Patterns of exchange induced by mitomycin C in C-bands of human chromosomes. II. High frequency of Y-Y exchange in XYY cells.

In XY cells, exchanges in the Y C-band were rare (less than 0.8%). However, in XYY cells we observed 58 Y chromosomes involved in exchange out of 208 (28%). Most were Y-Y interchanges. Therefore mitomycin C does affect Y C-bands, but subsequent interchange requires the presence of a homologue. This may be the consequence of interphase association of the homologues, or by homologous interaction of an affected DNA sequence in the two Ys. If the latter, the proposed sequence cannot be localized to proximal or distal parts of the Y C-band, as exchange points were observed in both. Also, it may be oriented in one direction in the Y, as the Y-Y exchanges which could be unambiguously classified were X-type events.