THE INHERITANCE OF FAMILY TRAITS 177 



strain) will not show the abnormality in their offspring. 

 This expectation is indeed realized in most of the pedigrees 

 published; as for instance in that of Parker and Robinson 

 (1887, Clin. Soc. Trans., Vol. XX., p. 181), Fig. 160. 



I DtO 



DDrO D[OOraDTO D[# DiO OfQ 



Fig. 160. — A pedigree of syndactylism, or "split foot/' All affected per- 

 sons are from an affected parent; hence the trait is a positive one. Little is 

 known about the condition of the digits in the first generation. Parker and 

 Robinson, 1887. 



The general conclusion is that, while a syndactyl individual 

 will transmit his trait, normals from a syndactyl strain have 

 little chance of doing so. 



h. Brachydactylism. — This is a condition of shortened 

 digits due to the presence of only two segments to the digit — 

 so that all fingers are like thumbs. The middle phalanx is 

 usually a more or less rudimentary bone attached to the 

 base of the distal phalanx. Inheritance follows the laws of 

 syndactylism. Two normal parents produce only the normal 

 condition; no generation is skipped. 



i. Other deformities of the hands. — From time to time 

 other digital peculiarities have been recorded and these are 

 usually strongly inherited. Thus Dobell has described a 

 family in_ which the hands are double jointed, all joints 

 thick, ring and little finger crooked from the last joint. The 

 pecuharity is distinguishable at birth. The law of inheritance 

 is the same as for syndactylism; viz., normal parents have 

 no offspring with the defect; but one affected parent tends 

 to transmit the defect to half (rarely all) of his offspring 

 (Fig. 161). The tendency of the great toe to grow under 

 the others occurs in at least one family strain (Fig. 162) and 



