n6 MENDELISM chap. 



are inherited on the same lines as the grossulariata and 

 lacticolor patterns of the currant moth, but with one essen- 

 tial difference. The factor which repels the red-eye factor 

 is in this case to be found in the male, and here conse- 

 quently it is the male which must be regarded as hetero- 

 zygous for a sex factor that is lacking in the female. 



In order to bring these cases and others into line an 

 interesting suggestion has recently been put forward by 

 Bateson. On this suggestion each sex is heterozygous for 

 its own sex factor only, and does not contain the factor 

 proper to the opposite sex. The male is of the constitu- 

 tion, Mmff and the female 

 Mmff Ffmm Ffmm. Each sex produces 



gives gives 



gametes gametes two sorts of gametes, Mf and 



Mf ^fm\ productive mf in the case of the male, and 



Mf Fm\ a *• Fm, fm in that of the female. 



iyij rm I unproductive ' J 



m f >f m J' fertili sations But on this view a f urt her sup- 

 position is necessary. If each 

 of the two kinds of spermatozoa were capable of fertilis- 

 ing each of the two kinds of ova, we should get individuals 

 of the constitution MmFf and mmff, as well as the normal 

 males and females, Mmff and Ffmm. As the facts of or- 

 dinary bisexual reproduction afford us no grounds for 

 assuming the existence of these two classes of individuals, 

 whatever they may be, we must suppose that fertilisation 

 is productive only between the spermatozoa carrying 

 M and the ova without F, or between the spermatozoa 



