The sickle cell anemia is controlled by a single pair of allele; $$mathrm{Hb}^{mathrm{A}}$$ and $$mathrm{Hb}^{mathrm{S}} .$$  If both the parents are heterozygous $$left(mathrm{Hb}^{mathrm{A}} mathrm{Hb}^{mathrm{S}}\right)$$ then the offspring can suffer from this disease.  The offspring should be homozygous ( $$mathrm{Hb}^{mathrm{S}} mathrm{Hb}^{mathrm{S}}$$ ). Heterozygous individuals are carriers of this ofthis disease.  Heterozygous individuals are advantageous in terms of adaptation. Due to this, sickle cell anaemia is persisting in the human population.