xiv] Irregular Dominance 255 



3. Irregularities of Dominance. 



It must be anticipated that irregularity or fluctuation 

 in dominance will prove a frequent source of exceptions. 

 Neither among animals nor plants is material often to be 

 found so homogeneous that the heterozygotes formed from 

 various individual pairs are quite identical, and the domi- 

 nant factors thus produce their effects in varying degrees. 

 Reference has already been made (p. 236) to the fact that 

 dominance may sometimes be so complete that the hetero- 

 zygote may pass for the pure homozygous type, while in 

 other individuals the distinction can be recognized. Such 

 cases present no real difficulty in analysis, for the two 

 classes can be taken as one in the counts of F 2 . Serious 

 difficulty is caused however in a few examples by the fact 

 that a dominant factor may be present without producing 

 any effect by which its presence can be perceived. Such 

 dominant individuals are indistinguishable from recessives, 

 and analysis thus becomes difficult or impossible. Con- 

 spicuous examples of this ambiguous phenomenon have 

 occurred in two of the subjects investigated on a consider- 

 able scale. The first is the heredity of the extra toe in 

 fowls. In our work we have found the extra toe usually to 

 be a dominant. Such a 5-toed breed as Dorking or Houdan 

 crossed with a normal 4-toed breed commonly gives F l birds 

 with extra toes developed to an extent somewhat less than 

 that found in the normal 5-toed type, and F 2 from such birds 

 is of the usual composition, averaging 3 birds with extra 

 toes more or less developed, to i normally 4-toed. But 

 it is by no means rare to meet with birds of the 4-toed type 

 which when crossed with an extra-toed breed will throw 

 many F^ with 4-toed feet, or with traces only of the extra 

 digit. The two feet of the same bird may also differ, pre- 

 senting every combination. Such birds have in several 

 instances been proved by their progeny to be ordinary 

 heterozygotes. At first one might be disposed to attribute 

 the peculiarity to interference caused by some epistatic 

 factor repressing the development of the dominant charac- 

 teristic ; bat this account cannot very readily be reconciled 

 with the actual numbers, for there is no consistent diminution 

 in the number of the dominant offspring of such birds. The 



