L. DONCASTER 13 



and yet the hypothesis most widely adopted, especially in America, for 

 the explanation of sex-limited transmission assumes that the sex-factor 

 and the factor for the sex-limited character are borne by the same 

 chromosome. 



In the hope of obtaining fresh evidence on these two points I have 

 collected and analysed a number of pedigrees of sex-limited affections 

 in Man — Colour-blindness, Night-blindness, Nystagmus and Haemo- 

 philia'. A preliminary examination of these showed that all four 

 affections had the same characteristics as regards their inheritance — 

 an apparent disturbance of the sex-ratio among the offspring of 

 transmitting females, an excess of affected over unaffected males in 

 affected fraternities, and occasional exceptions to the ordinary rule of 

 sex-limited transmission among the children of affected males. On 

 tabulating a considerable series of jjedigrees, however, it became clear 

 that there were several imj)ortant sources of error, which it is difficult 

 to eliminate, arising partly from the incompleteness of the pedigrees, 

 and partly from the fact that it is impossible in most cases to know 

 that a woman is a transmitter of the affection unless she has at least 

 one affected son. While I was engaged on the work, I received a paper 

 by Lenz^ which points out that the apparent excess of males in affected 

 fraternities, and also the excess of affected over unaffected sons, is 

 possibly due to the fact that of necessity the totals of offspring of 

 transmitting females are compiled from fraternities including at least 

 one affected male. Fraternities consisting chiefly of daughters, with no 

 affected son, are excluded, since they provide no evidence that the 

 mother is a transmitter of the affection, and thus there arises a 

 preponderance not only of males over females, but of affected over 

 unaffected males. With the object of testing this suggestion I have 

 tabulated the fi'aternities from my data for the four affections in which 

 there are at least seven children, and find that in these totals the 

 apparent excess of males is much reduced in each case, and in some 

 vanishes altogether. The prepondei'ance of affected over unaffected 



^ Colour-blindness — Nettleship, Tram. Ophthalm. Soc. xxvm. 1908, p. 220 ; xxvi. 

 1906 ; and unpublished cases kindly given me by Mr Nettleship and by Mr S. P. Hayes 

 of Mount Holyoke College, U.S.A. 



Night-blindness and Nystagmus — Nettleship, Trans. Ophthalm. Soc. xxix. 1909, p. Ivii; 

 Jixxi. 1911, p. 1-59 ; xxvni. 1908, p. 220 ; xxxii. 1912, p. 21 ; and Royal London 

 Ophthalm. Hospital Reports, xvn. p. 333. 



Haemophilia — Bullock and Fildes, Treasury of Human Inheritance, Parts v. and vi. 

 London, 1911. 



- Lenz, Ubcr die Krankhaften Erbanlagen des Mannes. Jena (G. Fischer), 1912. 



