In the common Mediterranean variant, mutations decrease production of the β-chain (β-thalassemia). In α-thalassemia, which is relatively frequent in Africa and several other countries, production of the α-chain of hemoglobin is impaired, and there is relative over-production of the β-chain. Individuals homozygous for β-thalassemia have severe anemia and are unlikely to survive and reproduce, so selection against the gene is strong. Those homozygous for α-thalassemia also suffer from anemia and there is some degree of selection against the gene. The lower Himalayan foothills and Inner Terai or Doon Valleys of Nepal and India are highly malarial due to a warm climate and marshes sustained during the dry season by groundwater percolating down from the higher hills. Malarial forests were intentionally maintained by the rulers of Nepal as a defensive measure. Humans attempting to live in this zone suffered much higher mortality than at higher elevations or below on the drier Gangetic Plain. However, the Tharu people had lived in this zone long enough to evolve resistance via multiple genes. Medical studies among the Tharu and non-Tharu population of the Terai yielded the evidence that the prevalence of cases of residual malaria is nearly seven times lower among Tharus. The basis for resistance has been established to be homozygosity of α-Thalassemia gene within the local population. Endogamy along caste and ethnic lines appear to have prevented these genes from being more widespread in neighboring populations. There is evidence that the persons with α-thalassemia, HbC and HbE have some degree of protection against the parasite. Hemoglobin C (HbC) is an abnormal hemoglobin with substitution of a lysine residue for glutamic acid residue of the β-globin chain, at exactly the same ß-6 position as the HbS mutation. The "C" designation for HbC is from the name of the city where it was discovered—Christchurch, New Zealand. People who have this disease, particularly children, may have episodes of abdominal and joint pain, an enlarged spleen, and mild jaundice, but they do not have severe crises, as occur in sickle cell disease. Haemoglobin C is common in malarious areas of West Africa, especially in Burkina Faso. In a large case–control study performed in Burkina Faso on 4,348 Mossi subjects, that HbC was associated with a 29% reduction in risk of clinical malaria in HbAC heterozygotes and of 93% in HbCC homozygotes.