INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1179 



cases. This is not so. Hence, either these translocations are simple, or 

 the Y is especially likely to break very close to one of its ends. Since the 

 discovery by Patterson, Stone, Bedichek, and Suche (109) that chromo- 

 somes are likely to break near the free ends, this argument loses some of 

 its force. 



The possibihty of the so-called "lateral attachment" of chromosome 

 fragments suggests also that simple translocations do occur. By lateral 

 attachment is meant an attachment of a fragment not to an end of a 

 broken or an unbroken chromosome, but to its side. Lateral attach- 

 ments lead to formation of branched (Y-shaped) chromosomes. The 

 existence of a lateral attachment is conclusively proved genetically in one 



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s 





ens 



ru 



5t 



ru 



f i 



I I 



5t 





M M 



I cot I 

 N N 



sp 

 SI JlHcci! 



mIsl tit 



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Fig. 3. — A, the second chromosome (stippled) and the third chromosome (black) of a 

 normal Drosophila melanogaster; B, the second and third chromosomes of an individual 

 carrying a translocation in heterozygous condition; C, same of an individual homozygous 

 for the same translocation. Below — chromosome plates of each of these three kinds of 

 individuals. 



of the translocations in Drosophila (Sturtevant's ^'-translocation — 

 Dobzhansky and Sturtevant, 36), and has been observed cytologically in 

 Allium (Levan, 57). Translocations involving lateral attachments can 

 be reciprocal only if a very short section of the recipient chromosome is 

 excised interstitially, and both the free end of the recipient chromosome 

 and a fragment of the donor chromosome are attached to the broken end 

 thus made available. An excision of a chromosome section and an inter- 

 calation of a section of another chromosome in its place have, to be sure, 

 been observed in Drosophila (Oliver, 88, and an undescribed translocation 

 studied by Sturtevant, which has the chromosome structure shown in 

 Fig. 3). 



