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 F» 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, ete.—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 
female but absent in the male. The formulas now become? 
(as Spillman has indicated) XG (¢ grossulariata), GG (¢ gross.) 
XG (¢ lacticolor) and GG (¢ lact). XG x GG then gives 
in F, XG and GG (gross. ¢ and «), G having passed from the 
female to the male. The following cross, XG x GG gives in F, 
the four types XG, XG, GG and GG,—~.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 
(# 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 
° 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. 
