508 



THE AMERICAN NATURALIST [Vol. XLV 



may be interpreted as an X chromosome attached to one member 

 of a pair of Y chromosomes, thus one of the members being an 

 XY combination, the other Y. 



He then calls attention to the very interesting case of the 

 inheritance of white eye found by Morgan in one of his Droso- 

 phila mutants. This mutation appeared first in the male. "When 

 this male was crossed with the normal red-eyed female the pro- 

 geny all had red eyes; but in the next generation some of the 

 males showed white eye again. Morgan has shown that in a cross 

 of this kind it is only the female progeny which transmits the 

 ;ihm»nii;ility, and although she transmits it to half of both sexes 

 of her offspring it does not appear in the female. It is possible, 

 however, to secure females with white eyes. 



These facts are all explainable on the following basis. Let WF 

 represent the X element in which white eye is associated with the 

 female determiner. RF would then represent the X element in 

 red-eyed individuals. The formula for the white-eyed male 

 would be WF.Y. The formula for the normal female is RF.RF. 

 These mated give WF.RF females and RF.Y males; but since R 

 is dominant to W these females have red eyes. 



If now these heterozygous females be mated with ordinary 

 males, the following four types of progeny are produced : 

 WF.RF females, RF.RF females, WF.Y males, RF.Y males. 

 Thus half of the males in this generation will have white eyes. 

 But if a heterozygous female be mated with a white-eyed male, 

 half of the female progeny and half of the male progeny will 

 have white eyes. 



Wilson points out that these phenomena exactly parallel be- 

 havior which he has found in the X and Y chromosome elements 

 of certain species, where the X element plainly consists of an X 

 chromosome united to a Y chromosome, while the synaptic mate 

 is a Y chromosome. If in such a species a mutation like that of 

 the white eye of DrosophUa occur as a result of the loss of some 

 characteristic on the part of the Y chromosome which constitutes 



correspondence of all this with the ciiu-nc <>t' >,-xdimited heredity 

 of this type that it is difficult to resist the conclusion that we have 

 before us the actual mechanism of such heredity— in other words. 



