What is the difference between sickle beta-plus and beta-zero thalassemia?

Both conditions are forms of sickle beta thalassemia, which in turn is a subset of sickle cell disease. These diseases have in common the production of sickled red blood cells, which are induced by a mutation on the gene for hemoglobin. In the most common form of sickle cell disease, sickle cell anemia, the same mutation occurs two times; if it occurs only once, not much happens. In sickle beta thalassemia, the allele with the sickle mutation is paired with an allele that usually causes a separate disease: beta thalassemia. Beta-plus and beta-zero are the names for two categories of damage that beta-thalassemia can do. Beta-zero describes a complete block of the production of beta-globin, while beta-plus refers to a decrease. Sickle beta-plus thalassemia is thus less severe almost by definition; its non-sickling allele is closer to normal.