146 



ARE DEFICIENCIES DUE TO UNEQUAL 

 CROSSING OVER? 



The "section-deficiencies" described 

 by Bridges (1917, 1919) and by Mohr 

 (1919, 1923 b) are probably to be in- 

 terpreted as due to losses of definite 

 sections of chromosomes. It will be 

 observed that bar reversion has here 

 been treated as due to the loss of a 

 very short section; it may accordingly 

 be described as a deficiency that is 

 too short to show the lethal eff^ect and 

 other properties of the previously de- 

 scribed deficiencies. When the case is 

 stated in this way, the question at 

 once arises: is it probable that notch 

 and other deficiencies have also arisen 

 through unequal crossing over? If so, 

 the contrary crossover should be a 

 chromosome that was double for a re- 

 gion corresponding to the deficient 

 section. Such a chromosome has never 

 been identified, but it may be doubted 

 if it would be detected even if present. 

 Furthermore, it might well be lethal 

 even in heterozygous females, in which 

 case it would not be capable of detec- 

 tion. 



There is evidence that deficiencies 

 may arise in other ways than by un- 

 equal crossing over. In at least one 

 case (Bridges and Morgan 1923) the 

 section missing from a second chro- 

 mosome was found to be present, but 

 attached to a third chromosome. In 

 this case, then, the deficiency can not 

 have been due to unequal crossing 

 over. The first deficiency described, 

 that for forked and bar (Bridges 1917), 

 occurred first as a single female that 

 had obtained the deficient X from her 

 father. Here the deficiency arose 

 (either in a male or very early in the 

 cleavage of a female zygote) at a time 

 when crossing over (and bar rever- 

 sion) does not normally occur. In the 

 case of notch, also, there is evidence 

 that the deficiency may originate at 

 stages other than maturation. Lance- 



STURTEVANT 



field (1922) records the occurrence 

 of a notch (probably corresponding 

 to that of D. melanogaster) in Dro- 

 sophila obscura; the mutation was first 

 detected as two females from a pair 

 mating that gave numerous ofi^spring. 

 In this case the deficiency must have 

 originated in the gonial cells of one 

 parent, unless the two notch females 

 received their notch chromosomes 

 from the father, in which case it is 

 just possible that they came from two 

 sperms derived from a single sperma- 

 tocyte. But in this case the hypothesis 

 of unequal crossing over remains as 

 improbable as before. I have observed 

 two cases in D. melmiogaster that 

 represent "somatic" (i.e., not occur- 

 ring at the maturation divisions) oc- 

 currences of notch. In one case three 

 notch females were produced from a 

 single mother. The X's of the mother 

 were attached, and the notch daugh- 

 ters, like all their sisters, did not carry 

 a paternal X. These three females were 

 all sterile, so here it was not possible 

 to demonstrate that the new type was 

 actually notch; but the numerous 

 characters of notch make the iden- 

 tification very probable. The other 

 case also occurred in a line in which 

 the females all had attached X's. A 

 female, from a line with no notch 

 ancestry, was notch in the left wing 

 but not in the right. The offspring 

 showed that this female was, like her 

 mother, heterozygous for several sex- 

 linked genes. These included scute, 3 

 units to the left of notch, and cross- 

 veinless, 10 units to the right of it. 

 Some of the eggs of the mosaic carried 

 notch, but many of them did not. 

 Furthermore, tests showed that scute 

 and crossveinless were in opposite 

 chromosomes in both types of eggs; 

 that is, the mutation to notch occurred 

 at a cleavage division, and was not 

 accompanied by crossing over between 

 scute and crossveinless. From these 

 three instances we may conclude that 



