xv MAN 175 



Besides cases such as these where a simple form of 

 Mendelian inheritance is obviously indicated, there are 

 others which are more difficult to read. Of some it may 

 be said that on the whole the peculiarity behaves as 

 though it were an ordinary dominant ; but that exceptions 

 occur in which affected children are born to unaffected 

 parents. It is not impossible that the condition may, 

 like colour in the sweet pea, depend upon the presence or 



rf i ^9 9 9 9 9 -rf * 9 9 9 ^ 



1 i ) i ill r~i i 



Hi 



FIG. 35- 



Pedigree of a haemophilic family. Affected (all males) represented by black, and 

 normals of both sexes by light circles. (From Stahel.) 



absence of more than one factor. In none of these cases, 

 however, are the data sufficient for determining with cer- 

 tainty whether this is so or not. 



A group of cases of exceptional interest is that in which 

 the incidence of disease is largely, if not absolutely, re- 

 stricted to one sex, and so far as is hitherto known the 

 burden is invariably borne by the male. In the inheri- 

 tance of colour-blindness (p. 117) we have already dis- 

 cussed an instance in which the defect is rare, though not 



