APPENDIX B 369 



makes the mistake, so to speak, of treating them as sexual. Of 

 course, however, the alternative inheritance of large non-sexual 

 differences is not certain, not so " clean " as that of sexual 

 differences, the alternative inheritance of which has been 

 established by a long course of selection. 



It nearly approaches perfection in the case of haemophilia, 

 which, when it occurs, is almost as much a male character as a 

 beard. But in most instances anomalies occur which tend to 

 link up the perfectly alternative type of inheritance with the 

 perfectly blended type. Thus, as in the case of Mr. Hurst's 

 poultry hybrids, partial blending was very common. Again, the 

 alternation of Mendelian characters is not so perfect as that of 

 the sexual characters; one of a pair is generally so dominant 

 over the other that when the two are brought together, the latter 

 is almost always reduced to latency for example, when an albino 

 is crossed with a normal individual, the offspring are generally 

 all pigmented, the albino only reappearing, if at all, in subsequent 

 generations. 1 Lastly, the various items of a set of Mendelian 

 characters do not tend to hang together with the persistency 

 which usually characterizes the sexual characters. Unlike the 

 latter they behave as discrete allelomorphs, each of which is 

 independently capable of displacing its opposite number. This 

 is only what might be expected ; it has been a main " object " of 

 Natural Selection to evolve a definite combination of sexual 

 characters, but not of Mendelian characters. 



A circumstance which tends to confuse our thinking on this 

 subject, and so to obscure our recognition of the essential simi- 

 larity between the Mendelian and the sexual phenomena, is the 

 following : When we cross two types that possess alternative 

 Mendelian characters, the first generation consists of impure 

 dominants. If these are self-fertilized or bred together, as they 

 usually are in Mendelian experiments, we get pure dominants, 

 impure dominants, and pure recessives, in more or less definite 

 proportions. The pure dominants can then be bred together, 



1 It should be noted that, in man at least, the dominance of the pigmented 

 type over the albino is by no means complete. Suppose an individual varies 

 so from his parents as to be a pure albino. The chances are he will mate with 

 a pigmented individual. His children will be pigmented impure dominants. 

 The allelomorphs will separate in the gametes. Albinos are so few that they 

 practically always mate with pigmented persons. Therefore, if the dominance 

 of the pigmented type were complete, all the offspring and descendants should 

 be pigmented, except in the very rare cases when an albino allelomorph meets 

 another of the same kind. But the descendants of albinos are so frequently 

 albinos, that the latter hypothesis seems excluded. It appears clear, therefore, 

 that the albino descendant of an albino is probably not always a pure 

 recessive, but that he is often an impure dominant in whom the albino 

 character has the upper hand. In this imperfect dominance of the pig- 

 mented type, albinism resembles the sexual characters which are alternatively 

 dominant. 



B B 



