246 GENETICS 



tion and gave 4.7%, or the sum of the other two per- 

 centages, as shown in Figure 75. 



If upon breeding yellow and bifid a percentage of 

 2.3% had been obtained instead of 4.7% as was ac- 

 tually found, then the order of the genes would have 

 been yellow-bifid-white instead of yellow-white-bifid. 



In the eloquent frontispiece of The Mechanism of 

 Mendelian Heredity, by Morgan, Sturtevant, Bridges 

 and Muller, there are drawn four straight parallel lines 

 representing the "chromosome maps" of Drosophila 



yellow 







white 4.7% 



9 



bifid' 



FIG. 75. An illustration of the proof of gene-localization from 

 cross-over percentages obtained by breeding. 



as known in 1915. It is doubtful if in any book there 

 may be found four straight lines that mean so much. 

 The work of gene-localization is quite comparable to 

 that done by mathematicians and astronomers in deter- 

 mining the distances that separate the stars in the 

 heavens from each other and is perhaps equally in- 

 comprehensible to the layman. In gene-localization it 

 is the infinitely small instead of the infinitely great 

 that one must observe. When it is remembered that 

 Drosophila is a very tiny fly; that occupying only a 

 small part within its abdomen are paired reproductive 

 organs; that each of these reproductive organs in the 



