56 A NEW TYPE OF BRACHYPHALANGY IN MAN. 



we should expect the majority of cases to be of an intermediate type 

 between the extreme ones examined. 



A possible suggestion that the different occupations of the individuals 

 might be of importance for the development of one or the other type 

 is clearly disproven by the facts. In the family record the occupation 

 of the brachyphalangous individuals is given, and it will be seen there 

 that both types occur within the most different occupations. There 

 is no support for the belief that hard physical work or lack of physical 

 work might favor the development of one type or the other. 



The question arises, accordingly, whether or not this occurrence of 

 two types can be explained on a genetic basis. To be able to give an 

 answer we will have to study the distribution of the two types within 

 the part of the family where our information is extensive enough with 

 regard to this special point. This is the case within line 1 for the four 

 generations including ll.Sd 71 C. A. B. H. and his descendants, and 

 within line 9 for most families descending from 19.4 9 L. E. 0. 



In text-figures 3 and 4 the pedigree is given for these parts of the 

 family. A glance at these pedigrees reveals the following points : 



The difference in types bears no relation to sex; neither of the types 

 is sex-linked. 



A B !-type brachyphalangous individual may have B-type brachy- 

 phalangous offspring and vice versa. A good illustration of this fact 

 is found in line 1, where ll.Sd 1 is brachyphalangous of the B !-type, 

 his daughter 115.1 9 of the B-type, her daughter 1151.2 9 of the 

 B-type, and her son 11512.1 cf of the B !-type. 



Striking, also, is the fact that the B !-type prevails in line 1, while 

 the B-type prevails in line 9. In line 1 only 2 individuals belong to the 

 B-type as against 9 of the B !-type. In the part of line 9 here studied 

 we know, on the contrary, 10 B-type brachyphalangous individuals 

 and only 1 of the B !-type. 



It is clear from these pedigrees that we are not dealing with two 

 independent genes, one causing the B-type and the other the B !-type. 

 The question, then, is whether the presence of a specific modifying 

 gene may account for the appearance of the two different types. 



One of the most remarkable features in modern genetic work is the 

 steadily increasing amount of information concerning the very frequent 

 occurrence of specific modifiers which influence the somatic appear- 

 ance of other Mendelian characters. As Morgan puts it (1918) : 



"Most interesting of all the mutations that are now engaging the attention 

 of students of mutation are those in which genetic factors or genes occur, or 

 appear, whose most obvious action is to enhance or diminish some other, more 

 conspicuous character. These genes may be called specific modifiers. They 

 do not differ from ordinary genetic factors in any essential respect, but since 

 their presence can not be detected except when other factors are themselves 

 producing some particular effect on the individual, it is convenient to give 

 them a special designation." 



