AN ANALYSIS OF THE EFFECT OF SELECTION. 



29 



The 4 not-Dichset flies produced by uncrossed descendants of 1264 

 appeared in cultures 1516, 2424, 2571, and 2851. Since most of the 

 flies of this line are heterozygous for other factors in chromosome III, 

 it should be possible by an examination of these 4 flies to determine on 

 which side of Dichset the lethal lies; for these flies are evidently cross- 

 overs between Dichset and the lethal, and should show certain rela- 

 tions with the other characters, depending on the locus of the factor. 



The 4 cultures in question gave the results shown in table 22 (both 

 parents in 1264 being rough, all these flies are rough). 



TABLE 22. 



Since the other characters were not noted in 2851, that bottle is 

 useless for our present purposes. The constitution of the parents in 

 the other three cultures must have been as follows : 



1516: 



D' 



hus, e* 

 2424: ^ 



2571: 



lmp p s a 



Since there can be no crossing over in the male, there must in each 

 case have been a crossover between D' and l ni in the female. 1516 

 indicates that l m is to the right of D'; 2424, to the left if the individual 

 was a single crossover. But the distance D' s s , here involved, is 

 known to be long enough so that double crossovers sometimes occur 

 in it. In 2571 the distance involved is D' p p , which is too short for 

 a double crossover, therefore l m is to the right of D'. The position 

 of l m being thus obtained, the not-D' produced by 2424 must have 

 been a double crossover. 



The next problem is: How far from Dicheet is the lethal locus? 

 The mating is always 



X 



lm lm 



There being no crossing over in the male, the sperm are of two 

 kinds only D' and l m . The non-crossover eggs are of the same 



