SEX-LINKAGE 21 



of her granddaughters. These facts can be explained most 

 readily on the assumptions that the genes for the characters 

 barred and non-barred are being distributed by a mechan- 

 ism that at the same time is distributing the elements of a 

 sex-determining mechanism and, secondly, that the male of 

 the domestic fowl has in his constitution the sex-determin- 

 ing element in duplicate whilst the female possesses it in the 

 single state and is heterogametic. Homo- and heterogamety 

 require that there shall be a qualitative or quantitative 

 difference of this kind between male and female. 



In man there is a form of the disease haemophilia that 

 behaves in inheritance in exactly the same way and which 

 goes far to prove that the male is heterogametic and that 

 haemophilia is a sex-linked recessive character, its gene 

 being X-borne. Evans and Swezy (1929) offered cytological 

 proof that man has the Lygaeus type of sex-chromosome 

 constitution, the male being XY. 



It will be noted that according to this explanation (on 

 p. 22) there can be two kinds of males, haemophiliacs and 

 normals, and three kinds of females, normals, carriers and 

 haemophiliacs. This is so because the male has but one X- 

 chromosome and the female two. On any one X there can 

 be either the gene for normality or else the gene for haemo- 

 philia. In the case of the female the haemophilia gene can 

 be present in neither, in one or in both of the X's. A male 

 cannot be a carrier. It is because the carrier female is so 

 difficult to identify that she constitutes a danger to her off- 

 spring by a normal male. A female can be haemophilic only 

 when her father is a haemophiliac and her mother either 

 a carrier or else a haemophiliac. Haemophilia is seldom 

 encountered in the human female for the reason, it would 

 appear, that female haemophiliacs die in utero. 



Non-Disjunction. That the mechanism that is concerned 

 with the distribution of these sex-linked characters is at the 

 same time the mechanism which in its functioning is in- 

 volved in the determination of the sex of the individual was 

 proved beyond all doubt by the work of Bridges (19 16) on 

 non-disjunction in Drosophila melanogaster. 



