60 A NEW TYPE OF BRACHYPHALANGY IN MAN. 



that he had no B !-type children by his second marriage. This is well 

 in accord with our hypothesis: 194.2 cf himself being B-type brachy- 

 phalangous is heterozygous for B. His first wife was heterozygous 

 for the modifier M B , his second was not. 



As to the expected numerical ratio between B-type (B) and B !-type 

 (BM B ) individuals, the numbers are evidently far too small to be of 

 significance. The expectation would be that a B !-type individual in 

 marriage with individuals who do not carry the modifier should have 

 B '.-type (BM B ), B-type (B), and normal children in the ratio 1:1:2, 

 half of the latter carrying the modifier M B in heterozygous condition. 



But as the hypothesis demands that this modifier must be quite 

 common, that is, numerous normal people must be heterozygous for M B , 

 we have always the possibility of this gene being introduced by both 

 parents. This will change the expected ratio to 3 BM B : 1 B : 4 normals, 

 3 of which carry the modifier in homozygous or heterozygous condition. 



On the assumption that we are dealing with a dominant modifier 

 we have seen that two normal individuals married into line 1 and one 

 married into line 9 must have been heterozygous for this gene. We 

 have no evidence of any relationship of these individuals to each other 

 or to the members of the family. An alternative hypothesis assuming 

 the presence of a recessive instead of a dominant specific modifier 

 would demand a much more general occurrence of the modifying gene 

 among normal individuals, since the family has not remained in one 

 locality but has crossed with many other stocks. 



As an analogous example of specific modifiers in mammals we might 

 mention the inheritance of the mutant character rough coat in guinea 

 pigs. Rough is dominant over smooth in a typical way, but in crossing 

 smooth individuals from certain (mostly wild) stocks to rough, Castle 

 (1905), Detlefsen (1914), and Wright (1916) obtained so-called "par- 

 tial-rough" individuals among the offspring. Wright (1916) showed 

 that the "partial-roughs" differ from ordinary full roughs by possessing 

 an independently inherited gene (or genes) introduced by the smooth 

 individuals from the special stocks. In this case the specific modifier is 

 incompletely dominant. 



In finishing the discussion of the two types it many be mentioned 

 that the investigation of line 9 alone might easily have led to the 

 entirely false conclusion that the character showed a "weakening" 

 through the later generations a statement often met with, especially 

 in earlier publications concerning heredity in man. A limited examina- 

 tion might indeed have given a conception of a very marked "weaken- 

 ing" of brachyphalangy when the data collected from now living mem- 

 bers of this line were compared with the description of the character 

 found in the family book and with the photograph from generation II 

 (fig. 3). The introduction of modifying factors in later generations 

 (e. g., as that found in this case) would give exactly this impression. 



