[Chromosome 21 mosaicism. A review].

Despite the great efforts contributed to studies on the trisomy 21 etiology, basic mechanisms of nondisjunction are still poorly understood. Even less is known about mosaic variant of trisonomy 21. In this paper, some problems of masaicism are considered: (1) estimation of the prevalence of mosaicism in patients affected with Down syndrome, (2) determination of population rate of parental mosaicism, (3) evaluation of the role of ovarian mosaicism in etiology of Down syndrome, (4) origin of the extra chromosome, (5) mechanisms of production of mosaic lines, (6) phenomenon of skewed sex ratio (female predominance, in fetuses particularly), (7) effect of maternal age.