Recombination, interference and sequence: comparison of chromosomes 21 and 22.

The euchromatic regions of chromosomes 21 and 22 are almost completely sequenced and have similar lengths (33.7-34.6 Mb). This similarity effectively controls for the influence of length, making comparisons of recombination and interference interesting. For both chromosomes, there is less male than female recombination, and male recombination is associated with GT/CA repeats. The striking sex difference may result from greater condensation of chromosomes in paternal meiosis, possibly restricting recombination to regions with longer repeat tracts and/or higher repeat densities. Chiasma interference in both sexes for chromosome 22 and in females for chromosome 21 is close to the genome average. Chromosome 21 is significantly different in male meiosis, with near complete interference, suggesting that even when double recombinants occur they are widely spaced. We propose that this difference is related to the different distribution of GT/CA dinucleotides. These repeats are widely distributed on chromosome 22, perhaps offering greater opportunities for double recombinants to occur within smaller regions, whereas they are largely subtelomeric in distribution on chromosome 21.