228 Irregular Phenomena [ch. 



one case of direct transmission from an alkaptonuric 

 parent, and it is of course improbable that this extremely 

 rare condition should have existed on the other side of 

 the parentage. Though the evidence is scarce, it is very 

 important, since the question of the actual mode of 

 descent followed by such a "chemical sport," as Garrod 

 has called it, is of absorbing interest. Anyone who has 

 an opportunity should not fail to use every effort to trace 

 the descent of alkaptonuria. 



Miscellaneous irregular Phenomena of Inheritance 



hi Man. 



There remain great numbers of pedigrees of malforma- 

 tions and diseases which in our present ignorance seem 

 altogether irregular. Of these some, for example, poly- 

 dactylism, are perhaps to be regarded as due to dominant 

 factors which can be inhibited or suppressed as a result of 

 the presence of other factors. In poultry we know by 

 experiment that the presence of extra toe may behave as a 

 dominant, following the simple rule with fair regularity, but 

 in other families the number of dominants produced is too 

 small and transmission may occur through normals destitute 

 of extra toes Such facts point to the existence of some 

 unknown complication in those families. 



Several pedigrees of ectrodactylism, the deformity of 

 hands or feet by the absence of digits, are recorded in 

 medical literature. Transmission in these cases usually 

 occurs through the affected members, but the degree of 

 malformation is exceedingly irregular, and the number of 

 affected persons is higher than expectation. On the 

 evidence it is improbable that any simple Mendelian scheme 

 will express all these descents'^. In Fotherby's "split-hand" 

 family a case of polydactylism also occurred, and the con- 

 dition is not very rarely recorded as associated with the 

 more extreme forms of human monstrosity. 



A vast literature exists relating to the heredity of various 



"^ See for instance Fotherby, Brit. Med.Jour.^ 1886 (i), p. 975; Tubbv; 

 Lancet, 1894 (i), p. 396 ; for a full and more recent collection, also Lewis 

 and Embleton (171). 



