A person with sickle cell disease
normally has two copies of the sickle cell mutation in each of their beta-hemoglobin genes (hb ss).
Those with sickle trait have one unaffected copy and one sickle (hb as, where "a" is normal). Other variants of beta-hemoglobin can be inherited along with one copy of the sickle gene (hb sc, sd, se, or s/beta-thal). These can give less severe disease than hb ss.