Positional control of chiasma formation. Spermatocytes at the Diakinesis/Metaphase I stage from a normally fertile human male with normal mitotic karyotype (top) in comparison to that in a male with non-obstructive azoospermia (bottom). Note the extremely low number of chiasmata in the spermatocyte from the azoospermic male; some chromosomes lack chiasmata altogether (I) while two relatively large bivalents show a single chiasma a medial position (arrowed, left). There are also a number of univalents (arrow heads) as well as two bivalents with two chiasmata (arrowed, right). Reproduced from .