15G 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 cf is mated to an eosin 9 the Fi is eosin 1 and F 2 consists of 

 3 eosin:! white. When a red-eyed 9 is mated to an eosin-eyed d", 

 Fi is red, and F 2 segregates in the ratio 3 red:l 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 

 iv. On this basis the analysis of the genetic constitutions of these 

 different races is as follows: 



(WX)(WX) = red 9 (WX)Y = red <? 



(WX)(WX) = eosin 9 (ivX)Y = eosin <? 



(wX)(wX) = white 9 (wX)Y = white cf. 



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 c , w 1 , tw 6 

 and ttf 6 " respectively. These seven factors therefore display a particular 

 type of behavior depending upon the fact that they occupy the same 

 locus in the .^-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 



'The FI 9s actually have an intermediate eye-color, "white-eosin compound". 



