A clinical genetic examination of 22 cases of hereditary microcephaly in 11 families was carried out. Parent consanguinity (first and second cousins) was ascertained in 7 families. Genetic analysis of the experimental material performed by Weinberg's method showed significantly recessive course of the genuine microcephaly inheritance. High frequency of low degree mental retardation (30%) among probable recessive genes carriers of microcephaly (of the parents and their siblings) was ascertained and that confirmed the possibility of gene heterozygous state manifestation. The gene frequency fluctuated from 0,0042 to 0,024. About 3500 people might show low forms of mental retardation in connection with probable non-penetration (about 30%) of the microcephaly genes being in heterozygols state.
