In recent years, geneticists have studied variation in rates of recombination and found that levels of recombination vary widely—among species, among and along chromosomes of a single species, and even between males and females of the same species. For example, about twice as much recombination takes place in humans as in mice and rats. Within the human genome, recombination varies among chromosomes, with chromosomes 21 and 22 having the highest rates and chromosomes 2 and 4 having the lowest rates. Researchers have also detected differences between male and female humans in rates of recombination: the autosomal chromosomes of females undergo about 50% more recombination than do the autosomal chromosomes of males.
Geneticists have found numerous recombination hotspots, where recombination is at least 10 times as high as the average elsewhere in the genome. The human genome may contain an estimated 25,000 to 50,000 such recombination hotspots, and approximately 60% of all crossovers take place in them. For humans, recombination hotspots tend to be found near but not within active genes. Recombination hotspots have been detected in the genomes of other organisms as well. Other chromosomal regions, such as those near centromeres, often display reduced rates of recombination.
Rates of recombination vary among species, among and along chromosomes, and even between males and females.