322 INTRODUCTION TO CYTOLOGY 



Among simple translocations involving non-homologous chromo- 

 somes the first case known both cytologically and genetically was one in 

 which a portion of the X-chromosome was transferred to one of the long 

 chromosomes (III) in Drosophila.^^ As an example of this general class of 

 translocations may be taken one in which the "right" end of chromosome 

 III was transferred to chromosome II (Fig. 184). By crossing flies 

 having this translocation with normal ones, " hyperdiploid " flies were 

 obtained with a normal complement plus the translocated piece. Studies 

 _ on the linkage relations in these flies showed 



^ ^ % between what genes the original break had 



^^I^S f /^^^ occurred and also that the hyperdiploid flies 



^^^ ^ •^^ j]- possessed an excess of genes belonging to the 



^' V^ "right" end of chromosome III (duplication), 



as was expected (Painter and Muller, 19296). 

 Moreover, it is stated that in most such cases 

 of translocation the crossover frequency is 

 reduced in the limb of the chromosome in 

 which the break occurs and in one limb of the 

 receiving chromosome, whereas in each chro- 

 », mosome the limb beyond the spindle-attach- 



^ ^^^^ ment region is not so affected (Dobzhansky, 

 193 la6). In flies having an extra fragment 

 of chromosome II attached to the F-chromo- 

 some it has been observed that for some reason 

 2 crossover frequency is reduced between the 



Fig. 183.— Translocation of two normal chromosomes II (Rhoades, 1931c). 



portion of chromosome 11 to 



chromosome IV in Drosophiia. Many such translocations are known in 

 Shortened chromosome II plants. In Zea, where the miuute morphology 



marked jS. 1, metaphase in 1.0./ 



oogonium; 2, metaphase in of the chromosomes has been studied during 



neurocyte. {After Dobzhansky, ^J^g prophase, it is oftcn possible tO reCOguize 



translocated pieces at this stage with the 

 microscope directly, as well as genetically in altered linkage relations. 

 Thus in one case it has been shown that a translocation which at 

 first appeared to be simple was in reality reciprocal, one of the exchanged 

 pieces carrying only a single characteristic chromomere (or at most two) 

 and apparently being too small to complete the synaptic configuration 

 ordinarily distinguishing reciprocal translocations (Burnham, 1932). 

 This naturally' raises a question regarding other supposed simple 

 translocations. 



Certain translocations throw an interesting light upon the problem of 

 the forces concerned in meiotic disjunction. In Drosophiia it has been 

 reported that a pair of chromosomes, one of which carried a translocated 

 piece, did not assort at random with the pair from which the piece was 



1" Muller (1926). See Painter and Muller (19296). 







X 1 



