Position Effect 265 



of the two homologues as in the normal female. If it has two 

 segments in one chromosome, it has bar eyes. It may be homo- 

 zygous or heterozygous. If homozygous, it will have four of 

 the chromosomal segments, two in each homologue. The double 

 bar type has three such segments in succession in one chromo- 

 some, whereas the homologous chromosome of the female may 

 have only one. Such a fly has a total of four of these similar 

 segments, just as the homozygous bar, but it has three in one 

 chromosome and one in the other instead of two in each. The 

 phenotypic effects of the two types of distribution of these 

 segments are very different, for the eye of the double bar fly 

 is much narrower than the bar's. In other words, the arrange- 

 ment of such segments in an individual, not their number, is 

 the important consideration. Three segments following one an- 

 other on one chromosome have a very different effect from two 

 segments. Similarly, a bar-eyed male has two segments on the 

 X chromosome and none on the Y. Its eyes are much smaller 

 than a wild-type female's, even though the female has just as 

 many segments, for the two segments of the normal female are 

 on separate chromosomes. This relation of the segments at 

 the bar locus indicates that at least sometimes the effect of a 

 gene ma)^ be determined in part by the place it occupies in 

 relation to the other genes on the chromosome. In other words, 

 its effect may be influenced by its position in relation to other 

 genes. 



Another example of the position effect is shown by the hairy 

 wing mutant in Drosophila melano g aster . Like the bar eye situ- 

 ation, this mutant is also the result of the duplication of a small 

 part of a chromosome; but whereas the bar eye duplication in- 

 volves a segment of the X chromosome long enough to include 

 six bands, hairy wing is a duplication of only one band lying 

 very near the tip of the left end of the X chromosome. Flies that 

 show the hairy wing character have extra bristles along the veins 

 of the wings and on the head and thorax. Demerec and Hoover 

 showed that normal, wild-type females (+/+) have just one 

 band at that region on each X chromosome and have no extra 

 hairs, whereas males with one such band on their single X chro- 

 mosome also have no extra hairs. Heterozygous hairy wing 

 females which have two bands on one X chromosome and just 



