THE MECHANISM OF CROSSING-OVER. IV 



HERMANN J. MTJLLER 

 Columbia University 



The "map" of the first chr 

 experiments, is shown below : 

 y w« b ci 



The figures represent the distances of the factors from 

 yellow, the first one in the line, and are calculated merely 

 by adding together the intermediate distances. This map 

 gives almost exactly the same proportionate distances be- 

 tween the different loci as does that obtained by com- 

 bining the results of linkage experiments performed by 

 other workers, in which usually the inheritance of only 

 two or three factors was followed at one time. Each set 

 of ratios, therefore, confirms the accuracy of the other. 

 The absolute distances in the present map are, however, 

 somewhat shorter, being % the length of those in the com- 

 posite map. This was caused mainly by the comparatively 

 large number of non-cross-overs produced by a few fe- 

 males; in the rest, the crossing-over frequencies were 

 about normal. It may, therefore, be concluded that chro- 

 mosomes which differ in regard to eleven pairs of factors 

 behave in the same way, so far as crossing-over is con- 

 cerned, as those which are alike except for two factors. 

 This is contrary to a suggestion made by Punnett. More- 

 over, the fact that chromosomes differing in so many fac- 

 tors behave normally is here especially noteworthy, be- 

 cause 11 of the 12 recessive factors were in the same 

 chromosome. 



The results of the experiments with the second chromo- 

 some may now be tabulated. 462 offspring of females 

 421 



