36 H. LUXDBORG 



of his cases, and after that arrived at such a low figure. The later 

 and more carefully the investigations have been carried out, the 

 more clearly is it recognized that the acquired deaf-mutism is 

 far more common than the inheritable. Meningitis and scarlatina 

 are the most frequently occurring causes of the former. 



In a treatise of the year 1912 I discussed the question of the 

 heredity of constitutional (= genotypical) deaf-mutism *. 



I availed myself of the extensive evidence collected by Fay in 

 U. S. A. (Marriages of the deaf in Arr-erica, 1898). To this end I 

 had to spend much time in drawing up numerous comparative tables. 



In opposition to Hammerschlag and others I then arrived at 

 the conclusion that the inheritable form of deaf-mutism probably 

 obeys Mendel's law as a recessive and monohybrid character. 



In his Vererbungslehre, published shortly after my treatise, 

 Plate bases his opinions on the incomplete records and analyses 

 of some older observers, and puts forth the hypothesis that deaf- 

 mutism is not a monohybrid, but a dihybrid character. He supposes 

 that 2 conditional factors are required in order to produce the abnormal 

 condition (deafness). This hypothesis of Plate's is probably not to be 

 maintained, as I shall show a little further on. 



In his treatise just mentioned Bergh refers to my view, but 

 seems to think that his own observations, at least in part, are in 

 contradiction with it. This is, however, a mistake on his part caused 

 by an insufficient knowledge of the laws of heredity. Bergh evi- 

 dently considers that in human life we must get exactly 25 % 

 affected or abnormal descendants, respectively, in marriages between 

 (phenotypically normal) heterozygous individuals. That would no doubt 

 be the case if we were able outwardly to tell heterozygotes from 

 normal homozygotes, but unfortunately we cannot do that. Only 

 on the basis of the nature and character of the offspring are we 

 authorized to assume that in the cases given the parents must be 

 heterozygotes. 



Those heterozygotes, on the other hand, who have only a small 

 number of normal children (= those equipped with perfectly nor- 

 mal senses) escape our observation. Children of these marriages 

 are not included in a calculation of the percentage of the genotypical 

 deaf-mutes in relation to the healthy individuals and of the pheno- 



^ H. Li'NDBORG, Ueber die Erblichkeitsverhältnisse der konstiiutionellen (here- 

 ditären) Taubstummheit und einige Worte über die Bedeutung der Erblichkeitsfor- 

 schung für die Krankheitslehre. Arch. f. Rassen- und Gesellschafts-Biologie 1912. 



