PHYSICAL CHARACTERS IN MAN 119 



daughter directly in the female line — that is, it is 

 sex-linked or, better, female sex-linked. It may be 

 spoken of as matrilineal in descent . The appearance 

 of the character in heterozygous females shows it to 

 be a dominant. We may think of such females as 

 having one affected and one normal X chromosome 

 (see p. 23). This would be transmitted to half the 

 sons, none of whom are affected, presumably because 

 the presence of the Y chromosome in some way 

 neutralises its activity.* Half the sons should then 

 be capable of transmitting colour-blindness to all 

 their daughters. Unfortunately the records of the 

 family furnish no data regarding the male lines of 

 descent in this famil3\ But it seems likely that if 

 such transmission had occurred it would have been 

 noticed, because the facts are evidently carefully 

 recorded. It is possible, therefore, that transmission 

 through the males may fail, just as it appears to fail 

 to be inherited through two successive generations 

 in the female line in the night-blind family studied 

 by Newman (191 3) (see p. 115). Another possible 

 explanation would be that only sons who have re- 

 ceived a normal X chromosome are viable. This 

 should lead to a deficiency of males in the offspring. 

 It is very interesting to find colour-blindness trans- 

 mitted as a sex-linked dominant in this family and 

 a sex-linked recessive in others. In this Belgian 



family Madame Th (III. i. Fig. 24) and her aunt 



and grandmother could not distinguish blue from 

 red (" rouge "), while her descendants confounded 

 blue with " cerise." The eyes of affected individuals 

 were very sensitive to light. 



Sedgwick (1861) states that inability to distinguish 

 colours is often associated with inability to distinguish 



* There is some evidence (see p. 94) that in man and in fishes, 

 unUke insects, the Y chromosome plays an active role in in- 

 heritance. 



