THE ORIGIN OF VARIATION 



genes near Bar, Sturtevant and Morgan showed that this simultaneous mu- 

 tation from Bar to Double-Bar and to normal was based upon unequal 

 crossing over. Thus, while each chromosome of the pair ought to contain 

 the Bar gene, after the unequal crossing over, one chromosome had two 

 Bar genes in tandem, and the other chromosome had none at all. Those 

 zygotes which got the chromosome with no Bar gene were wild type, 

 while those having the chromosome with the two Bar genes showed the 

 Double-Bar phenotype. It appeared then, that two Bar genes in the same 

 chromosome caused a different phenotype than did two Bar genes located 

 one in each of a pair of chromosomes ( Double-Bar and Bar, respectively ) . 

 To describe this unexpected phenomenon, it was Sturtevant and Morgan 

 who introduced the term position effect. 



The analysis of the Bar "gene" could be completed only after the intro- 

 duction of the salivary gland chromosome technique. It turned out that 

 there is, in the region of the X chromosome which is indicated by cross- 

 over tests to include this gene, a series of about six bands which is present 

 only once in wild-type flies, but twice and three times in Bar and Double- 

 Bar flies, respectively ( Figure 81 ) . It appears, then, that the Bar "gene" 

 is actually a duplication for a small segment of the X chromosome. If 



16A 



16 A 1.2 



-> 



WILD TYPE 



WILD TYPE 



Figure 81. The Bar "Gene" in the Salivary Gland Chromosomes of Drosophila. 

 (After Bridges and Sutton in White, "Animal Cytology and Evolution," 2nd Ed., Cam- 

 bridge University Press, 1954. ) 



220 



