Representative images of pachytene spermatocytes from (A) BALB/cBy (2N = 40), (B) CBy.RBF(8.12) (2N = 38), (C) RBF/DnJ (2N = 34), and (D) ZALENDE/EiJ (2N = 26) males. Cells were immunostained for the mismatch repair protein MLH1 (green foci), which localizes to most sites of crossing over; the synaptonemal complex protein SYCP3 (red); and for centromeric proteins (CREST; blue). The cells displayed have 26, 27, 20, and 21 autosomal MLH1 foci, respectively. Foci on the sex bivalent were not considered in this study.