THE RELATION BETWEEN X-RAY DOSAGE AND THE FREQUENCY OF SIMULATED HEALING OF CHROMOSOME BREAKAGES IN DROSOPHILA MELANOGASTER FEMALES

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Abstract
Irradiation of late oocytes, having the "snoc" attached-X chromosome and containing no Y chromosome, with 1000 r and 4000 r of X-rays has produced a large number of cases in which the X was broken and one of the arms produced persisted in the offspring. The incidences of such individuals among the F1 females at these doses were 0.67 [plus or minus]0.10 and 426 [plus or minus] 0.53 per cent, respectively. Analysis of these data and of their components shows that there is no basis for believing that the broken chromosomes became stable by a process of new telomere formation following single breakage. The frequencies of those exceptions which might according to their phenotype, have represented single breakages, being in the ratio of 1:8.8, clearly represent gross chromosomal rearrangements in which eucentric union occurred between one breakage point in or near the proximal heterochromatin on one of the arms of the X chromosome and another breakage point located subterminally, sometimes in the X, but probably more often in other, chromosomes. The present data, indicating that all the exceptional flies may be considered to carry gross rearrangements, make it possible to state definitely what was before only suggested[long dash]that gross rearrangements occur more frequently after X-raying oflcytes than after X-raying oogonia. The fact that gross rearrangements have been detected only rarely in other studies after treating oOcytes is probably due to the unsuitability of the genetic methods employed to detect changes of the types occurring in this material.

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