Attached-X females lacking a Y chromosome (= C(1)RM, y v bb/O) and having differentially marked fourth chromosomes (Dp(1;4)1021, y+ ac+/ciD or Dp(1;4)1021, y+ ac+/spapol) were irradiated. Offspring having detachments were analyzed to identify, where possible, cases involving C(1)-4 interchange and to determine in those cases whether a free maternal fourth chromosome was also recovered and, if so, whether it was the homologue or the sister of the broken 4. In approx. 85% of the cases where a free 4 was recovered, it was the homologue; in about 10%, the sister of the broken 4. In the remaining cases either a recombinant 4 or both 4's were recovered. It is inferred that a usual consequence of heterologous interchange is the formation of an heterologous quasi-bivalent, with the interchange elements separating at the first meiotic division, while the homologous 4 segregates more or less at random, these being processes that would leak to the kinds of recoveries most commonly experienced. Recovery of the sister 4 does not necessarily require that the elements of an heterologous quasi-bivalent fail to separate at first division. The occurrence of multibreak interchanges involving only C(1) and 4 and leading to the formation of quasitrivalents must account for some portion of these cases; it is postulated that other conceivable kinds of multibreak interchange, involving major autosomes in addition to C(1) and 4, might also contribute to recovery of the sister 4. The consequences of such meiotic behavior following interchange induction for the detection of reciprocal translocations and for induced nondisjunction are discussed. © 1969.
