SOME RADIATION EFFECTS ON SEGREGATION IN DROSOPHILA

Translocation induced in the immature oocyte, in meiotic prophase, affects division I orientation and segregation, the usual result being that the two halves of translocations are directed to opposite poles. Since interchange is usually (if not exclusively) between chromatids, this is to be expected from the creation of illegitimate conjunctions. Good agreement is obtained between patterns of segregations deduced from recovered half-translocation bearing exceptions and the kinds of disomic gametes expected as alternative recoveries from the same division I configurations. Inferences drawn from the study of compound-X females have been found to apply as well in the case of females of normal karyotype. Numerical errors occur predominantly, possibly exclusively, in division I. The rate of induced nondisjunction of specific chromosome pairs varies in relation to the structure of the entire complement, as required if radiation-induced nondisjunction is interchange dependent, but which would be unexpected if the mechanism involved effects on individual spindle fibers, chromosomes, or chromosomal bivalents.