Beta-Thal is present in many populations, but primarily in those where malaria is (or was) an endemic occurrence. In this section, the distribution of the many beta-thal alleles are reviewed in Mediterranean countries (Table III, Table IV, Table V), East European (Table VI) and West European populations (Table VII), North African (Table VIII), Middle Eastern (Table IX, Table IX-a), Lebanese and Israeli populations (Table X), the population of the Indian continent (Table XI-a, Table XI-b), East Asian countries (Table XII), in Chinese (Table XIII), Japanese and Koreans (Table XIV), and finally, in Black Americans and some South American populations (Table XV).
A review of all this information indicates that only some 20 alleles account for the great majority of all beta-thal determinants (>80%). Each population has its own major beta-thal types as is indicated in Table II and further detailed in Tables III through XV. The unique distributions of the beta-thal alleles greatly facilitate prenatal diagnosis of beta-thal through the direct determination of specific alleles. Furthermore, this information also explains why beta-thal in some countries (Italy, Greece, China, India, for instance) is usually a serious disorder (no beta-chain synthesis), while in other populations (Blacks from Africa and the United States) mild beta+-thal types prevail.
FIG. 6. Map of the Iberian Peninsula.
FIG. 7. The distribution of beta-thal alleles among the Portuguese population. Some dots and triangles represent more than one person.