Different between sickle cell anaemia and beta-thalassemia?

Beta thalassemia: Is a quantitative deficiency of beta globin, caused by alterations in regulatory elements governing beta globin expression. The mutation that causes sickle cell anemia is a qualitative abnormality of beta globin. Two hgbs alleles causes sickle cell dz. But you can inherit one hgbs and one beta thal allele (so called sickle-beta thal). Dz severity depends on beta globin expression from thal allele.