Recombination doesn't happen everywhere with the same probability. Brassica napus, for example, seems to actively suppress (somehow) recombination between home

Recombination doesn't happen everywhere with the same probability. Brassica napus, for example, seems to actively suppress (somehow) recombination between homeologous chromosomes (chromosomes that came from the same ancestor chromosome) to make sure that reproduction runs flawlessly: http://www.plantcell.org/content/22/7/2265.full http://www.plantcell.org/content/22/7/2265.full In most species recombinations are suppressed around centromeres: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000326 http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjo... Sometimes there are specialized proteins involved undoing or blocking crossovers, sometimes it just seems to be a mechanical problem - for example, you can't have crossovers happening close to other crossovers because the machinery is so large that a secondary set of chromosomes can't dock for a distance of few thousand base pairs.