ANANALYSIS OF THE EFFECTS OF SELECTION. 1 



INTRODUCTORY SUMMARY. 



The present paper describes a series of experiments aimed a1 de- 

 termining the causes of the variability in bristle number observed in 

 Dich«t, a mutant race of Drosophila melanogaster (ampelopki 

 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 2 . 



(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 diflferen- 

 already present; and that genes are relatively stable, not being con- 

 taminated in heterozygotes, and mutating only very rarely. 



DICH/ET. 



The mutant character known as Dichset 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, 19146). 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 

 "Dichset," thus showing the character to be dominant. 



Bridges's unpublished data show that the Dichaet gene is in the third 

 chromosome, approximately 5 units to the left of junk. 



The data published by Muller (1916) give the locus as 9.7 from Bepia 

 (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 My own (unpublished) data give: 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 Dich*t, ^- = 14.9 p. ct. Dichaet spineless, -— - 13.1 p. ct 



13b9 • ° ' 



U am indebted to Mr. J. W. Gowen for much advice and assistance in eonneotion with the 

 statistical treatment of the present problem. He has done a pari of the actual calculation*, 

 but is not responsible for any arithmetical slips, as I ha%-e myself done all tbfl Checktaf. 



