MEIOTIC RECOMBINATION AND GERM-CELL ANEUPLOIDY

Data on human trisomic conceptuses suggest that the extra chromosome c ommonly has a maternal origin, and the amount and position of crossing -over on nondisjoined chromosomes is commonly altered. These observati ons may provide important clues to the etiology of human germ cell ane uploidy, especially in regard to evaluating whether environmental fact ors play a role. There is concordance of effects of environmental agen ts on fungi, plants, and animals, which suggests that the overall proc ess of meiosis is well conserved and that chemical and physical agents con affect meiotic recombination, leading to aneuploidy. It seems lik ely that meiosis in humans will fit the general pattern of meiosis in terms of sensitivity to radiation and chemicals. Thus studies on other organisms provide some insight into the procedures necessary for obta ining useful human date. For example, frequencies of spontaneous meiot ic recombination ore not uniform per physical length in Drosophila, an d different regions of a chromosome respond differently to treatment. Treatments that relieve constraints on the distribution of meiotic exc hange, without changing greatly the overall frequency of exchange, may increase the number of univalents and give the impression that there are chromosome-specific responses. Recombination studies that monitor one or a few relatively short genetic regions may also give a false im pression of the effects of a treatment on recombination. In addition, meiotic mutants in Saccharomyces and Drosophila highlight a number of processes that are important for production of an exchange event and t he utility of that event in the proper segregation of both homologues and sisters. They also suggest that tests for pairing at pachytene, ch iasmata at diplotene, and genetic crossing-over may give different res ults. (C) 1996 Wiley-Liss, Inc(star)