Translocations 



409 



2 and 18 had exchanged places. Because PT2 is homozygous 

 for both these translocation chromosomes, it shows nothing but 

 twelve pairs of chromosomes. By an observation of the chromo- 

 somes or of the meiotic divisions of these two types by them- 

 selves, there is no indication that they differ by a reciprocal 

 translocation. It is only the hybrid that reveals this informa- 

 tion, for the hybrids are heterozygous for the translocation and 

 form the circle: 



18 



18 



17 



17 



Another prime type is PT3. This type also has nothing but 

 paired chromosomes and therefore is homozygous for any in- 

 versions that it might include. When it is crossed with PTl, it 

 also forms one circle of four in the hybrid, showing that it, also, 

 differs from PTl by one interchange. The interchange that dif- 

 ferentiates PT3 from PTl may or may not be the same inter- 

 change that differentiates PT2 from PTl. The only way to 

 determine this is to cross plants of PT2 and PT3. When it is 

 done, the hybrid shows two circles of four chromosomes. If both 

 PT2 and PT3 differed from PTl by the same interchange, their 

 chromosomal segments would have a similar arrangement and 

 the hybrid between PT2 and PT3 would show nothing but paired 

 chromosomes. The fact that two circles are formed shows that 

 they differ by two interchanges involving four chromosomes. 

 Thus PT2 differs from PTl by having 1-18 and 2-17 instead of 

 1-2 and 17-18 chromosomes, whereas PT3 differs from it by 

 having two chromosomes that are 11-21 and 12-22 instead of 

 11-12 and 21-22. When PT2 and PT3 are crossed, the chromo- 

 somes in the hybrid pair as: 



11 



11 



12 



12 



21 



21 



22 



22 



