ii8 • Experime7ital Zoology 



If polydactylism is dominant and the normal condition is reces- 

 sive, the chances are that any polydactyl person has had one nor- 

 mal parent and the germ-cells are therefore P and N.^ Paired 

 with a normal individual (N + N), half the children should be 

 polydactyl and half normal. In the above case there were in 

 fact in the first generation four normal and four polydactyl chil- 

 dren. In the second generation when normal offspring paired 

 with normal, 5 polydactyl children and 21 normal were pro- 

 duced ; and when the polydactyl descendants paired wdth nor- 

 mal, 7 polydactyl children and 12 normal were produced. 

 For the small number recorded, the latter result is not very 

 different from 1:1, the Mendelian ratio. Again in the third 

 generation when P was mated to N, 5 normal and 5 polydactyl 

 children were born. 



Struthers gives the following case of polydactyl inheritance in 

 man : — 



I. Px ? 



II. 



iP(xN) 10 P 



3F iP(xN) 



III. 4N 4P 



The result can only be explained on the Mendelian view by assum- 

 ing that both parents of the first generation were polydactyls, i.e. 

 produced germ-cells bearing polydactylism. It is necessary to 

 make this assumption in order to account for the second genera- 

 tion that descended from one of the first filial generation. Here 

 a polydactyl parent married a normal individual and produced 

 only polydactyl children, showing that no normal germ-cells 

 were present in one parent. Had there been some normal germ- 

 cells, some normal children would be expected, provided the 

 numbers are really large enough to give this result a chance to 

 appear. In the third generation an equal number of the two kinds 

 of offspring are expected, and such are found. A third case is 



1 Or P(N) and N(P) on my view. 



