156 GENETICS IN RELATION TO AGRICULTURE 
Multiple Allelomorphism in Drosophila.—A typical case is that 
centering around the locus for eye color in Drosophila which we have 
called W. This locus is situated in the X-chromosome at a distance of 
one unit from the locus Y for body color. The first mutations in Dro- 
sophila involved a change in W such that white eyes were produced, a 
mutation recessive to the normal red-eyed condition. This factor is 
called w and its inheritance has been dealt with in previous chapters. 
Later some flies arose in a white-eyed culture which had eosin eyes. 
When a white & is mated to an eosin 9 the F, is eosin! and F» consists of 
3 eosin:1 white. When a red-eyed 2 is mated to an eosin-eyed o, 
F, is red, and F, segregates in the ratio 3 red:1 eosin. The facts are 
explainable on the assumption that the factor W has been changed in a 
different fashion to produce the factor for eosin which we designated as 
w*. On this basis the analysis of the genetic constitutions of these 
different races is as follows: 
(WX)(WX) = red 9Q (WX)Y =red oe 
(w°X)(weX) = eosin 2 (wX)Y =eosin 3 
(wX)(wX) = white @ (wX)Y = white 3. 
A change in the same locus has occurred in the mutation to white and 
to eosin, but the change has been different in each case. Later four 
other changes in this locus occurred giving eye colors which have been 
named cherry, tinged, blood and buff, and these fulfil the same conditions 
as those pointed out for eosin. The factors are designated w*’, w', w® 
and w" respectively. These seven factors therefore display a particular 
type of behavior depending upon the fact that they occupy the same 
locus in the X-chromosome. They form together a system of septuple 
allelomorphs. 
In Drosophila there are at least three other such systems of multiple 
allelomorphs. One of these centers around the Y locus in the X-chromo- 
some which may change to y giving a yellow-bodied fly in place of the 
normal gray body or may change to y* when a spot-bodied fly is produced. 
Another system of triple allelomorphs for eye color is located in the 
third chromosome; it consists of the factors for pink and peach eye 
color, and the normal allelomorph of these which is concerned in the 
production of red eyes. A fourth such series of allelomorphs is that 
of the factors for ebony and sooty body color and their normal allelo- 
morph concerned in the production of gray body color. This series is 
also located in the third chromosome. 
Assuming that more than two factors may occupy identical loci in 
homologous chromosomes there are several simple relations which 
must be fulfilled in order to establish the case experimentally. The 
1The F, 9s actually have an intermediate eye-color, ‘“white-eosin compound’’. 
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