CROSSING-OVER ANALYSIS AT PACHYTENE IN MAN

The distribution of anti-MLH1 (MutL homologue 1) antibody labelling wa s studied in human prophase meiocytes. A labelling pattern consisting of distinct foci, always associated with the synaptonemal complex (SC) and never in closely juxtaposed pairs, was observed. Comparison of th e number and general positions of autosomal foci with previous studies of the number and positions of autosomal chiasmata indicates that the anti-MLH1 antibody marks sites of crossing over in human pachytene sp ermatocytes. A mean number of 50.9 autosomal foci was observed from 46 human pachytene spermatocytes corresponding to a genetic length of 25 45 cM. Division of these spermatocytes into sub-stages revealed that t he number of foci remains stable throughout pachytene. A focus was fou nd on the XY bivalent in 56.5% of the nuclei. The presence or absence of foci from the XY bivalent could not be correlated to pachytene sub- stage.