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 Y-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 Y-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, 7.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, 7.e., when a white 
eyed female is mated to a red-eyed male the results are different. In 
the F,; of such a mating the female flies have red eyes and the males 
all have white eyes. When the F) flies are bred together an F2 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 
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