BAStS 01= CHANGE 



ALLELO- 

 MORPH 



Wild -Type 



(+) 



Bar 



(B) 



Double- Bar 



(BB) 



SALIVARY 



CHROMOSOME 



REDUPLI. 

 CATION 



ORIGIN 



UNEQUAL 

 CROSSING OVER 



Average Number of Facets Per Eye in 

 Females Males 



PHENO- 

 TYPES 



BB/+45^56-f-/B 



25 36\ 68 

 BB/BB BB/B V B/B 



+ 738 



B 91 



BB 29 



THESE TWO TYPES HAVE THE SAME GENIC 



CONTENT. THE PHENOTYPIC DIFFERENCE IS 



DUE TO DIFFERENCE IN ARRANGEMENT 



The Position Effect 



Fig. 21. — The Bar gene, which reduces the size of the eye in Drosophila mclanogastcr , 

 is itself a duphcation for a segment of the X clironiosonic containing at least tour 

 distuict bands in the salivary chromosome. Unequal crossing-over in females 

 homozygous for the Bar duplication leads to triplication of the segment, which 

 thus gives the enlianccd eftcxt described as l^ouble Bar. Females heterozygous for 

 the wild type chromosome and Double Bar have the same total gene content as 

 females homozygous for Bar, though the genes are distributed differently between 

 the chromosomes. The eyes of these two classes of fly have different average sizes, 

 showing that the arrangement of the genes in the chromosomes has its effect on 

 their action. This is the Position Effect. 



108 



