STUDIES ON CHROMOSOMES 97 



into the female, and the male is normal ; but the female of course 

 likewise shows only the normal (dominant) character. In the 

 following F2 generation (5) there are four possible combinations 

 XX, XX, XY and XY, two of each sex. Though X is present in 

 half of each sex, the character appears only in the males, XY, 

 again because of its recessive nature. By crossing together males 

 of the composition XY and females of composition XX, some of 

 the resulting females will have the composition XX, and the sex- 

 limited character is thus made to appear in the female. 



When the female is the heterozygous or digametic sex as in 

 sea-urchins, in Abraxas, the Plymouth Rock fowls, etc. exactly 

 the converse assumption has to be" made. Here, as Spillman 

 ('08) and Castle ('09) have pointed out, the observed results 

 follow if the sex-limited character (e.g., lacticolor color-pattern 

 in Abraxas) be allelomorphic to, or the synaptic mate of, a sex- 

 determining factor, X, that is present as a single element in the 

 fe'male but absent in the male. The formulas now become 9 

 (as Spillman has indicated) XG (9 grossulariata), GG (cT gross.) 

 XG (9 lacticolor) and GG (d 1 lact). XG X GG then gives 

 in FI XG and GG (gross. 9 and cf), G having passed from the 

 female to the male. The following cross, XG X GG gives in F 2 

 the four types XG, XG, GG and GG, i.e., grossulariata appear- 

 ing in both sexes but lacticolor only in the female. By crossing 

 XG with GG some of the progeny will have the composition GG 

 (d" lacticolor). The other combinations follow as a matter of 

 course. 



This interpretation is in all respects the exact converse of 

 that made in the case of Drosophila, which is also the case with 



9 These formulas are in substance the same as those stated by Mr. Spillman in 

 a private letter to the writer, and are a simplified form of those suggested by Castle 

 ('09). The interpretation thus given seems both the simplest and the most satis- 

 factory from the cytological point of view of all those that have been offered. It 

 obviates the cytological difficulties that I urged ('09) against Castle's formulas, 

 and renders unnecessary the secondary couplings that I suggested. All these 

 ways of formulating the matter conform, of course, to the same principle and 

 differ only in details of statement. Whether the synaptic mate of X is directly 

 comparable to the Y-chromosome of other insects (in which case the female formula 

 becomes XY and the male YY) is an open question. 



