LINKAGE 



387 



a chromosome from one parent bearing the genes for the three linked 

 characters, Y, W, and M, and a chromosome from the other parent with 

 the' genes for the three linked characters, G, R, and L. The first three 

 characters are allelomorphic to the last three respectively, and the two 

 chromosomes are homologous. 1 Upon breeding from the females of 

 these hybrids it is found that in the majority of cases (first column) the 

 F 2 flies show the same genetic consitution as do the grandparents (with 

 respect to these particular characters). This is taken to mean that the 

 two homologous chromosomes forming the synaptic pair and separating 

 at reduction have maintained their substance intact there has been 



DROSOPHIUA 



YEU.OW lelY N f C.KHY ojr 



WH.T. .... X { ~ 



nimoTnt WIHM J ^ LON WINI 



1361 2O89. 



FIG. 152. Diagram illustrating the evolution of new linkage groups through crossing over. 

 Explanation in text. (Adapted from Morgan.) 



no crossing over. In a certain number of cases (second column) new 

 groupings of the characters in question are observed in the F 2 flies : some 

 have Y, W, and L, while others show G, R, and M . This is interpreted 

 on the assumption that a break has occurred along a new plane (dotted 

 line) at a point of contact, so that at reduction some of the gametes, 

 and hence the F% flies to which they give rise, receive a chromosome with 

 Y, W, and L, while others receive G, R, and M . In a smaller number 

 of cases (third column) the new combinations YRL and GWM are formed 

 in a similar manner, the break and reunion occurring at another point. 

 In a very small number of cases (fourth column) the combinations 

 YRM and GWL appear, which can be explained on the basis of a double 



1 The hybrids have gray bodies, red eyes, and long wings, because G, R, and 

 L are dominant over Y, W, and M . (Ordinarily the genes have other designations.) 



