The genes for normal beta globin and sickle shaped beta globin are co-dominant. A person who inherits sickle cell trait inherits one gene for sickle cell and one gene for normal red blood cells. Therefore, they are heterozygous. For sickle cell anemia to be present in an individual, there must be two sickle cell genes in that person's genotype or genetic makeup. Therefore, they are homozygous for sickle cell anemia and only make sickle shaped red blood cells. Because the individual with sickle cell trait only has one sickle cell gene, their body will produce half sickle shaped red blood cells and because the other gene is normal, they would also produce half normal shaped red cells. With this situation, because some of the red cells are normal, the symptoms you would expect of a sickle cell anemia patient- would be lessened. Having trait is considered a benign condition although rare complications can include, kidney and spleen complications and possible sudden death during physical exertion. However, the individual with sickle cell trait wouldn't have the crises and frequent hospital visits that are associated with sickle cell anemia.
The correct terminology here is sickle cell trait. Sickle Cell trait is a condition that is inherited by children from their parents. Those who have sickle cell trait produce two types of hemoglobin in their red blood cells, the normal hemoglobin called Hemoglobin A and an abnormal type of hemoglobin called Hemoglobin S. But as the number of red blood cells producing the normal hemoglobin is higher, people with sickle cell trait do not develop sickle cell anemia and are generally healthy.
In the case of people who have sickle cell anemia, a majority of the red blood cells contain hemoglobin S. These cells have a shortened life and as they are distorted in shape, they cannot pass through small blood vessels as easily as normal red blood cells can. This decreases the quantity of blood that reaches different parts of the body which leads to their damage.