AN ANALYSIS OF THE EFFECT OF SELECTION. 51 



arising in the hooded factor, then the "mutant" variation is evidently 

 the only case of that sort that Castle has reported. 



GENERAL CONCLUSIONS. 



That many characters may be influenced by more than one pair of 

 genes has long been recognized, and this is the essence of the multiple- 

 factor view. That genes exist which require the action of other genes 

 before they produce visible effects has also been long known. Further- 

 more, that there are genes which produce very slight visible effects 

 is now another commonplace. Given these three facts, and the 

 hypothesis (which is supported by much specific evidence) that most 

 races are heterozygous for a number of such genes is all that is re- 

 quired to complete the conception that is held by most adherents of 

 the view that multiple factors or modifying genes are responsible for 

 the results of selection. 



In specific cases, the existence of definite modifying genes has been 

 demonstrated by Dexter, Bridges, Muller and Altenburg, and the 

 author. All other data in question fit in with the view that selection 

 ordinarily acts only by isolating modifiers. 



Modification of factors by selection, crossing, fractionation, or 

 similar means is undemonstrated in any given case, and has been 

 shown not to occur in other cases that are typical of the results usually 

 obtained. Factors do change, and more than two forms are possible 

 for certain loci; but there is no known method of inducing such changes, 

 and they are ordinarily quite rare and definite. 



SUMMARY. 



(1) Dichset is a dominant character, the gene being lethal when 

 homozygous (yellow-mouse case). The gene is in the third chromo- 

 some. 



(2) Dichset flies are more variable in bristle number than are not- 

 Dichsets. This variability is partly environmental, partly genetic. 



(3) Selection was effective in isolating both plus and minus Dichset 

 lines. 



(4) A cross between two separate inbred plus lines gave no increase 

 in variability and no increase in parent-offspring correlation. There- 

 fore the two lines were presumably of very similar constitution, though 

 independent in origin. 



(5) A cross between an inbred plus line and an inbred minus line 

 gave the results characteristic of such crosses — increased variability 

 in F2 and increased parent-offspring correlation. 



(6) Linkage tests demonstrated that modifying genes exist in the 

 selected lines. Several lines were shown to differ in one or more sec- 

 ond-chromosome modifiers, and at least one of these modifiers was 

 shown to cross over from the speck gene. 



