120 Nature of the Genetic Material 



aa. The Bar case 



The name "position effect" was originally given by Sturtevant 

 (1925) in his classic paper on unequal crossing over of the Bar 

 "gene," a dominant mutant reducing the eye facets of Drosophila. 

 By unequal crossing over this could double in one chromosome and 

 thus quadruple in the homozygous condition (BB/BB) the pheno- 

 typically extreme, double, or ultra Bar. Another allele, infra Bar, 

 could do the same. The effect of these combinations followed in a 

 general way the dosage of 1, 2, 3, 4 Bar genes. (The details will be 

 discussed in a later chapter on genie dosage.) Moreover, it made a 

 considerable difference whether, in the case of two doses, these 

 were heterozygous double Bar (BB/+) or homozygous Bar (B/B), 

 since the former has more of an inhibitory effect on the formation of 

 facets, which is the Bar effect. Sturtevant expressed this not as we 

 did just now, but by saying that two Bar genes in one chromosome 

 have a greater action than the same two located in two chromo- 

 somes. Thus the position of the Bar genes made a difference which 

 might mean in a general way the location in the chromosomes (one 

 or two) or, more specifically, a different neighborhood, namely, a 

 "foreign" gene on both sides of B in the case of B/B, but on 

 only one side of each B in the case of BB/+. This was the original 

 meaning of the term "position effect," location of a gene in different 

 neighborhoods. 



It is important to see clearly that this interpretation, which was 

 justified in its time, does not cover our present knowledge of so- 

 called position effect. This is necessary at the outset because the 

 old interpretation has come up again in important recent work 

 which we shall have to analyze below. We now know many cases 

 of "position effect" in which the phenotype of a locus at some distance 

 from a break appears. Here the locus has remained in its normal 

 chromosomal neighborhood, but is nevertheless changed in its action 

 by a break at some distance. This shows clearly that it is not the 

 direct position of a locus that is involved but its relation to a dis- 

 turbed order within the chromosome nearby. We shall soon see 

 that the classic Bar case also is included in this statement. We 

 should forget, in analyzing the phenomenon, the position of an 

 assumed gene, and think only in terms of chromosomal breaks and 

 changes of order within the chromosome. Only thus will a unified 

 understanding of the entire group of facts be possible. 



