CHIASMA-BASED GENETIC-MAP OF THE MOUSE X-CHROMOSOME

The X chromosome pair was identified in diakinesis/metaphase I stage mouse oocytes using a repeat sequence DNA probe and fluorescence in situ hybridisation. Chiasma positions along the X bivalent were mea sured in 57 oocytes from 4 females. Overall, our observations showed that while there were no obvious ''hotspots'' for chiasma formation along the X chromosome, there was a tendency to favour the distal end. Minimum inter-chiasma distances were substantial indicating the occurrence of strong genetic interference. Estimates of both genetic distances and recombination fractions for any interval along the chromosome can be calculated from the chiasma data. The average chiasma frequency for the X bivalent was 1.37 giving an estimated total genetic map length of 68.5 cM. In general, the pattern of chiasma distribution along the X chromosome resembled that anticipated from recombination distances in published consensus linkage maps. There were, however, some intriguing differences between the two approaches. The reason for these discrepancies are unknown but may be related to lack of precision in cytogenetic mapping of loci, inter-strain and/or interspecies differences in the genetic controls over the distribution of crossover events. One advantage of the chiasma analysis approach is its suitability for investigating these problems.