1 These people have normal red blood cells, unless they have some other disease. 2  One gene that makes hemoglobin A and one that makes hemoglobin S. These people carry the sickle cell trait (and are called a carrier ), but they do not have sickle cell disease. 3  Sickle cell trait is usually a harmless condition. These people have normal red blood cells, unless they have some other disease. 2  One gene that makes hemoglobin A and one that makes hemoglobin S. These people carry the sickle cell trait (and are called a carrier ), but they do not have sickle cell disease. 3  Sickle cell trait is usually a harmless condition.