56 
GENETICS: F. PAYNE 
extra bristles were mated. This method of selecting the high grade parent 
has been continue4 for 38 generations. A rise in the per cent of extra bristled 
flies and also in the mean bristle number has been produced until in the last 
generations of selection no normal flies were found and the mean number of 
bristles in the twenty-ninth generation was ,9.084. From the twenty-ninth 
to the thirty-eighth generation the mean remained practically the same. 
The highest bristle number found in any one individual fly was 15. 
I cannot give details, but I think these figures are sufficient to show that 
selection has been effective. How then has it been effective? Have the re- 
sults been produced by selecting merely somatic variations? Have they been 
produced by selecting the variations of a single gene, or have they been pro- 
duced by getting rid of or by piling factors? The possibility that selection 
can act upon somatic variations, I believe, can now be dismissed without 
much consideration. Practically every one admits that variations must be 
germinal in order to be inherited. As to the second and third possibilities, 
my evidence is in favor of the third. 
In the selection line the bristle number in the females is slightly higher 
than in the males. This indicated that there might be a sex-linked factor 
present, which when homozygous produced a different effect than when hetero- 
zygous. An experiment was devised to eliminate the X-chromosome but 
retain the others. This was done as follows: A male from the high selection 
line was mated to a bar female (bar is a sex-linked dominant). The Fi males 
will get their X-chromosome from the bar female and hence will be bar. Of 
the other three pairs of chromosomes, one of each pair will come from the 
high selection line and the other from the bar line. Some of these Fi males 
have extra bristles. These were mated to bar females from stock. All flies 
from this cross will get their X-chromosomes from the bar line. Some of 
them may get one member of each of the second, third, and fourth pairs from 
the high selection line. Since extra bristle number is partially dominant to 
the normal, such flies might have extra bristles if there are any genes for 
extra bristles in the second, third; or fourth chromosomes. Some of the flies 
from this cross did have extra bristles. They were mated and the line has 
been inbred to see whether the bristle number could be increased until it 
reached the mean in the high selection line. If so, then the X-chromosome 
could carry no gene for extra bristles. If the mean could not be raised, then 
it would indicate that the X-chromosome carried such a gene. Five gen- 
erations of inbreeding has failed to raise the mean above 5.2. 
Further, by crossing the high selection line to eosin, miniature and to 
eosin ruby forked (sex-linked characters), and mating the Fi females back to 
the recessives, it comes out clearly that a gene influencing bristle number is 
located in the X-chromosome, somewhere near eosin. Of the cross-overs 
between eosin and miniature and between eosin ruby and forked, miniature 
and forked flies have a higher bristle number than the eosin, and eosin ruby. 
