NEW DATA. 33 



we may take 300 as the number of the males which died. There 

 must have been, then, about 2 per cent of crossing-over, which makes 

 lethal I a lie about 2 units from lethal i. This location of lethal 

 IS is confirmed by a test that Miss Rawls made of the daughters of 

 the high-ratio female. Out of 98 of these daughters none repeated the 

 high sex-ratio and only 2 gave i 9 : i cf ratios. The two daughters 

 which gave i : i ratios are cross-overs. There should be an equal number 

 of cross-overs which contain both lethals. These latter would not be 

 distinguishable from the non-cross-over females, each of which carries 

 one or the other lethal. In calculation, allowance can be made for them 

 by doubling the number of observed cross-overs (2X2) and taking 

 98 — 2 as the number of non-cross-overs. The cross-over fraction 



—-7 gives 2.6 as the distance between the two lethals. Lethal 



ifl is probably to the right of lethal i at 0.7-1-2.6 = 3.3. 



SPOT. 



(Plate II, figures 141017.) 



In April 1 91 2 there was found in the stock of yellow flies a male 

 that differed from yellow in that it had a conspicuous light spot on the 

 upper surface of the abdomen (Morgan, 1914a). In yellow flies this 

 region is dark brown in color. In crosses with wild flies the spot 

 remained with the yellow, and although some 30,000 flies were raised, 

 none of the gray offspring showed the spot, which should have occurred 

 had crossing-over taken place. The most probable interpretation of 

 spot is that it was due to another mutation in the yellow factor, the 

 first mutation being from gray to yellow and the second from yellow 

 to spot. 



Spot behaves as an allelomorph to yellow in all crosses where the two 

 are involved and is completely recessive to yellow, i. e., the yellow-spot 

 hybrid is exactly like yellow. A yellow-spot female, back-crossed to a 

 spot male, produces yellows and spots in equal numbers. 



In a cross of spot to black it was found that the double recessive, spot 

 black, flies that appear in F2 have, in addition to the spot on the abdo- 

 men, another spot on the scutellum and a light streak on the thorax. 

 These two latter characters ("dot and dash") are very sharply marked 

 and conspicuous when the flies are young, but they are only juvenile 

 characters and disappear as the flies become older. The spot flies 

 never show the "dot and dash" clearly, and it only comes out when 

 black acts as a developer. These characters furnish a good illustration 

 of the fact that mutant gens ordinarily affect many parts of the body, 

 though these secondary eflFects often pass unnoticed. 



In the F2 of the cross of spot by black one yellow black fly appeared, 

 although none are expected, on the assumption that spot and yellow 



