A high incidence of mitotic chiasmata in endoreduplicated Bloom's syndrome cells.

The frequency of mitotic chiasmata is compared in endoreduplicated and non-endoreduplicated Bloom's syndrome fibroblasts and in endoreduplicated Fanconi's anemia lymphocytes. The incidence of mitotic chiasmata in BS diplochromosomes is greatly increased over that in diploid BS cells and is much higher than in FA or normal diplochromosomes. The distribution of chiasmata among the BS diplochromosomes is not significantly different from that expected if crossing-over occurs at random along the chromosomes. This is in contrast to the distribution of chiasmata in chromosomes of diploid BS cells which is highly non-random among chromosomes and chromosome regions (Kuhn 1976). Mitotic crossing-over is increased in endoreduplicated cells from all sources compared to diploid cells, but the incidence is highest in endoreduplicated BS cells. This provides evidence against the idea that the high rate of mitotic crossing-over in diploid BS lymphocytes is primarily due to an increase in chromosome pairing. BS chromosomes apparently have a greater tendency to undergo mitotic exchange than normal of FA cells, both in diplochromosomes and in accidentally paired homologous segments in diploid cells.