AN ANALYSIS OF THE EFFECT OF SELECTION. 23 
transference to a Dichet gene and must materially affect the result 
produced by that Dichet gene. 
The hypothesis that modifying genes are responsible for the result 
meets with no such complications. 
LOCATION OF MODIFYING GENES. ie 
The selection experiments recorded 
above have demonstrated that Dichet 
lines exist that are genetically different » 
with respect to bristle number. The “ 
cross between the 1002 plus line and 
1331 minus line showed that there is an 2 
increase in variability in F, when two 
such lines are crossed. Both these facts 
are consistent with the view that modi- 
fying genes, other than the Dichet gene 
itself, have influenced the bristle num- 
ber of Dichet flies. But it would also ,, 
be possible to interpret the result as 
due to variations in the Dichet gene it- 
self, and to contamination of that gene 
in crosses. (See above.) 
It has been pointed out by Muller and 
Altenburg (Morgan, Sturtevant, Muller 
and Bridges, 1915, p. 191), by Dexter “ 
(1914), and by Muller (1917) that there 
is a method of distinguishing between 2 
these two possibilities. The truncate 
race of Drosophila with which Muller 
and Altenburg worked is characterized 
= 
q 
1 2 3 4 5 6 7 8 
Fie. 12.—Results of crossing 1002 in- 
by shortened ‘‘truncated” wings. The 
race does not breed true for the trun- 
cate character, but the percentage of 
truncates produced and the degree of 
truncation shown are both capable of 
bred plus and speck (1331) minus 
lines. The P, curves represent the 
last few generations of each parent 
race. All four curves are reduced to 
the percentage basis; the ordinates 
represent percentages and the ab- 
scisse bristle numbers. 
modification by selection. Muller and 
Altenburg showed that this race contains a mutant gene in the second 
chromosome that is primarily responsible for the truncate character. 
By means of linkage experiments involving readily classifiable mutant 
characters they were able also to show that there are modifying factors 
for the truncate character in the first and in the third chromosomes. 
Furthermore, when the stock was by special methods kept uniform in 
constitution with respect to the truncate gene itself and with respect to 
these modifiers, selection was without effect. In this way the genetic 
variability of the race was shown to be due to modifying factors. 
