410 



Intrachromosomal Aberrations 



Another interesting prime type is PT94. When this is crossed 

 with PTl, the offspring show a circle of six chromosomes and 

 nine pairs. How could a circle of six be produced? If one inter- 

 change between 1-2 and 17-18 produced an interchange as 1-18 

 and 17-2, a second interchange between the new 1-18 chromo- 

 some and another one such as 13-14 would result in three new 

 chromosomes, 1-14, 13-18, and 17-2, no one of which would be 

 found in PTl. If this new type was then crossed with PTl, the 

 offspring would form a circle of six, as: 



If this hypothesis were true, PT94 would differ from PT2 by 

 only one interchange and should therefore produce offspring 

 with it that had only a circle of four. Actual crosses between 

 these two prime types result in hybrids that have only four chro- 

 mosomes in a circle. On the other hand, PT94 should then dif- 

 fer from PT3 by two translocations in three chromosomes and 

 another in a separate pair of chromosomes. Cytological observa- 

 tions of hybrids between these two types show one circle of six 

 and a separate circle of four. The chromosome ring configura- 

 tions in these hybrids are: 



14 14 



13 



11 



12 



13 



11 



18 21 



12 



22 



17 17 



18 



21 



22 



It is by crossing together all these prime types that the various 

 translocations that have arisen in them can be identified. It 

 happens that in Datura terminalization is practically complete 

 so that alternate chromosomes go to the same pole. Therefore, 

 these translocation heterozygotes are usually highly fertile, al- 

 though some combinations give about 25 per cent sterility and 

 the environment has a considerable effect on the fertility of 

 some of the hybrids. 



