AN ANALYSIS OF THE EFFECT OF SELECTION. 29 
The 4 not-Dichzt 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 Dichzt the lethal lies; for these flies are evidently cross- 
overs between Dichet 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, 
D’. D’ ss so. D’ so. | not-D’. not-D’ ss. | not-D’ so. 
1516 | 106 6 4 1 0 0 
2424 | 78 Dichets, some ss. 0 1 Ae 
2571 | 49 Dichets, with some D’ pe ss so. 0 0 1 
2851 | 41 Dichets, 1 not D’; other characters not noted. 
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: 
, / 
Ir118s e° lis: e° 
/ 
2424: ey D 
hiSs li18s 
D’' D’' 
a lip? s, ef Ip? s, e8 
Since there can be no crossing over in the male, there must in each 
case have been a crossover between D’ and l,, in the female. 1516 
indicates that l,,, is to the right of D’; 2424, to the left if the individual 
was a single crossover. But the distance D’ 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?, which is too short for 
a double crossover, therefore l,, is to the right of D’. The position 
of U,,, being thus obtained, the not-D’ produced by 2424 must have 
been a double crossover. 
The next problem is: How far from Dichet is the lethal locus? 
The mating is always— 
eet 
fm olin 
There being no crossing over in the male, the sperm are of two 
kinds only—D’ and l,,. The non-crossover eggs are of the same 
