CH. XII] 



Collecting Evidence 



231 



should be recorded with as much care as the abnormals. 

 In all cases, where possible, inquiry should be made re- 

 specting the children of the normals. The sex and age as 

 far as possible of each individual should be noted. If the 

 condition studied be not a congenital one, the age at which 

 it appeared in each individual should be entered. 



Dominant characters will in general be recognized as 

 such from the fact that they are transmitted through affected 

 persons only. The dominants will as a rule have had one 

 parent affected with the peculiarity and one parent free 

 from it. It is then to be expected that the children of such 

 dominants, resulting from their marriages with unaffected 

 persons, will be in equal numbers affected and normal. 



Sex- limited dominant characters such as colour-blindness 

 and haemophilia affect one sex, generally the male, most 

 often. 



~> 



Ox 





9 



X 



Lx L* 



(j) O Ox 



Fig. 34. Tentative representation of the descent of colour-blindness, 

 drawn up on the hypothesis stated p. 195, note. Symbols as in 

 Fig. 33. On this scheme a homozygous colour-blind male cannot be 

 produced even by the union of two colour-blind parents. 



