ANANALYSIS OF THE EFFECTS OF SELECTION.’ 
INTRODUCTORY SUMMARY. 
The present paper describes a series of experiments aimed at de- 
termining the causes of the variability in bristle number observed in 
Dichet, a mutant race of Drosophila melanogaster (ampelophila). 
These experiments are discussed under several headings, as follows: 
(a) Selection of plus and of minus variants was carried out. Both 
plus and minus lines were obtained and were used in the further ex- 
periments. 
(b) A plus line and a minus line were crossed, and an increase in 
variability was observed in F;. 
(c) Linkage tests were made, and by this means it was demon- 
strated that modifying genes were present in the selected lines. 
(d) Evidence against the hypothesis of contamination of allelo- 
morphs was obtained. 
(e) This evidence, and that obtained by other investigators, is then 
utilized in a general discussion of the selection problem, and of the 
hypothesis of contamination of genes. The conclusions are drawn 
that selection is usually effective only in isolating genetic differences 
already present; and that genes are relatively stable, not being con- 
taminated in heterozygotes, and mutating only very rarely. 
DICHET. 
The mutant character known as Dichet was discovered by Dr. 
C. B. Bridges, July 3, 1915. In an experiment involving the sex- 
linked characters sable, forked, and cleft there appeared a single 
female that had wings extended and bent backwards near the base, 
like those of the mutant bent (Muller, 1914b). In addition it was 
observed that this female had only 2 dorso-central bristles, instead 
of the 4 usually present. When mated to a male having the mutant 
character eyeless, this female produced 48 normal offspring and 46 
“Dichet,” thus showing the character to be dominant. 
Bridges’s unpublished data show that the Dichzt gene is in the third 
chromosome, approximately 5 units to the left of pink. 
The data published by Muller (1916) give the locus as 9.7 from sepia 
(the locus farthest to the left of those as yet discovered), and 11.0 
from spineless, on the right. My own (unpublished) data give: 
Sepia Dichet, 208 = 14.9 p. ct. Dichzt spineless, 122 _ 13.1 p. ect. 
1369 931 
1] am indebted to Mr. J. W. Gowen for much advice and assistance in connection with the 
statistical treatment of the present problem. He has done a part of the actual calculations, 
but is not responsible for any arithmetical slips, as I have myself done all the checking. 
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