204 MENDELISM chap. 



not absolutely, restricted to one sex, and so far as is 

 hitherto known the burden is generally borne by 

 the male. Colour-blindness is a case in point, and, 

 as already mentioned (p. iii), the clue has been 

 provided by Drosophila. Other examples of this 

 form of inheritance are to be found in haemophilia, 

 in night-blindness (some pedigrees), in certain ocular 

 defects, and in several diseases of the nervous system. 

 The peculiar nature of the sex-limited inheritance 

 finds its expression in pedigrees such as that 



r 1 1 1 



9 9 9 9 



I — I — 1 — i — I — I I 1 — I — I I — I 1 — I — \ 



^^^999 9 9^^99 9^rf 



I r— ^ Children J—H l—'-l 



(?)J J 9 all healthy (?)J J(?) ^9 



Fig. 52. 



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

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



illustrated in Fig. 52. In rare instances an affected 

 female occurs as the result of a mating between an 

 affected male and a female carrier. Such females 

 mated with normal males produce affected sons only, 

 while their daughters, being all heterozygous, are all 

 carriers (cf Fig. 36, p. i 1 1). 



Though by far the greater part of the human 

 evidence relates to abnormal or diseased conditions, 

 a start has been made in obtaining pedigrees of 

 normal characters. From the ease with which it 

 can be observed, it was natural that eye -colour 

 should be early selected as a subject of investigation. 



