44 A NEW TYPE OF BRACHYPHALANGY IN MAN. 



VI. HEREDITARY TYPE OF THE CHARACTER STUDIED. 

 1. GENERAL DISCUSSION. 



On the basis of the data given in the family record, it is now possible 

 to state the hereditary type of the malformation with absolute cer- 

 tainty. It is not necessary to emphasize that we are dealing with a 

 clear-cut case of Mendelian heredity (cf. the pedigree). The funda- 

 mental Mendelian principle, the principle of segregation, is strikingly 

 demonstrated. The character behaves in every respect as a typical 

 mutant character. 



In line 1 we have been able to follow the brachyphalangy for 6 gen- 

 erations through heterozygous individuals, who may have brachy- 

 phalangous or normal children. The latter are free from the gene 

 causing brachyphalangy and their descendants will be normal 

 provided no intermarriage with brachyphalangous individuals takes 

 place. A good illustration of this fact is 11.4 9 L. A. H. H., a daughter 

 of the brachyphalangous 1.1 9 G. H. 0. Her hands were normal and 

 all the 28 individuals of her descendants in the following three genera- 

 tions are free from brachyphalangy, as was to be expected. 



The character is not sex-linked, as may be easily seen from the 

 pedigree. 



The fact that the brachyphalangy appears in heterozygous individ- 

 uals shows that it is a dominant character, and the material we are 

 dealing with furnishes us with interesting facts for a further discussion 

 of this special point. 



The individuals showing a dominant character in human material 

 will in the large majority of cases be heterozygous for the factor 

 involved. Intermarriage is in general rare and it is obvious that 

 individuals affected by the same hereditary malformation are even 

 more unlikely to marry each other. 



Although the data concerning dominant hereditary malformations in 

 man, therefore, are generally restricted to heterozygous individuals, it 

 is nevertheless usually taken for granted that the dominant character 

 described is the same that would appear in homozygous individuals. 

 Besides this, another statement is very often met with, namely, that 

 the characteristic for a dominant character is the fact that it never 

 skips a generation, in the sense that only individuals that show the 

 character can transmit it to their children. Both these points are open 

 to serious criticism. A reservation is necessary, especially because 

 both matters are of importance from a medical point of view. 



It must first be strongly emphasized that a knowledge of the effect of 

 a gene when heterozygous by no means entitles us to conclude that the 

 effect will be the same when it is homozygous. The complete domi- 

 nance found, for instance, in mice, where a gray individual heterozy- 

 gous for albino can not be somatically distinguished from homozygous 



