170 GENETICS IN RELATION TO AGRICULTURE 



genetic constitution (bT)(Bt)(pT 3 )(Pta)XY, and since no crossing-over 

 occurs in the male it produces the following series of gametes: 



(bT)(pT)X (bT)( P T,)Y 



(bT)(Pt t )X (bT)(Pt,)Y 



(Bt)(pT t )X (Bt)(pT t )Y 



(Bt)(Pt t )X (Bt)(PtJY 



When, therefore, such an Fi 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 t and t 3 . 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 directly 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 miniaturs 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- 



