SCOPE OF GENETICS 43 



women who are colour-blind have, so far as 

 we know, no sons who are not colour-blind ^ 



Mendelian analysis of these facts shows 

 that colour-blindness is due, not, as might 

 have been supposed, to the absence of some- 

 thing from the composition of the body, but 

 to the presence of something which affects 

 the sight. Just as nicotine-poisoning can 

 paralyse the colour sense, so may we conceive 

 the development of a secretion in the body 

 which has a similar action. The comparative 

 exemption of the woman must therefore mean 

 that there is in her a positive factor which 

 counteracts the colour-blindness factor, and 

 it is not improbable that the counteracting 

 element is no other than the femaleness- 

 factor itself 2. 



' We have knowledge now of seven colour-blind women, 

 having, in all, 17 sons who are all colour-blind. Most of these 

 cases have been collected by Mr Xettleship. 



* An alternative and perhaps more satisfactory interpretation 

 of the ssime facts h:is been proposed by Doncaster {Jour. Genetics i, 

 Pt 4, p. 377). Until more progress has been made with the 



