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The leading cause of Down syndrome (DS) is nondisjunction of chromosome 21 occurring during the formation of gametes. In this review, we discuss the progress made to identify risk factors associated with this type of chromosome error occurring in oogenesis and spermatogenesis. For errors occurring in oocytes, the primary risk factors are maternal age and altered recombination. We review the current progress made with respect to these factors and briefly outline the potential environmental and genetic influences that may play a role. Although the studies of paternal nondisjunction are limited due to the relatively small proportion of errors of this type, we review the potential influence of paternal age, recombination and other environmental and genetic factors on susceptibility. Although progress has been made to understand the mechanisms and risk factors that underlie nondisjunction, considerably more research needs to be conducted to dissect this multifactorial trait, one that has a considerable impact on our species., (Copyright 2005 S. Karger AG, Basel.)

Within the last decade, aberrant meiotic recombination has been confirmed as a molecular risk factor for chromosome nondisjunction in humans. Recombination tethers homologous chromosomes, linking and guiding them through proper segregation at meiosis I. In model organisms, mutations that disturb the recombination pathway increase the frequency of chromosome malsegregation and alterations in both the amount and placement of meiotic recombination are associated with nondisjunction. This association has been established for humans as well. Significant alterations in recombination have been found for all meiosis I-derived trisomies studied to date and a subset of so called "meiosis II" trisomy. Often exchange levels are reduced in a subset of cases where the nondisjoining chromosome fails to undergo recombination. For other trisomies, the placement of meiotic recombination has been altered. It appears that recombination too near the centromere or too far from the centromere imparts an increased risk for nondisjunction. Recent evidence from trisomy 21 also suggests an association may exist between recombination and maternal age, the most widely identified risk factor for aneuploidy. Among cases of maternal meiosis I-derived trisomy 21, increasing maternal age is associated with a decreasing frequency of recombination in the susceptible pericentromeric and telomeric regions. It is likely that multiple risk factors lead to nondisjunction, some age dependent and others age independent, some that act globally and others that are chromosome specific. Future studies are expected to shed new light on the timing and placement of recombination, providing additional clues to the link between altered recombination and chromosome nondisjunction., (Copyright 2005 S. Karger AG, Basel.)