SOME BELIEFS OF PRACTICAL BREEDERS 575 



qualities to all his female offspring regardless of the genotypes of the 

 females to which he is mated, whereas one of the genetic constitution 

 (Zm)(Zm)ll would transmit low egg laying capacity to such an extent 

 that among his daughters even from high producing hens none would 

 fall in the high producing class. For favorable characters the validity 

 of this interpretation depends upon dominance of the determining factors, 

 a condition by no means universally fulfilled. 



The Relative Factor Potency Interpretation. There is some evidence 

 that the potency of a given factor sometimes varies with the source from 

 which it is derived. Pearl has suggested for example that the factor L 

 when derived from the Cornish Indian Game has a lower absolute fecun- 

 dity value than that of the same factor in the Barred Plymouth Rock. 

 The suggestion amounts to an application of the hypothesis of multiple 

 allelomorphism, a graded series of multiple allelomorphs of differing 

 potencies, or different relations with respect to dominance, being con- 

 ceived to determine the absolute degree of expression of the factors. We 

 recall here Detlefsen's work with the agouti factor of the wild Cavia 

 rufescens which was recessive to the agouti pattern of the tame guinea- 

 pig, and less decided in its phenotypic expression. The conclusions of 

 Goldschmidt that races of the gypsy moth exist which have sex factors 

 of various potencies, such that crosses between them give series of 

 intersexual forms, while less definite with respect to the actual factors 

 involved, provides some evidence in support of the belief that some of the 

 phenomena of prepotency are dependent upon actual differences in the 

 factors themselves. 



The Hereditary Complex Interpretation. The characters for which 

 families are prepotent are evidently often complex, as for example 

 speed in horses, total butter-fat production in dairy cows, beef con- 

 formation in cattle, and so on. They must, therefore, depend upon 

 a favorable genetic constitution with respect to series of factors. 

 This interpretation is based upon the conception that factors form 

 physico-chemical reaction systems and it follows the lines which have 

 been developed in the application of this hypothesis to species hybrids. 

 We have pointed out for instance that varieties of Nicotiana tabacum 

 impress their total set of characters upon the hybrids with N. sylvestris 

 because of the dominance of the tabacum reaction system. Certain 

 characters which are recessive within the tabacum group are expressed 

 in such species hybrids apparently because of their interrelations with 

 other factors in the tabacum group. This idea is also borne out by 

 certain of the Drosophila experiments. Thus Morgan notes that the 

 factor for truncate wings, usually recessive, is dominant in races which 

 have the black factor. The hypothesis rests upon a belief that sometimes 

 factor interrelations determine whether a particular member of an 



