HEREDITARY TRANSMISSION OF GENOTYPICAL DEAF-MUTISM 39 



cases = dominant homozygote). From the tabular representation 

 we find that in 358 cases out of 409 the statements of the relatives 

 perfectly agree with this expectation, in 21 cases the necessary in- 

 formation could not be supplied, in 30 cases again jt has been stated 

 that both parents were born deaf. Of course, as I have before stated, 

 this does not exclude the possibility that in several cases the deaf- 

 mutism was acquired during the fetal life, in other cases again the 

 statement is no doubt incorrect. Fay himself writes on this point 

 (p. 36 ff.): »In estimating the value of all statistics on this subject 

 it should be remembered that in many cases the fact of congenital 

 or adventitious deafness cannot be determined with certainty. In- 

 deed, the fact of congenital dea^fness cannot be determined with ab- 

 solute certainty in any case. — — — Deafness truly congenital is 

 probably of much rarer occurrence than is indicated by even the 

 most trustworthy statistics.» 



Taken into account this assertion of Fays we should not be 

 surprised if in some cases the deaf-mutism of one of the parents has 

 happened to be designated as congenital instead of acquired (before or 

 after birth), all the more as in some of these cases the parturition of 

 the parents has had to be dated far back into the past, or else they 

 have already been dead before the statistics were collected, which needs 

 must entail unreliable statements. 



The tables in question thus decidedly point to the conclusion 

 that Mendels law is applicable to this large group also. No less 

 than 2111 cases are concerned. 



Now we will proceed to examine the problem from another 

 point of view with the aid of the literature. Lemcke has found 

 that in consanguineous unions the percentage of descendants born 

 deaf amounts to no less than 31,5, which closely corresponds to 25 % 

 in reality. His material is, however, somewhat scanty (28 out of 

 89). Uchermann, on the other hand, who commands a much larger 

 material, has found exactly 25 % (236 born deaf out of 944). This 

 figure is somewhat too low, it is true, but Uchermann himself, in 

 the introduction to his work, remarks that in some children of young 

 ages the deaf-mutism could not be ascertained when he made up 

 the statistics, but that this can only be done some time later on. 

 To this must be added that Uchermann has so h*igh a percentage 

 of acquired deaf-mutism as 18,55 in the very same consanguineous 

 marriages. It is quite possible that in some of these cases there has 

 been confusion between acquired and congenital deaf-mutism. 



