24 AN ANALYSIS OF THE EFFECT OF SELECTION. 



Dexter (1914) showed by similar methods that the chief gene for 

 the variable character beaded is in the third chromosome, and that 

 there is a modifier in the second chromosome. These results have 

 been verified by Muller (1917). 



In making such tests for modifying factors it is very desirable that 

 the test stocks, as well as the selected stocks that are to be tested, 

 should be homogeneous for modifying genes. This is desirable in 

 order that the tests may be repeated, and in order that results ob- 

 tained with different stocks may be compared. It was for this pur- 

 pose that the speck minus, or 1331, line of this paper was obtained. 

 (See above for history of this line.) This line, in the later generations, 

 was homozygous for the recessive mutants speck (chromosome II) 

 and sepia and rough (chromosome III). Since it had been inbred 

 and minus selected for several generations, it was probably uniform 

 in constitution with respect to modifiers. Since the other selected 

 lines also became homozygous for rough in later generations, it was 

 desirable to have a uniform not-rough line. For this purpose a pair 

 was taken from the speck stock, wild type in other characters. From 

 this pair a line was established, and continued by strict brother-sister 

 pair matings, in order to obtain a stock nearly or quite homozygous 

 for all its genes. This material is designated "speck stock." All 

 individuals from it that were used for test purposes came from 8 or 

 more successive brother-sister matings. 1 



Sex-linked modifiers would become apparent in Fi when two races 

 were crossed, since the males from reciprocal crosses would differ and 

 each type would resemble the maternal race. There is no clear evi- 

 dence of the existence of such modifiers in this experiment, so the sex 

 chromosome will be ignored in the discussion that follows. 



The method used for detecting second-chromosome modifiers is 

 as follows: Two lines are crossed, one of which contains speck, the 

 other not; one or both have Dichset (fig. 13). The Fi Dichaets are 

 then heterozygous for speck, and for any second-chromosome 

 modifiers in which the two lines were different. If an Fi male is now 

 mated to a speck not-Dicha^t female, there will be no crossing over 

 between speck and the modifiers. Therefore the not-speck Dichsets 

 produced will receive second chromosomes from their father which 

 will be identical with those present in the Pi not-speck race (in the 

 diagram the Dicha^t race), while the speck-Dichset back-cross 

 individuals will receive the second chromosome that came from the 

 other Pi race. Since the two types are alike in their third chromosome 

 constitution, and since they have been reared in the same culture bot- 

 tles, so that environmental influences were the same, any differences 



1 With the exception of culture 1737 (see Appendix), all were from 10 or more successive brother- 

 sister matings. All these specks were from the same Fs pair. All those before culture 2430 were 

 from the same Fs pair. All those after 2430 were from a different Fs pair, but were themselves 

 from the same Fn pair. Fu and Fie were mass cultures instead of pairs. 



