170 GENETICS IN RELATION TO AGRICULTURE 
genetic constitution (b7')(Bt)(pT'3)(Pts)X Y, and since no crossing-over 
occurs in the male it produces the following series of gametes: 
(bT) (pT's)X (6T) (pT 3) ¥ 
(bT) (Pts) X (bT) (Pts) Y 
(Bt) (pT 3)X (Bt) (pT 3) Y 
(Bt) (Pts)X (Bt) (Pts) Y 
When, therefore, such an F; male is mated back to a black long pink 
female the results are as recorded in the checkerboard in Fig. 80. Of 
the male flies only the gray reds bear both the factors ¢ and t3. Such 
flies are long or truncate winged, but they should behave in the same fash- 
ion in further breeding tests unless the factors themselves are variable. 
Actually it was found that continued breeding back of these gray red 
males to black pink females gives approximately the same proportions 
of truncate to long in every generation. This method of taking advan- 
tage of the linkage relations and using the pink factor so that a given 
genotype could be determined without fail has in this series of experiments 
been the means of analyzing a case which otherwise would have baffled 
investigation, for the results clearly point to the fact that the genotypic 
differences which exist between the long and truncate flies of a selected 
culture are due to the fact that the lower vitality of truncate flies homo- 
zygous for the three factors direetly concerned in the expression of this 
character favors the survival of heterozygous individuals, and it is, there- 
fore, practically impossible to secure a strain of truncate winged flies 
which will breed true. 
The Factor Explanation of Reversion.—Many phenomena included 
under the term reversion can be explained satisfactorily as instances of 
complex factor interaction. Reversion in general is a term applied to 
sudden return to an ancient, generally wild form, whether by hybridiza- 
tion or from other causes. 
The Mendelian explanation of reversion is most simply illustrated in 
Drosophila, for in Drosophila the relation of any particular form to the 
wild type is known accurately. Thus for example a form of Drosophila 
with miniature wings arose as a mutation directly from the long-wing 
type. Likewise several other wing characters have arisen from the long- 
wing type by a single mutation, among them vestigial wings. When now 
a vestigial-winged female is mated to a miniature male, the progeny all 
have long wings. This phenomenon may be explained by the fact that 
in a vestigial fly, a mutation has occurred in the locus V, which changed 
it to v without affecting the normal allelomorph of the miniature factor. 
Similarly the miniature fly bears the normal allelomorph of the vestigial 
factor, so that when the two are mated the original series of factors of the 
long-winged type is reunited and consequently the characters of the 
original wild form are reproduced. This is the principle on which rever- 
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