Relationship Between Various Chromosomal Changes in Drosophila melanogaster

抄録

From a total of 61 Jethals affecting known loei of the X-chromosome, and induced by X-ray treatment of 2500 to 3000 r-units, 26 or 42.6 per Cent had ehromosomal aberrations such as inversions and translocations. In 92.3 per cent of cases one breakage point of the chromosomal aberration coincided with the region where the lethal change took place.
Of 30 visible mutations induced by a similar treatment only one carried a chromosomal aberration. This, however, did not coincide with the region where visible change took place.
Data available an 80 spontaneous Jethals show that none was conneeted with either an inversion or a translocation. Among Jethals induced by X-ray treatment, the frequency of such chromosomal aberrations increases with the increase of dosage.
It has been determined by salivary chromosome studies that all investigated lethals affecting known loci are minnte deficiencies. It is suggested that either all or a great majority of random lethals are also deficiencies, and that some of the small deficiencies may not have a lethal effect.
Coincidence between a breakage point of a chromosomal aberration and the place where a deficiency has occurred indicates that these two processes may be induced by the same mechanism. Chemical changes producing small deficiencies are responsible for breaks in chromosomes. Free ends produced by such breaks have a tendency to join again. If the frequency of breaks in a nucleus is low, as in Gase of spontaneous changes, chromosomal rearrangements have little chance to occur, but if it is high, as in Gase of high dosage X-ray treatment, the opportunity for the origin of chromosomal rearrangements (inversions and translocations) is also high.