THE FACTORS WHICH DETERMINE SEX 677 



mating (4) results in offspring which are either grossulariata males 

 or laeticolor females. Furthermore, whereas lacticolor females 

 mated with wild grossulariata males (1) produce offspring which 

 are of both sexes, but all of the grossulariata variety, the converse 

 cross (6) yields grossulariata males and lacticolor females. It is 

 concluded, therefore, that the wild and presumably pure grossulariata 

 females are heterozygous for sex, femaleness being dominant and 

 maleness a homozygous recessive character. All the females are 

 believed to produce male-bearing and female-bearing gametes in 

 equal, numbers, whereas all the males appear to produce only one 

 sort of spermatozoon. In confirmation of this conclusion Doncaster 

 since discovered a pair of unequal chromosomes in the female 

 Abraxas, corresponding therefore with the X and Y chromosomes of 

 the other insects described. 



The results obtained by Miss Durham in her experiments on 

 cinnamon canaries are explicable on a similar hypothesis. When a 

 cinnamon male was mated with a green female, the female offspring 

 were cinnamon and the males green ; but when a cinnamon female 

 was paired with a green male, all the offspring of both sexes were 

 green. 1 Where, however, a green cock of the second generation (the 

 Fj generation produced by crossing) was mated with a cinnamon 

 hen, both green and cinnamon birds of both sexes were produced ; 

 but when a green cock of the second (F a ) generation was crossed 

 with a green hen the resulting male birds were all green, but the 

 females were of both types. A more complex case of a like kind 

 has been brought to light by Bateson and Punnett in their investiga- 

 tion on the heredity of the black pigmentation of the silky fowl in 

 its crosses with brown Leghorns and other fowls with light shanks. 

 Here two allelomorphic characters, in addition to the two sex 

 determinants, are concerned, but Bateson and Punnett state that 

 the facts point very clearly to some such solution as that indicated 

 by Doncaster's experiments with Abraxas. A clearer case is that 

 of the factor for barring in the Barred Plymouth Rock. If a barred 

 cock is mated with an unbarred hen (Cornish Indian G'ame) all 

 the chicks of both sexes are barred ; if a barred hen is mated 

 with an unbarred cock, all the males are barred and all the 

 females unbarred. 2 Similar results have been obtained with 

 other breeds. 3 



The mode of sex-inheritance shown by Abraxas and the other 



1 Durham, Report to the Evolution Committee of the Royal Society, IV., London, 

 1908. 



2 Pearl and Surface, " On the Inheritance of the Barred Pattern in Poultry," 

 Arch. f. Entwick.-Mech., vol. xxx., 1910. The sex-linked factor for high egg- 

 production has been already mentioned (p. 644). 



3 See Doncaster, The Determination of Sex, Cambridge, 1914. 



