Human aneuploidy: incidence, origin, and etiology.

Chromosome abnormalities occur with astonishing frequency in humans, being present in an estimated 1030% of all fertilized eggs. Of the different classes of chromosome abnormality, aneuploidy (trisomy and monosomy) is by far the most common and, clinically, the most important. For example, over 25% of all miscarriages are monosomic or trisomic, making aneuploidy the leading known cause of pregnancy loss. Furthermore, among those conceptions that survive to term, aneuploidy is the leading genetic cause of mental retardation. Over the past 25 years, a considerable body of information has accrued on the incidence of aneuploidy in human gametes, fetuses, and newborns. More recently, the application of molecular biological techniques to the study of aneuploidy has begun to uncover some of the underlying causes of human aneuploidy. In this review, we first summarize the cytogenetic data on the incidence of aneuploidy in humans, and then discuss recent molecular data on the mechanism of origin of different aneuploid conditions, the basis of the maternal age effect on aneuploidy, and the importance of aberrant genetic recombination to the genesis of aneuploidy.