sickle-cell disease is passed on in a recessive manner. Sickle cell anemia affects about 72,000 people in the United States. Most Americans who have sickle cell anemia are of African descent. The disease also affects Americans