PHYSICAL CHARACTERS IN MAN 107 



are said to be the most common causes of acquired 

 deaf-mutism. In addition to the difhcuh}^ in dis- 

 tinguishing between genotypical (inherited) and 

 phenotypical (acquired) deaf-mutism a statistical error 

 is introduced by the fact that in small families the 

 children may all be normals, even when the parents 

 are heterozygous for the condition. Since these 

 children are not added to the offspring of matings of 

 heterozygotes, the result is more than 25 per cent, of 

 abnormals. This can be corrected by applying a 

 formula of Weinberg, and it then closely approxi- 

 mates to the expected 25 per cent. 



From Fay's evidence of marriages of the deaf in 

 America (1898), twenty-two families had four to 

 nine children each, all deaf. The children numbered 

 112. Hence it appears that genotypical deaf-mutes 

 will have only deaf-mute children, and from this and 

 other evidence it appears that the character is prob- 

 ably a simple recessive. The difficulty in all statistics 

 on this subject is to distinguish between genotypic 

 and phenotypic deaf-mutism. Love (1920) also 

 reaches the conclusion that sporadic congenital 

 deafness is inherited as a Mendelian recessive, from 

 a stud}^ of an Ayrshire famil}^ which has branches in 

 America and Australia. This famil}' shows five 

 affected generations descended from a common 

 ancestor three generations further back. Yearsle}' 

 (1920) gives an account of a family with a history of 

 '' acquired " deafness, which produced offspring born 

 deaf. But as the acquired deafness is apparentl}' 

 hereditary otosclerosis, this is merely a case of the 

 earlier appearance of the abnormality. Home (1909), 

 in a study of deaf-mutism, says that the number of 

 deaf-mutes in Europe is estimated at about i in 1,350 

 of the population. In England the frequency (from 

 statistics) is i in 2,043, Scotland i in 1,860, Ireland 

 I in 1,398. Seven pedigrees are recorded, but 



