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PHYSIOLOGICAL GENKTKS 



such an inquiry necessary arc grouped around a phenomenon 



that -rather unfortunately — has been called position effect. 



A. The Position Effect 



This phenomenon was discovered by Sturtevant (1925). We 

 have reported in different chapters the details of the Bar-eye case 

 in Drosophila. Bar is a duplication of a small segment in the 

 X-chromosome. By unequal crossing over, two Bar genes (if we 

 continue using this term for descriptive purposes) may come 

 together in one chromosome; when homozygous, this is called 

 Double-bar or Ultrabar. At the same locus there is a mutation 

 Infrabar, which also may be obtained by unequal crossing over as 

 Double-infrabar and together with Bar as Bar-infrabar. We 

 have analyzed the quantitative effects of all these combinations 

 expressed in facet numbers and found them proportional to the 

 number of genes present. A combination B/B and a combination 

 BB/ have both two Bar genes and ought to have the same 

 effect. Sturtevant found it to be different (see Table 22). 



Table 22 

 (From Dobzhansky) 



This means that two Bar genes in the same chromosome 

 prevent facet formation in a higher degree than do the same 

 genes in different chromosomes. This might also be expressed 

 thus: Two adjacent Bar genes reinforce each other's action, as 

 compared with two opposite Bar genes. The position of the 

 gene has an influence upon its action. 



This effect came into the foreground of genetical interest when 

 chromosome breaks, translocations, inversions, and such rear- 

 rangements of chromatin material were obtained by X-raying 

 and studied extensively. Other cases of this effect came to light, 

 and the same term was applied to a number of related phenomena. 

 We may arrange the facts in groups which are not very natural 

 and are only provisional, as new facts are frequently being 

 discovered. Very probably the different groups will turn out to 



