Sickle cell trait

Sickle cell trait is a blood disorder related to sickle cell anemia. In the trait form, the patient has inherited one copy of the defective gene from the parent, instead of the two that lead to the anemia.
Sickle cell trait is generally a benign condition. Although the body produces abnormal hemoglobin from the defective gene, it also produces normal hemoglobin from the normal gene. As such, it rarely develops into anemia. However, in very rare cases, a person with the trait can suffer the same type of collapse of red blood cells as a person with both copies of the gene. This is more common in persons who engage in intense athletic activities, and in pregnant women. As such, persons who have the trait must be carefully monitored in these situations.
Apparently, persons with the trait are more resistant to malaria than people with normal hemoglobin. As such, it is believed that evolution has played a role in the spread of the gene where malaria is common. Moreover, although the anemia is almost exclusively limited to persons of African descent, the trait is much more widespread, appearing in about 25% of persons of West African descent, a lower percentage of people of Mediterranean descent, and is not unknown in South Asians, Arabs, North Africans, Turks, and even Native Americans.
Sickle cell trait at Wikipedia
Sickle cell trait