INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1193 



chromosome breakages constitutes an altogether new field of research in 

 genetics. Exploration of this field has merely begun, and no firmly 

 established conclusions have been so far arrived at. Nevertheless, some 

 of the facts already obtained are highly suggestive. 



An especially interesting group of cases is that in which chromosomes 

 are broken close to the loci of certain known genes, causing these genes 

 to mutate. Stern and Ogura (129) observed mutations at the bobbed 

 locus in the X-chromosome of Drosophila that took place simultaneously 

 with attachments of fragments of the F-chromosome in the vicinity of 

 that locus. A mutation at bobbed w^as also observed by Sidorov (123) 

 in a translocation in which the X-chromosome is broken close to the locus 

 of bobbed. Several duplications for sections of the X-chromosome 

 including bobbed and certain genes normally located in the left end of 

 the chromosome were studied by Sivertzev-Dobzhansky and Dobzhansky 

 (124). Since these duplications were obtained in the offspring of wild- 

 type males treated with X-rays, all of them should contain the wild-type 

 allelomorph of bobbed, or should not carry the bobbed locus at all. 

 Contrary to this expectation, the analysis of these duplications showed 

 that none of them has the wild-type allelomorph of bobbed. Instead, 

 allelomorphs of bobbed of various strength were found to be present. 

 It follows that in all these duplications a mutation at the bobbed locus 

 took place apparently simultaneously wdth the breakage of the chromo- 

 some in the vicinity of bobbed. Whether or not such "mutations" take 

 place every time the chromosome is broken in that region remains to be 

 studied. Dubinin and Sidorov (41) have described an even more remark- 

 able case of this sort. They have studied a series of translocations 

 involving the fourth chromosome of Drosophila melanog aster. These 

 translocations were obtained by irradiating flies which were homozygous 

 for the wild-type allelomorph of the fourth-chromosome gene cubitus 

 interruptus; hence, the fourth chromosome in these translocations must 

 also contain the wild-type allelomorph of this gene. Nevertheless, if 

 flies carrying the translocations are made heterozygous for cubitus 

 interruptus, the characteristics of this recessive gene sometimes show up, 

 indicating that the wild-type allelomorph of it has somehow been weak- 

 ened owing to the translocation. About one-half of the translocations 

 studied by Dubinin and Sidorov show this effect, it being apparently 

 immaterial which chromosome besides the fourth is involved in the 

 translocation. 



Muller (75), Patterson (105), Van Atta (136, 137), and Glass (43, 44, 

 45) found a series of dominant eye-color mutations in Drosophila melano- 

 gaster, most of which show patches of a differently colored tissue in the 

 eye. A most remarkable fact is that all of these dominant eye colors 

 are associated with some kind of chromosome rearrangements (inversions 

 or translocations). Two groups can be distinguished among these eye- 



