Voi,. 7, 1921 
GENETICS: C. B. BRIDGES 
191 
bent and for eyeless were crossed to Diminished males, and the offspring 
furnished the equivalent of 17 crossovers in a total of 1987, or 0.86 =>= .14 
per cent of crossing over. 
FIG. 1 
An oogonial metaphase plate from each of eight "diminished" individuals, two from 
each of four strains, all figures showing a single round chromosome instead of the 
usual pair. 
V. The cytological finding in the case of the haplo-fourths has put the 
theory of deficiency on a sound basis ; for, each of the features met with in 
the cases described as deficiency has been met with in the case of the 
haplo-fourths. Thus, it has been shown, not simply that these character- 
istics might be produced by loss, but that in this specific case they were 
actually produced by loss. 
VI. Both the character complex and the exaggeration effects of Dimin- 
ished are due to a change in balance of the genes. Each character of the 
wild-type is the result of a balance between many genes that tend to mod- 
ify the character in various directions. These "plus and minus modifiers" 
are apparently distributed at random in the chromosomes, so that if a 
particular short section of chromosome is removed, as in deficiencies, usually 
more plus than minus modifiers (or vice versa) of some character will be 
removed. In that case more minus than plus modifiers will remain in 
action, and a new character will appear, differing from the old in a minus 
direction. 
VII. The genetic and cytological evidence in the case of Diminished has 
provided a positive identification of a particular chromosome — the small 
round one — as the carrier of the genes of particular Mendelian characters — 
bent and eyeless. Non-disjunction of the sex-chromosomes has previously 
furnished a similar identification of the rod-like chromosome as the 
carrier of the genes of certain sex-linked characters. Further work on non- 
disjunction of the X-chromosome has extended the identification to in- 
clude so many sex-linked characters without exception that it may be ac- 
cepted as general. Since the behavior of non-sex-linked characters had 
been shown to be the same as that of the sex-linked characters in a large 
range of particulars — in all points except those connected with the fact 
that the male is virtually haploid for this chromosome — the demonstration 
