The progress of (Tenetics since the rediscovery of Mendel's papers. 375 



mauy examples in which dominance is imperfect and several in which 

 it is, so far as is at present known, irregular. AVhen dominance is 

 imperfect F, may not improperly be described as a blend form, such 

 that the heterozj^gotes may be distinguished without difficulty from 

 the two homozj^gous types. It is for the most part in connection 

 with the occurrence of such blendings that the more important irre- 

 gularities of dominance have been found. In most of these examples 

 segregation has been proved to occur, though it will readily be 

 understood that when dominance is liable to a fluctuation so great as 

 to make determination of the homozj^gous types in Fo impossible, the 

 proof of complete segregation becomes unattainable. Of this latter 

 class very few illustrations are yet known. 



As illustrating irregular dominance with occasional appearance 

 of blends in Fj the following cases may be given. 



LIST II. 



1. Tall and dw^arf in Phaseolns. Here tall is usually dominant, 

 but the height of F^ is less than that of the tall type 

 {Tschermak(lOl)). 



2. Hard pods with "parchment" and soft pods without '"parch- 

 ment" Pisiim (R. E.G. (10 a)). Mendel regarded the hard as 

 dominant. Generally it is so, but the hardness is almost al- 

 ways distinctly impaired in heterozygotes. 



3. The same is true for Phaseolns, but the dominance is subject 

 to much greater fluctuations (Emerson (54)). There are two 

 pairs of characters concerned in the texture of the pods, which 

 Emerson calls (1) stringiness and stringlessness (2), tough- 

 ness and tenderness. The second pair corresponds with the 

 two types found in Pisiim pods. In both respects the do- 

 minance is very irregular and blend-forms occur, but segre- 

 gation may be complete. 



4. The extra toe in Fowls is usually dominant over the normal 

 foot, the F^ form in such cases being sometimes indistingui- 

 shable from the pure five-toed type, though generally having 

 the extra toe of reduced size. Occasionally however the 

 heterozygote shows no trace of the extra toe, which is then 

 a recessive character and may reappear in the next gene- 

 ration. (R.E. C. (10,10 a)) Polydactylism in the Guinea pig 

 has a somewhat similar behaviour (Castle (17)) and the 

 evidence in cases of human polydactylism suggests heredity 

 of the same type. Attention may be called to the scarcity 

 of clear evidence of segregation in regard to Meristic 

 characters. That observed by Farabee in the segmentation 



