58 A NEW TYPE OF BRACHYPHALANGY IN MAN. 



The clear definition of the ''specific modifiers" as given above — the 

 term was first used by Bridges (1916) — ^was, however, first given after 

 they had been found in Drosophila, where their conclusive demon- 

 stration is possible through linkage tests. 



In Drosophila these genes have been studied by Morgan, Dexter, 

 Bridges, Sturtevant, MacDowell, Muller, and Altenburg, and the 

 work along this line has proved beyond doubt that they are extremely 

 common in this material. Bridges has, for instance, isolated not less 

 than 9 of them, which all modify the ''eosin" eye color, while they do 

 not affect the red color of the eye of the wild fly. At present more 

 than 25 specific modifiers are known in Drosophila — ^according to Dr. 

 Bridges's personal information — and whenever a character shows 

 marked variability it seems practically only a question of time before it 

 will be possible to isolate, through inbreeding, a modifier, or modifiers 

 which influence the character. 



In a general way, it can safely be stated that this material is not at 

 all exceptional in this respect. The discussion regarding the effect of 

 selection and the large amount of experimental data underlying it 

 furnish conclusive evidence for the very common occurrence of genes 

 of this kind throughout the whole animal and plant kingdom.^ All 

 individuals are probably heterozygous for a multitude of them, and it 

 is a priori unthinkable that man here should represent an exception. 

 Evidence in support of the hypothesis that skin color in man depends 

 on multiple genes of this type is already found in Davenport's investi- 

 gations on crosses between races with different color (Davenport, 

 G. C. and C. B., 1910; Davenport, C. B., 1913). 



In reviewing the literature concerning brachyphalangy in man 

 (p. 13) we noticed that the majority of cases showed a very striking 

 variability in the somatic appearance of the character. Not only was 

 the degree of shortening very different in different members of the 

 same family, but quite different phalanges or metacarpi showed the 

 shortening and some members showed a combination of the brachy- 

 phalangy with hyperphalangy or anchylosis, others not. In fact the 

 case we are here dealing with is exceptional in respect to regularity of 

 the inherited character. 



The striking irregularities in most of the other reported cases can 

 hardly be explained without the assumption that the normal indi- 

 viduals who married into the families must have been heterozygous for 

 different modifying genes. This would not be surprising. When, 

 for instance, we remember that within comparatively few years more 

 than 50 Mendelian genes have arisen which influence the eye color 

 in Drosophila, more than 40 affecting the shape or length of the wings, 

 it seems very likely that a high number of genes must be involved in 

 the formation and development of any other organ, such as a normal 



^ As to this we may refer to the discussions by East (1912), MacDowell (1914, 1915, 1916), 

 Morgan (1918), Sturtevant (1918), and Bridges (1916.). 



