Behaviour of the Ring-Chromosome inDrosophila Melanogaster

Abstract

SynopisThe genetical studies on ring-chromosomes inDrosophilashow that there is no interlocking of chromatids at mitosis.Dicentric ring-chromosomes formed at meiosis on being pulled apart do not become broken with subsequent reunion of the broken ends. This may be due to the failure of these chromosomes either to rejoin or to become broken at all.The results make it probable that there is a gene or genes located in one of chromocentral regions of the X-chromosome ofDrosophila, probably in the region at the distal end, which has a function in ensuring the orderly segregation of the chromatids in the early cleavage stages of the fertilised eggs. Hence the absence of this chromosome region results in a tendency to gynandromorphism.Results indicate the incorrectness of the view that the inviable zygotes and the patroclinous males found in the ring-chromosome stock arise from different kinds of eggs (single cross-over and progressive double cross-over respectively, as proposed by L. V. Morgan). Both arise from eggs that have for any reason failed to receive a monocentric X-chromosome. This result occurs in all cases of digressive double crossing-over and in half the cases of progressive double crossing-over. Such virtually X-less eggs, when fertilised by an X-bearing sperm, form patroclinous males, and when fertilised by a Y-bearing sperm, form inviable zygotes. The classes thus found are therefore equal in number.