Comparison of human genetic and sequence-based physical maps

Recombination is the exchange of information between two homologous chromos omes during meiosis. The rate of recombination per nucleotide, which profou ndly affects the evolution of chromosomal segments, is calculated by compar ing genetic and physical maps. Human physical maps have been constructed us ing cytogenetics(1), overlapping DNA clones(2) and radiation hybrids(3); bu t the ultimate and by far the most accurate physical map is the actual nucl eotide sequence. The completion of the draft human genomic sequence(4) prov ides us with the best opportunity yet to compare the genetic and physical m aps. Here we describe our estimates of female, male and sex-average recombi nation rates for about 60% of the genome. Recombination rates varied greatl y along each chromosome, from 0 to at least 9 centiMorgans per megabase (cM Mb(-1)). Among several sequence and marker parameters tested, only relativ e marker position along the metacentric chromosomes in males correlated str ongly with recombination rate. We identified several chromosomal regions up to 6 Mb in length with particularly low (deserts) or high (jungles) recomb ination rates. Linkage disequilibrium was much more common and extended for greater distances in the deserts than in the jungles.