74 GENETICS IN RELATION TO AGRICULTURE 



may be used to illustrate the history of this pair of chromosomes in hy- 

 bridization. Since chromosome relations have been determined more 

 definitely in Drosophila, we shall follow out in detail a selected case in 

 this species. We have pointed out previously that aside from the pair 

 of sex-chromosomes, the pairs of chromosomes in both the male and fe- 

 male of Drosophila are alike and bear the same factors. But in the male 

 the F-chromosome appears to have no effect upon the development of 

 the body characters so that the male depends upon a single X-chromo- 

 some for the development of those characters determined by the factors 

 borne in this chromosome. The F-chromosome may, therefore, be re- 

 garded as a neutral mate for the X-chromosome in the male. Since the 

 distribution of this pair of chromosomes is unique as we have pointed out 

 in the discussion of the inheritance of sex in Drosophila, the history of 

 factors carried by the X-chromosomes furnishes a beautiful illustration 

 of the parallelism existing between chromosome behavior and factor 

 distribution. The inheritance of white-eye color in Drosophila is a case 

 in point. Wild races of Drosophila have red eyes, but Morgan discovered 

 a few white-eyed male mutants in an inbred strain of "wild" flies, i.e., 

 flies which were directly descended from wild flies. From this muta- 

 tion it was found possible to establish a white-eyed race of flies which 

 breed true to this new eye character. When a white-eyed male is 

 mated to a red-eyed female the offspring all have red eyes, because 

 red eye in Drosophila is dominant to white. In F% red- and white- 

 eyed flies are produced in the proportion of 3 red : 1 white. All the fe- 

 males in this generation are red-eyed, but of the males half have red and 

 half white eyes. When the reciprocal cross is made, i.e., when a white- 

 eyed female is mated to a red-eyed male the results are different. In 

 the FI of such a mating the female flies have red eyes and the males 

 all have white eyes. When the FI flies are bred together an FZ is obtained 

 half the females of which have red eyes and half white eyes, and likewise 

 among the males half have red eyes and half white eyes. 



The explanation of this type of inheritance is shown diagrammatically 

 in Figs. 35 and 36. The factor for white eyes is represented by w and it 

 is borne in the X-chromosome. The factor W for red eyes, allelomorphic 

 to w, is carried by the X-chromosome of the red-eyed race of flies at ex- 

 actly the same locus as that of w in the white-eyed race. Since these 

 two factors occupy the same locus in the X-chromosome obviously they 

 can never be contained in the same chromosome. In Fig. 35, the two 

 X-chromosomes of the red-eyed female both contain the factor W for 

 red eyes. In a convenient shorthand system the genetic constitution 

 for such a fly may be designated (WX)(WX), the parenthesis indicating 

 that the factor W is carried by the X-chromosome. Each egg from such 

 a female will contain an X-chromosome with a factor for red eyes in 



