This condition occurs in people who have one of two possible genes (i.e., they are heterozygous for the allele) that code for the defective hemoglobin responsible for sickle-cell anemia. The codition is diagnosed by exposing an individual's red blood cells to a low oxygen environment; if the trait is present, the cells will turn to a sickle shape. People with this trait may suffer milder symptoms of sickle-cell anemia, or may have no symptoms. Some scientists believe the trait actually provides an evolutionary advantage in tropical environments because the slightly altered shape of the blood cells causes a person to be more resistant to malaria.