Chiasma-based genetic maps of chromosome 21.

The available cytogenetic data on meiotic chiasmata have been used to construct sex-specific genetic maps, showing the genetic distances and recombination fractions along the length of 21q. The male maps are based on direct observations of spermatocytes, while the female maps are derivations related to the increased chromosome length in oocytes. The male chiasma data have also been used as a frame of reference for ordering and positioning loci on the physical map with D21S110 as a fixed point.