Genetic Difference between Europeans and Indians: Tissue and Blood Types

When Colombus reached America, the continent probably was inhabited by 15 to 30 million natives. Mexico now has 68 different Indian tribes classified from a linguistic point of view; 5 million people speaking different languages are registered. However, the Mexican population is mainly composed of Mestizos (95%), who have a triracial admixture of Caucasian genes coming from the Spanish conquerors, black genes from the African slaves brought by the Spaniards to America, and an Oriental gene-pool derived from the natives. The admixture started around 1500, but at present it may be very difficult to distinguish phenotypically one group from another, and the Mestizos from the Indians. Therefore, polymorphic systems like the major histocompatibility complex (MHC) or several blood markers are very valuable tools and crucial elements to trace human migrations, to define degree of admixture, and to explore the impact of genetics on the epidemiology of the different populations. The distribution of blood group systems throughout western Europe is very homogeneous. In contrast, in the Mongolian the A₂ subgroup and the S allele almost disappear compared with European Caucasian. Although homogeneity also exists in Orientals, several groups have a very particular pattern, such as the Senoi from Malay, the Tharons from Burma and the Ainu from Japan. We analyzed four Mexican Indian tribes, and as in Amerindians group 0 is extremely high, almost all are ₂ Fy^a is increased as in Mongolians, and Di^a is an Oriental and Amerindian marker. The distribution of Lu and Kp suggest that the environment might have influenced the variability of these antigens found in Indians. Certain Rh genes (such as R₂) are present probably because they confer selective advantage to these groups.

The HLA complex encodes for molecules (class I and class II antigens) that function as recognition elements of foreign and self antigens. Thus, an immune response is only triggered in the context of HLA antigens. HLA genes also are involved in resistance and susceptibility to many autoimmune diseases and to certain infections, such as leprosy or tuberculosis. Because of its extraordinary polymorphism, the HLA genes are markers of individuality and are of great help in population genetics, anthropology, and to analyze diseases with a genetic background. We studied five different Indian groups (Nahuas, Tarahumaras, Mazahuas, Mixtecos, and Lacandones) and Mestizos. The genetic pattern of the Indians is very restricted and certain antigens seem to be Indian markers: A24, A31, Aw33, B39, B60, B62, Cw4, Cw7, DR4, DRw8, DRw14, DQw1, DQw7, and DPw4. The Mestizos are clearly a triracial group, and the Lacandones, who belong to the Mayan family are the most homogeneous tribe. DR4 is remarkably high in them, suggesting either selective advantage or fixation of the antigen because of genetic drift. The genetic distances demonstrate that Nahuas and Tarahumaras are closely related, and that Lacandones and Mixtecos, although belonging to different families, had a common origin. The ancient ancestors of Mexican Indians are Japanese, Koreans, Chinese, Thais, and Malays, as shown by the genetic distances. Disease-association studies in Indians indicate that ankylosing spondylitis depends strongly on the presence of B27, and populations with a low frequency of B27 have a low prevalence of the disease. Pima Indians do not suffer with type I diabetes (insulin-dependent diabetes mellitus [IDDM]), but non-IDDM is a common disease among them. It is now evident that class II genes (DQB, DQA, and in Mexicans, DRB as well) play an important role in the expression of IDDM. Hepatic cirrhosis is frequent in Mazahuas but not in others; we suggested that a Caucasian gene (HLA-B8) partially responsible for the disease, remained after an important admixture. All these are examples of how isolated populations provide fertile ground for studying diseases, migration patterns, and how the admixture has given rise to new ethnic groups where the genetics and environment have jointly acted to define the features of the peoples in the New World.