6o HEREDITY AND EUGENICS 



three generations all showing the mark were females, 

 but in the last three generations affected males and 

 females were equal in numbers, and inheritance was 

 thought to be through the males only. The data 

 indicate, however, that the peculiarit}^ is inherited 

 as a simple Mendelian dominant, chance accounting 

 for the absence of affected males in the early genera- 

 tions, and the small number of females who were 

 mothers in the later generations accounting for the 

 later lack of transmission through the mothers. 



Pearson (1921) has recentl}^ described another 

 very similar case, with a white patch or flare on the 

 forehead and hair. In this famity there were seven 

 affected members hi four generations. This family 

 also goes gre}" ver}^ earty, and there is some tuber- 

 culosis but no consanguinity. This feature, of course, 

 resembles the white " star " marking often seen in 

 the forehead of horses and cattle, and frequentl}^ 

 accompanied by white patches on the extremities. 

 In this family the original great-grandmother in the 

 pedigree must have been heterozygous. In the 

 succeeding three generations the numbers of members 

 which were normal or flared were respectively 3:2, 

 4:1, and 6:3, making a total of 13:6. This is a rather 

 wide departure from the ratio which would be ex- 

 pected if the trait were a simple dominant, the odds 

 against its occurrence being 17-5 to i. But since un- 

 affected members never transmit the flare, it cannot 

 be a regular recessive. If the character is, as in other 

 cases, a simple dominant, the small numbers showing 

 the flare might, perhaps, be accounted for on the 

 assumption of the decreased viability of such hetero- 

 z3'gous individuals. 



The occurrence and inheritance of a differently 

 coloured, usually white, patch of hair, is a fairl}- 

 common phenomenon. Darwin {Animals and Plants 

 under Domestication, chapter xii.) cites an English 



