28 AN ANALYSIS OF THE EFFECT OF SELECTION. 
1002 line contained a dominant plus modifier in the region of the 
rough locus.? 
There can be no question that the lines studied do differ in their 
constitution with respect to definite modifying genes that affect 
bristle number. In the case of the 1002 and 1331 lines there is at 
least one modifier, and probably two, located in different chromosomes. 
This gives the explanation of the increased variability observed in F, 
when these lines were crossed. The only other available explanation of 
that phenomenon—contamination of allelomorphs—has already been 
shown above to lead to complications in this case. (See also below.) 
Since it is both improbable and unnecessary, it may safely be dis- 
carded. 
TABLE 21.—9 Tests, Chromosome III. 
Source. Means. Distributions. 
Tested — 
against— Diff. 
Not-ro. ro. Not-ro. ro. Diff. PE. x? 
Sp. stock. 864 1331 |4.793+0.106/4.761+0.081| —0.032+0.133] 0.2 | 1.76] 0.60 
Pantie 1331 1331 |4.750+ .071/4.581+ .091)/+ .169+ .115) 1.5 | 1.09 .58 
Sp. stock. 1002 1331 |4.697+= .065|5.098+ .042)/+ .401+ .077| 5.2 |18.7 . 002 
Sp. stock.| cross-br. minus 1331 |4.323= .089/4.2764 .092)/— .047+ .127| 0.4 | 0.33 .999 
THIRD-CHROMOSOME LETHALS. 
Culture 1264, belonging to the third generation of the 1002 inbred 
plus line, produced, in the last 6 days it was counted, 60 Dichets and 
no not-Dichets. It seemed possible that one of the parents was 
homozygous for Dichet, so the line was continued. It was finally 
bred through about 18 generations, and produced 2,735 Dichets and 
only 4 not-Dichets. The 4 not-Dichets suggest the hypothesis that 
all the Dichets are really heterozygous as usual, but that they carry 
a lethal in the other chromosome that kills the not-Dichets.? That 
they are heterozygous has been shown by out-crossing them. When 
mated to Dichets of other strains the result was 211 Dichets to 103 
not-Dichets (4 cultures), the 2 : 1 ratio usually obtained when Dichets 
are mated together. When mated to not-Dichets the result was 207 
Dichets to 209 not-Dichets (6 cultures)—a normal 1:1 ratio. That 
there is a lethal in the stock has been shown by mating Dichets of 
this strain to Extended flies and inbreeding the not-Dichet offspring, 
which were found to carry a lethal as expected. (See below.) 
1 The second row of table 18 seems to contradict the conclusion that the 1002 and 1331 lines 
differed with respect to a modifier near rough. However, the experiment represents only a few 
flies, and did not give a significant result. Moreover, it was carried out before the 1002 line 
had been very long inbred (F's), and involved a not-rough chromosome from that line, which had 
not then become homozygous for rough. 
2 See Muller (1917) for a discussion of autosomal lethals. 
