Meiotic and mitotic chromosome counts conclusively established that an adult male mouse possessed 41 chromosomes. Karyotype analyses indicated that the extra chromosome was homologous to one of the smallest autosomes – probably chromosome number 16. The animal was phenotypically normal and was only detected by the fact that it was completely sterile. Spermatogenesis was normal until after meiotic metaphase I. Subsequently, few, if any, meiotic products could be identified. The trisomy was probably the result of triethylenemelamine-induced non-disjunction in a meiotic division of the fathe