CROSSINGOVER IN DROSOPHILA 151 



the base of the wing; were used in several experiments. The 

 order of these factors and the approximate percentage of cross- 

 ingover (corrected for double crossingover) is as follows: 



In addition to these second chromosome characters, some tests 

 have been made with a sex-linked — first chromosome stock — 

 vermilion-sable-garnet-forked. These results will be described 

 when the data are examined. 



The method used for testing the effect on linkage and cross- 

 ingover of the variations in the environment is very simple. 

 Virgin sister females from the normal wild stock known as 

 'Falmouth stock,' collected at Falmouth, Mass., were selected 

 and mated (in pairs) to males of the mutant stock to be used in 

 the experiment. The offspring of such pairs will be hetero- 



Star black-purple curve d spec k 



0-0 45.50.5 70. 100. 



<i 



1 I ' 



62.5 



vestigial 

 Fig. 1. Diagram of Chromosome II 



zygous for the mutant characters in question, and in the de- 

 veloping germ cells of the females hatching from these bottles 

 crossingover takes place. The variations in environmental 

 conditions are therefore applied while these Fi females are going 

 through their development, and any effect on crossingover will 

 be shown by the percentage of the different classes of the F 2 

 generation. The results of crossingover are best revealed by 

 back-crossing these F x females to males of the original mutant 

 stock. Controls of all experiments are also mated and are 

 generally sibs of the same Fi pairs tested. The cultures of both 

 control and experimentally treated females are kept under ex- 

 actly the same conditions and the females allowed to lay for a 

 uniform period of ten days, so that any differential effect of the 

 environment on the viability can not enter into the final result. 

 For the purposes of a more complete analysis this method was 

 varied slightly in later work, but the principle involved remained 

 the same. 



