Four patients with various phenotypic stigmata of gonadal dysgenesis are presented. Cytogenetic investigation revealed each to be mosaic for 45 XO and 46 XO+ minute element stemlines. In two patients the additional element proved to be a small ring, and in the remaining two the supernumerary chromosome was a minute centric fragment. Autoradiographic studies indicated that the ring chromosomes were late replicating, while the metacentric fragments were not. The correlation of the autoradiographic behavior of these abnormal chromosomes with their possible origin is discussed, and a new hypothesis concerning the evolution of the XO karyotype is proposed.