CHROMOSOMES AND MEN DELI AN HEREDITY 



305 



none of these cases was the distribution of genes followed as in Drosophila 

 and Zea. 



The behavior of knobbed chromosomes in Zea has served further to 

 show directly that in a diploid meiocyte an exchange may occur between 

 only two of the four chromatids in a given level (Creighton and McChn- 

 tock, 1932). In the plant in question there was a chromosome having 



Goimetes 



2ygo+e 



■ ■ ■ • 

 • !!• 



Or 



■ I ii 

 • III 

 II II 

 1 1 II 



Non- cross-over 



V 





*'^.»-:5 



Cross -over 



Synoip+Ic Configura+ion La+e Prophoise 



Fig. 175. — Diagram of cytological proof in Zea that crossing-over involves two of the four 

 chromatids. For explanation, see text. {From Creighton and McClintock.) 



a large knob at one end and a translocated piece at the other, and a 

 homologous partner with a small knob and no extra piece (Fig. 175). 

 While these chromosomes remained attached at the ends after opening 

 out from their laterally synapsed position, ^^ it could be seen that in some 

 of the sporocytes one small-knobbed chromatid had exchanged portions 

 with one large-knobbed chromatid. That it was not whole chromosomes 

 that had exchanged places was shown by the presence of the translocated 

 piece and the synaptic relations in the whole complex. For some time 

 " Thus forming a ring with two other chromosomes because of a reciprocal trans- 

 location between non-homologous pairs, as explained in Chapter XIX. 



