THE FACTORS WHICH DETERMINE SEX 639 



The offspring are almost invariably females. Castle concludes 

 that in those rare instances in which males are produced a 

 reduction of chromosomes probably takes place, the dominant 

 female character being then eliminated. 



Experiments by Doncaster and Others. Important evidence 

 has lately been obtained by Doncaster l as a result of breeding 

 experiments with certain Lepidoptera. He has shown that in 

 the moth Abraxas grossulariata, there is a rare variety, which 

 generally occurs only in the female sex. This variety, which 

 is called A. grossulariata lacticolor, is a Mendelian recessive, 

 so that when crossed with an ordinary grossulariata male, the 

 offspring are all typical, the lacticolor variety disappearing. 

 Experimental crossings yielded the following results : 



(1) Lact. 9 x gross. gave males and females all g4-oss. 



(2) Heterozygous 2 9 x heterozygous gave gross. ^ , gross. 



9 , and lact. 9 . 



(3) Lact. 9 * heterozygous gave all four possible forms 



(gross. , lact. $ , gross. 9 > and lact. 9 ),the lacticolor males 

 being the first ever seen. 



(4) Heterozygous 9 * lact. < gave gross. ^ and lact. 9 



(5) Lact. 9 x l act - c? gave lact. and lact. 9 



(6) Wild gross. 9 x lact. gave gross. and lact. 9 



It is shown, therefore, that males of the lacticolor variety 

 can be produced by mating lacticolor females with heterozygous 

 males (i.e. with males obtained by crossing the two original 

 varieties, and so presumably bearing two sorts of gametes), 

 but that the converse mating (4) results in offspring which 

 are either grossulariata males or lacticolor females. Further- 

 more, whereas lacticolor females, mated with wild grossulariata 



1 Doncaster, " Recent Work on Determination of Sex," British Associa- 

 tion Report, Dublin Meeting, 1908. See also Punnett and Bateson, "The 

 Heredity of Sex," Science, vol. xxvii., 1908. 



2 The term heterozygote has been given by Bateson to offspring result- 

 ing from the union of dissimilar gametes. Such organisms, according to 

 the Mendelian theory, produce more than one sort of gamete (see p. 194). 

 Homozygotes are formed by the union of similar gametes, and produce 

 gametes of one kind. Thus, homozygotes, as regards sex, are believed to 

 produce gametes bearing one sex character only (either male or female) ; 

 whereas heterozygotes, as regards sex, are supposed to give rise to both 

 male-bearing and female-bearing gametes. 



