QUALIFICATIONS OF MENDEL'S LAWS 155 



units of a set are supposed not to hang together but to be inherited 

 independently of one another so that new combinations may be 

 formed. They have been termed * allelomorphs,' alternative 

 inheritable units, each of which in one set of allelomorphs is 

 capable of displacing its opposite number in another set. (3) One 

 unit of every pair tends to be dominant over its alternative, so 

 that when both are present in a zygote the individual which springs 

 from it displays the dominant character only. Such, at any rate, 

 were the chief points of the Mendelian theory as originally pro- 

 pounded, and as held very recently. 



261. More extended study, however, has rendered it evident 

 that in all points the Mendelian doctrine needs more or less qualifica- 

 tion. Dominance is seldom if ever complete. As regards any 

 Mendelian character, the hybrids of the first generation (all impure 

 dominants) usually resemble the dominant parent more nearly than 

 the recessive ; but the influence of the latter may be observed, at 

 least very often. " Even in the pea it is not the case that the 

 heterozygote [mongrel] always shows the dominant allelomorph as 

 clearly and in the same intensity as the pure dominant, and, speak- 

 ing generally heterozygotes, though in numerous instances readily 

 referable to one or other of the allelomorphic types, exhibit these 

 types in a more or less modified form." 1 Frequently indeed a 

 combination of parental qualities amounting to particulate, mosaic, 

 or even blended inheritance is observable in offspring. Thus a 

 cross between black and white plumage in poultry may result in 

 barred offspring, as often happens when the white Leghorn fowl 

 is crossed with the black Cochin ; 2 or in a minute patchwork, as 

 in the case of the blue Andalusian fowl which results from crossing 

 a black bird with a ' splashed ' white ; or there may be complete 

 blending, as in the offspring of a white man and negress. 3 



262. Sometimes the usually recessive character may be 

 dominant. 4 Again, instead of one type being dominant over the 

 other, the character of neither parent is reproduced, but a common 

 ancestral form reappears. For example, when albino mice are 

 crossed with black-and-white Japanese waltzing mice "a grey 



1 First Report to Evolution Committee, p. 129. 



2 Davenport, Inheritance in Poultry, p. 40. 



3 Adherents of the Mendelian doctrine sometimes maintain that blended 

 inheritance is nothing other than a fine mosaic. Doubtless they are right ; but 

 I imagine their opponents mean nothing more by blended inheritance than that 

 it is a very fine mosaic a mosaic as fine, perhaps, as the mixture of colours in a 

 painter's brush. All blending, for example a mixture of alcohol and water, is a 

 mosaic, even if one of chemical molecules. 



4 First Report to Evolution Committee, p. 120. 



