GENETIC CONTROL OF CELL INTEGRATION 



339 



Normal 

 (Bar-reverted) 



Double-bar 



Normal 



FIGURE 11.17. Comparison of chromosomal segments in Bar-eye and wild-type 

 Drosophila. The segment 16A of the X chromosome consists of a characteristic 

 banding pattern in the wild type, which is present twice in Bar-eye flies, and three 

 times in double-Bar (from Bridges, 1936, Science, 83:210). 



work of Sturtevant on Bar-eye, a dominant mutation on the X chromo- 

 some in Drosophila. Sturtevant showed that "mutations" from Bar-eye 

 (B) to normal (b) and to an extreme form called double»-Bar (BB) result 

 from unequal crossing-over events. In a cytological study, Bridges found 

 that the Bar phenotype results from repeats of the Bar region; it is 

 present once in the normal, twice in Bar-eye, and three times in double- 

 Bar as shown in Figure 11.17. Sturtevant compared the phenotypic 

 effects of different numbers of repeats, (including another allele called 

 infra-Bar), when located on the same and on homologous chromosomes. 

 He counted the numbers of facets in the eye and found: 



Genotype Facets per Eye 



