Vol. 7, 1921 
GENETICS: C. B. BRIDGES 
187 
(1) A definite complex of mutant characters: Diminished differs so- 
matically from the wild-type in many characters, among which the principal 
are: smaller size throughout; bristles relatively shorter and more slender; 
body-color paler; thorax-pattern darker; eyes larger and roughish, some- 
what denuded of hairs ; aristae reduced or absent; wings slightly spread, 
blunter, and less clear in texture. 
(2) Low productivity. 
(3) Frequent stirility. 
(4) Dominance. 
(5) Heavy mortality: Outcrosses of "heterozygous" Diminished to 
wild- type gave an extremely variable proportion of Diminished; the bulk 
of the cultures ranged from 20 to 50 per cent, with a mean value of 34.5 
(1772 Dm : 3366 wild-type) . Since the Diminished offspring were expected 
to be as numerous as the wild-type, the mortality of the Diminished is 
very high, viz. 47%. 
(6) Delayed hatching: Diminished adults begin to emerge from the 
pupa cases four or more days later than wild-type sibs. The delay is ap- 
parently due to a slow metamorphosis. 
(7) Lethal when "homozygous:" When "heterozygous" Diminished 
was inbred there were 139 Diminished to 100 wild-type (1079 : 775). The 
expectation for such inbreeding, on the basis that "homozygous" Dimin- 
ished is lethal snd that mortality of the "heterozygous" Diminished 
is the usual 47%, is 106 : 100, which is somewhat under the observed 
ratio. The mortality of Diminished is extremely variable, and a mortal- 
ity of 31% would give the observed ratio. Further evidence in favor of 
the lethal view is the fact that continued inbreeding did not increase the 
percentage of Diminished; also no Diminished individual was found that 
gave all Diminished offspring upon outcrossing. 
(8) Inheritance as member of the fourth chromosome : 
(a) The Diminished mutant is not due to a change in the first, sec- 
ond, or third chromosomes : Reciprocal crosses gave like results, and out- 
crosses of Diminished male to wild female gave Diminished offspring among 
the sons, so the Diminished is known to be neither cytoplasmic nor sex- 
linked in inheritance. Male back-cross tests with such well-known second- 
chromosome mutants as Star and black, and third-chromosome mutants, 
such as Dichaete, Hairless, pink and ebony, showed free assortment of 
these mutants with Diminished. Since there are only four chromosome 
pairs in D. melanogaster the above evidence proves that Diminished is 
caused by a change in the remaining or "fourth" chromosome. 
(b) That Diminished is due to a fourth-chromosome change might be 
proved by means of male back-cross tests with the known fourth-chromo- 
some recessive mutants bent and eyeless. But when crosses of Dimin- 
ished to bent and to eyeless were made in order to secure the necessary 
