GENES—STURTEVANT 
tween flies from these strains show at 
once that they fall into two, and only 
two, types. If flies from two forked 
strains are crossed, the offspring are 
all forked; if flies from two singed 
strains are crossed, the offspring are 
singed, but if a forked strain is crossed 
with a singed one the offspring are 
“‘wild type.”’ When flies from a series 
of such types are obtained and exam- 
ined, a difference does appear; the 
females are sterile and lay abnormally 
shaped eggs in many of the singed 
types, but never in the forked ones. 
Similar observations have been made 
on other species. There are now seven 
species of Drosophila in which two such 
types are known, and in no species 
have more than two been found. 
Furthermore, in several forms one 
type—evidently singed—has been 
found to be sometimes associated with 
female sterility. There can, then, be 
no serious doubt that in these species 
the same two “wild type” genes are 
present and have similar effects on the 
development of bristles—even though 
the test of crossing to known D. melano- 
gaster types is here not possible. 
A frequently occurring mutant 
change in D. melanogaster is that which 
results in a completely white eye. 
There are many genes involved in the 
production of the red eye of the “‘wild 
type,” but only one is known to cause, 
when it mutates, a wholly colorless 
eye. There are other ways of produc- 
ing white eyes, but these require 
changes in more than one gene. The 
same gene of the “‘wild type” that may 
mutate to give a wholly white eye is 
also subject to other changes that give 
intermediate eye colors: eosin, cherry, 
buff, apricot, etc. Here again, in no 
species is there more than one gene 
known that is capable of changing in 
such a way as to produce white eyes, 
and this one is known in 12 species. 
Furthermore, in several of the other 
species intermediate stages are also 
known that are due to changes in this 
same gene. Therefore it seems safe to 
conclude, even without the crucial 
test of crossing to a known D. melan- 
817369—49—24 
295 
ogaster white, that we are dealing with 
changes in the same “‘wild type” gene. 
The principal bristles of the head 
and thorax are constant in their 
number, position, relative lengths, and 
in the directions in which they point in 
the “wild types” of the various species 
studied; they are in fact recognizable 
in a large proportion of all the higher 
Diptera, including, for example, the 
common housefly. There are, how- 
ever, a number of genes in the “wild 
type’? whose mutations affect this 
bristle pattern. One of the most 
frequent types of change involves a 
loss of particular bristles, and there 
are several different ‘‘wild type’ 
genes giving such changes. In gener- 
al, however, the new patterns result- 
ing from such changes are sufficiently 
characteristic in D. melanogaster so 
that, with practice, it is usually possi- 
ble to determine by simple inspection 
of a mutant specimen which one is 
concerned. Crosses to known types 
have consistently confirmed such iden- 
tifications. The two such types most 
frequently occurring in D. melanogaster 
are known as scute and hairless; types 
closely resembling both of them are 
known in other species (scute in 10, 
hairless in 4). Here the high degree 
of specificity of the patterns is the chief 
assurance of the identity of the genes. 
The examples just given show some 
of the ways in which homologies may 
be established between the genes of 
different species. By such means, and 
by others similar in nature, rather 
detailed comparisons have now been 
made possible between several forms. 
Within each species, study of the 
linkage, or association in heredity, 
between genes makes it possible to 
correlate the genes with particular 
chromosomes, and even to determine 
in which part of a chromosome each 
gene lies. It so happens that the 
chromosome configurations of the 
various species are not all alike, but 
they can all be interpreted in terms 
of six elements, lettered from A to F, 
which are variously attached to each 
other (fig. 1). When the genes of 
