POLYDACTYLY AND SYNDACTYLY 509 



are records it would appear that the brachydactylous 

 individuals were hybrid since they give two equal classes 

 of descendants when mated, i.e., back-crossed, to the 

 normal recessive. Doubtless individuals may occur who 

 are pure for brachydactyly and would have all their 

 children affected, i.e., produce a uniform Fi. 



Polydactyly and Syndactyly (Fig. 146). — In the 

 former there is an extra finger or toe. It not only occurs 

 in man but is also well-known in poultry. The gene for 

 this character is dominant over normal and is transmitted 

 usually to about half the offspring, because the affected 



I T \ — ' \ 



I i_ ^ ^ * 6Z,f: ' 



|T — ' — ^ — I T-^^ 5 Z \ _ 



^ L o o o o •6/:Z. id/iz ie/iz i6f:Z 



I I. r 1 r — I 



/r o 4syndaktyl f6^Z. ^ • O 



6 

 Fig. 146. — Inheritance of Supernumerary Digits. Affected indi- 

 viduals are indicated by dark symbols and unaffected by light ones. 

 F indicates extra fingers and Z extra toes. Females are indicated 

 by 9 and males by cf. , 



Plate, Vererbungslehre. Englemann. 



parent is ordinarily a hybrid. It occasionally happens 

 that apparently normal (i.e., recessive) offspring transmit . 

 the abnormality to their children. This would indicate 

 that other factors or conditions are concerned in the ex- 

 pression of this character. In syndactyly the fingers or 

 toes are united side by side, hence the common term 

 " web-foot." This condition is also due to a single domi- 

 nant factor difference. Affected individuals (usually be- 

 ing hybrid) transmit it to approximately half their 

 offspring, and the normal children (being pure reces- 

 sives) do not transmit it. 

 Skin Color presents a more complicated situation and 



