HUMAN INHERITANCE 193 



(near Montpellier in France) it has been traced to 

 the year 1637 and hence has been handed down for 

 about two hundred and fifty years. There are other 

 types of night-blindness that have a different in- 

 heritance not entirely made out. 



Haemophilia in man has been shown to be trans- 



T , , • 



Fig. 73. — Diagram to show the inheritance of night-blindness. 

 (After Nettleship.) 



mitted as a sex-linked character {fig. 74) . The blood 

 of affected individuals fails to coagulate quickly 

 when exposed to air, hence there is danger of the 

 individual bleeding to death. Several pedigrees have 

 been made out. It is a recessive character whose gene 

 is carried by the A^-chromosome. It appears in any 

 male whose single A^-chromosome carries the gene 

 for haemophilia. Its relative infrequency in women 

 is explained on the grounds that it can appear in 

 them only when the father and mother both possess 

 the character or when the mother herself has had a 

 haemophilic father — in other words when both X- 

 chromosomes carry the gene. 



