LSF Research Unit, Regional Genetics Services, Heartlands Hospital, Birmingham, UK

^aCurrent Address: Chromatim and gene expression group, Dept of Anatomy, The Medical School, University of Birmingham, UK.

^bCorrespondence (current address): MA Hultén, Molecular Medicine Research Centre, Dept of Biological Services, University of Warwick, Coventry, CV4 7AL, UK. Tel: 01203 528976; Fax: 01203 523701; E-mail: ceht@dna.bio.warwick.ac.uk.

The distribution of anti-MLH1 (MutL homologue 1) antibody labelling was 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 the 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 spermatocytes. A mean number of 50.9 autosomal foci was observed from 46 human pachytene spermatocytes corresponding to a genetic length of 2545 cm. Division of these spermatocytes into sub-stages revealed that the number of foci remains stable throughout pachytene. A focus was found 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.

Crossing over; genetic length; human; meiosis; MLH-1; oocyte; pachytene; synaptonemal complex; spermatocyte