1 66 MENDELISM chap. 



absolutely, restricted to one sex, and so far as is 

 hitherto known the burden is invariably borne by 

 the male. In the inheritance of colour-blindness 

 (p. 1 08) we have already discussed an instance in 

 which the defect is rare, though not unknown, in the 

 female. Sex-limited inheritance of a similar nature 

 is known for one or two ocular defects, and for 

 several diseases of the nervous system. In the 

 peculiarly male disease known as haemophilia the 

 blood refuses to clot when shed, and there is nothing 



-i > . 



9 9 9 



I I I I 1 I 1 — ' — I 



i i i'i 9 9 9 9 i i 9 9 9 i i 



i i i 9 ijj_9ii39i9 iii9ii9 



1 1 — 'n Children r""-! r-S 



{?)(? cJ 9 aU healthy •('')(? C?(?) ^9 



Fig. 35. 



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

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



to prevent great loss from even a superficial scratch. 

 In its general trend the inheritance of haemophilia is 

 not unlike that of horns among sheep, and it is possible 

 that we are here again dealing with a character which 

 is dominant in one sex and recessive in the other. 

 But the evidence so far collected points to a differ- 

 ence somewhere, for in haemophilic families the 

 affected males, instead of being equal in number to 

 the unaffected, show a considerable preponderance. 

 The unfortunate nature of the defect, however, forces 

 us to rely for our interpretation almost entirely upon 

 the families produced by the unaffected females who 



