SUMMARY The Authors have studied, from the standpoint of population genetics, twins born in the Aosta Valley from 1910 to 1957. This material, consisting in 980 pairs, has been examined from the standpoint of the frequency ot twin deliveries (Gemelliparity Index), of monozygosity (Monozygosity Index), and of patronymic (Patronymic Index). The Authors show that the region may be divided in geo-demographical departments, 6 of which may be considered « isolates » with a high rate of inbreeding.

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... eover, the usefulness of the twin-method in population genetics is demonstrated.