Exactness of Crossing Over 209 



Occasionally, however, crossing over is not so precise; the chief 

 exception to the rule is found at the locus of the so-called bar 

 "gene" (Fig. 65) in Drosophila melanogaster. The bar "gene" 

 is located between the loci of forked (/) and fused (fii). Occa- 

 sionally, when crossing over occurs between bar and one of these 

 genes, it is so inexact that the two bar "genes" are included in 

 the same chromatid, whereas the other chromatid has no bar 

 "gene" (Fig. 66a). Thus, if one chromatid has the genes forked, 

 bar, and fused, and the other has 

 nonforked, bar, and nonfused, one 

 of the crossover chromatids might 

 be forked, bar, bar, and nonfused, 

 and the other nonforked and fused. 

 Thus a new form, double bar, arises 

 which has the normally narrow eye 

 of bar even further reduced. At 

 the same time recessive wild-type 



(nonbar) flies appear even when the _ ^, 



. . , _ , .XI Fig. 65. The bar eve mu- 



origmal fly was homozygous for the ^^^^ ^^ Drosophila melano- 



dominant "gene" for bar. gaster. 



This situation for a long time 

 was very puzzling, for it seemed contradictory to the general 

 rule that crossing over is precise. With the discovery of the 

 salivary gland chromosomes, the bar locus could be examined 

 cytologically. It was then found that the bar "gene" was 

 not a gene, as we usually understand the term, but was the 

 result of reduplication of a small segment of one chromo- 

 some. The wild-type fly has this segment, but it is repre- 

 sented only once. In the bar fly, this segment is present 

 twice, and these two duplicate segments follow one another im- 

 mediately on the chromosome. Sometimes, in pairing, the lower 

 of the two segments of one chromosome happens to pair with 

 the upper segment of the homologous chromosome (Fig. 666). 

 A crossover will then produce a chromatid with three segments, 

 or double bar, and one that has only one segment, and is reverted 

 bar. Flies that are reverted bar are phenotypically like the 

 wild type. What had once been an unexplainable situation can 

 now be understood as caused by a chromosomal aberration. Not 

 only does this duplicated segment illustrate why crossing over is 



