THE DETERMINATION OF SEX 101 



In explaining these phenomena in the first paper this last 

 result was not known ; and it was suggested (in accordance with 

 Castle's hypothesis) that the germ-cells bore one or the other 

 sex, that fertilisation was selective, so that all individuals were 

 heterozygous in respect of sex and that in the eggs the 

 lacticolor character was coupled with the female sex-determinant. 

 Later Bateson and Punnett 1 offered a modified hypothesis, which 

 is more in accord with the facts as known at present. They 

 suggest (i)that the sex-determinants behave as Mendelian units, 

 femaleness being uniformly dominant over maleness ; (2) that 

 female individuals are heterozygous in respect of sex, being of 

 the constitution ? $ and producing male-bearing and female- 

 bearing eggs in equal numbers ; males are homozygous in sex, 

 of the constitution c?cT, so that they produce only male-bearing 

 spermatozoa ; (3) that there is repulsion in oogenesis between 

 the dominant determinant for femaleness and the dominant gros- 

 sulariata (type) determinant, in consequence of which all male- 

 bearing eggs bear the type, all female-bearing eggs the lacticolor 

 character. 



This suggestion completely accounts for the facts and has 

 since been greatly supported by the discovery that all females 

 with the type {grossulariata) character are heterozygous and 

 produce lacticolor offspring when paired with a lacticolor male. 

 This fact compels us to assume that the lacticolor determinant 

 is present in all females of the species and is only prevented 

 from appearing because typical males bear normally only the 

 type {grossulariata) character, which dominates over lacticolor. 

 If, then, the males are homozygous and the females heterozygous 

 in respect of a character so intimately related with sex, it 

 strongly supports the idea that the same may be the case 

 with the sex-determinants themselves. 



If this instance stood alone it might seem rash to found on 

 it such a far-reaching theory of the nature of sex. But exactly 

 similar cases have been found elsewhere. Miss Durham 2 has 

 described almost identical phenomena in the case of canaries of 

 the Cinnamon variety, which have pink eyes. A pink-eyed hen 

 paired with a black-eyed cock gives offspring which are all black- 

 eyed ; but a black-eyed hen by a pink-eyed cock gives males 

 which are all black-eyed, and pink-eyed females, together with 



1 Science, vol. xxvii. 1908, p. 785. 



* Reports to Roy. Soc. Evolution Committee, iv. 1908, p. 57. 



