232 Heredity and Pathology [CH. 



D. For colour-blind woman marrying normal man : 



sons to be all colour-blind. Daughters to be 

 normal, but all capable of transmitting colour- 

 blindness to their sons. 



E. For normal brother of colour-blind person : all 



children normal. 



The descent of such a sex-limited condition is shown in 

 the diagram, and in it the various possibilities are repre- 

 sented in such a way as to show the probable results of 

 each kind of mating (Fig. 33). 



K" Recessive characters will be recognized by the fact that 

 they may appear in the children of parents not exhibiting 

 such characters, and especially among children born of 

 consanguineous marriages. Complete proof of the recessive 

 nature of a characteristic will only be obtained by evidence 

 that jill the children of affected parents exhibit the charac- 

 teristic. '-^ A"-* ^ ^//^ 



In all these descents occasional exceptions are to be 

 expected, which in the light of present knowledge cannot 

 be interpreted; but when the habitual course of the in- 

 heritance has been determined, an inquiry into the nature 



I of these exceptions may perhaps be undertaken with 

 success. 



A correct knowledge of the system which a hereditary 

 disease follows in the course of its descent will, it may be 

 anticipated, contribute to a proper understanding of the 

 pathology of these affections, and thus make a definite 

 advance in the general study of the physiology of disease. 

 If, for example, a disease descends through the affected 

 persons, as a dominant, we may feel every confidence that 

 the condition is caused by the operation of a factor or 

 element added to the usual ingredients of the body. In 

 such cases there is something present, probably a definite 

 chemical substance, which has the power of producing the 

 affection. Thus the abortion of the digits, or the inability 

 to distinguish certain colours, must be due to such added 

 factors. On the contrary, when the disease is recessive we 

 recognize that its appearance is due to the absence of some 

 ingredient which is present in the normal body. So, for 



