Mouse strains with the Psmb9 initiation site of meiotic recombination active on both, only one or none of the two homologous chromosomes. The Psmb9 hotspot is on mouse chromosome 17. The wm7 haplotype, drawn in orange (or dark grey), is required for Psmb9 activity. Fragments from B10 or B6 origin are in black and that from the A strain is in blue (or light grey). The initiation activity at Psmb9 on each homologue is indicated (+ or −). CO frequencies (percentage of recombinant molecules per total sperm DNA) in B10.AxSGR and R209xB10 are from the study by Baudat and de Massy (2007a). In R209, B6 and RB2 × B10 frequencies are in parentheses as they are predicted based on genetic analyses that showed that the presence of the R209 or RB2 chromosome (Grey et al, 2009) was sufficient to induce Psmb9 initiation activity on both homologues (Shiroishi et al, 1991; Baudat and de Massy, 2007a). A full-colour version of this figure is available at The EMBO Journal Online.