CH. xn] 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. The exoectations in such cases are as follows, 



to face p. 230] 



I regret that this representation of the inheritance of colour-blindness 

 contains a serious error. Mr Nettleship has called my attention to a fact 

 which must greatly modify the suggestions here given. The statement 

 made (p. 172) that colour-blind men frequently have colour-blind sons is 

 not true. On my scheme, half the sons of a colour-blind man should be 

 -colour-blind. In haemophilia and night-blindness there are cases of the 

 direct descent of the condition from father to son in families showing the 

 ordinary sex-limited descent, but as regards colour-blindness, though there 

 are instances of the direct descent from father to son, it is now obvious 

 that in all of them the affection was introduced by the normal-sighted 

 mother also. Apart from these, there is no case of a colour-blind man 

 having a colour-blind son known to me. On the contrary, from the 

 records, for the most part communicated by the kindness of Mr Nettleship, 

 there is now good evidence as to the condition of 23 sons of colour-blind 

 men, and all are normal (besides 1 5 more, also said to be normal on less 

 certain authority). Such a fact is very significant. Until more data are 

 collected I hesitate to suggest an emendation of the scheme. The 

 statement that colour-blindness is dominant in males and recessive in 

 females stands good, and the descent from females, whether normal or 

 colour-blind, is probably represented by the scheme correctly; but in the 

 fact that the sons of colour-blind men are normal we perhaps see the 

 complement of the peculiarity already ascertained in other sex-limited 

 conditions (p. 224), that the number of daughters transmitting is greater 

 than the scheme contemplates. It will be evident that the question of 

 a sexual dimorphism among the spermatozoa is thus prominently raised 

 (see p. 195). 



I shall be very grateful to any one who will send me authentic records 

 respecting the families of colour-blind persons. 



W. B. 

 April 1909. 



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