236 T. H. MORGAN. 



sex-linked, and belongs, therefore, in our first (or sex-linked) 

 group of factors. When a spot female was bred to sable male 

 the daughters were gray and the sons spot. Inbred these gave 

 the following results. 



Gray 9. Spot 9. Spot <?. Sable d*. Gray cf- Spot Sable cf. 



35 30 I? 20 24 8 



The analysis is the same as when the cross is made with yellow, 

 except that gray is formed by recombination here. The linkage 

 of spot and sable modifies the ratios somewhat. 



In the cross with ebony and in that with sable there appeared 

 in the FI generation a few exceptional cases that are due to 

 non-disjunction. The actual results are 



Fi 



These cases are like those described by Bridges and call for no 

 further comment here. They are explicable on the assumption 

 that the two sex chromosomes of certain spot females occasionally 

 stick together, and either pass out of the egg or else both remain 

 in it. In the first work done more than a year ago with spot, 

 some unexplained results were obtained but not then inter- 

 preted. It now appears that in part at least they may have 

 been due to non-disjunction. 



THE ORIGIN OF MULTIPLE ALLELOMORPHS. 



It is noteworthy that spot appeared in yellow stock. It may 

 seem that there is some causal connection here since spot is a 

 mutation in the same locus as yellow. Likewise eosin appeared 

 in white stock, and is another case of multiple allelomorphism. 

 Plausible as is the assumption it can not be proven, but the fol- 

 lowing considerations are not without interest. If the mutation 

 appeared first in the yellow stock, in a chromosome that went 

 into a male, the spot would be apparent on examination. If the 

 chromosome went into a female the mutation would not appear 

 for another generation and then in a male. If on the other hand 

 the mutation had appeared in a chromosome in the gray stock 



