The chiasma pattern in normal mouse chromosomes. Mouse spermatocytes (top) and oocytes (bottom) at the Metaphase I stage from mice with normal karyotypes after block staining (a, c) followed by C-banding (b, d). The XY bivalent of the spermatocyte is arrowed. Note examples of mono-chiasmate (I) and di-chiasmate (II) bivalents. Reproduced from .