34 AN ANALYSIS OF THE EFFECT OF SELECTION. 



' Matings of intermediates by intermediates gave both intermediates 

 and normals, showing that the character was either dominant or irreg- 

 ular in appearance. 



Matings of intermediate to specks and to black purples of other 

 stocks gave only normals, showing the character to be recessive. 



Mating together the Fi normals from the last type of matings gave 

 a few intermediates; but these were in no case speck or black or purple. 

 This is the usual behavior of a second-cliromosome recessive, due to 

 no crossing over in the Fi male. Hence ''intermediate" is a recessive 

 character, and lying in the second chromosome. Its occurrence in 

 the Star Dichset must have been only a coincidence, and can have had 

 nothing to do with the presence of Dichset in that stock. The differ- 

 ence between this character and Extended is a striking illustration 

 of the danger of arguments as to the identity of characters based on 

 similarity of appearance. 



NOT-DICHyETS FROM SELECTED LINES. 



As has already been pointed out, Dichset flies almost always have 

 fewer bristles than have normals. All Dichsets are heterozygous for 

 the normal allelomorph. Therefore, in such an experiment as this 

 one, in which Dichsets are repeatedly mated together, one obtains 

 normal flies the not-Dichset genes in which have been associated with 

 Dichset genes for many generations. The experiment is, then, suited 

 for a study of the question as to whether or not factors "contaminate" 

 their allelomorphs. If this contamination occurs, one might expect 

 the not-Dichset flies to show a tendency to have fewer bristles than 

 they normally have, and the Dichsets to have more. That Dichsets 

 tend to increase in bristle number is very improbable. The stock 

 has now been kept, always of necessity in heterozygous condition, for 

 more than 40 generations. There is no evidence that any progressive 

 change has occurred, though no selection has been used in keeping 

 the stock cultures. The modal class at present (5 bristles) is actually 

 lower than the class (6) of the original mutant.^ 



There are some data regarding the bristles of the not-Dichaets pro- 

 duced by selected Dichsets. Counts of these bristles have been taken 

 only occasionally (see table 24), but whenever a bristle number other 

 than 8 has been observed in such flies it has been noted on the record 

 sheet. Examination of these notes shows that in the minus-selected 

 series there are several records of 6 and 7 bristled not-Dichsets, but 

 none of numbers higher than 8. In the plus selected lines there are a 

 number of records of nines and tens, but no sixes and only 1 seven 

 (from 1190, an Fe of the crossbred plus series). The complete counts 

 taken of bristle numbers are given in table 24. 



^It may be pointed out that the familiar yellow mouse and several similar cases in Drosophila 

 afford evidence of the same sort against contamination. 



