4 INHERITANCE IN POULTRY. 



In typical Mendelian cases not only do the qualities segregate in the second 

 hybrid generation, but in addition, in the first generation, when two con- 

 trasted characteristics are bred together one of the two is patent in the 

 offspring ; the other does not appear. The first is the dominant quality ; the 

 second is recessive. 



It seems at first to have been assumed that when one of two antagonistic 

 characteristics was dominant over the other it was so in all cases. Recent 

 studies have, however, greatly expanded our notion of dominance and reces- 

 siveness. Even in alternative inheritance we have to admit various addi- 

 tional phenomena of which the following are examples : Prepotency of a 

 character, elsewhere recessive, in some individual or strain. Latency, as 

 Castle (1905, p. 24) uses the term, or the inactive persistence of a normally 

 dominant characteristic in a recessive individual or gamete. When the 

 recessive is cross-bred the latent characteristic may appear as a dominant. 

 Reversion, or the assumption of an atavistic character by a heterozygote. 

 This is illustrated by the case of the cross between albino and black-and- 

 white mice which throw gray. However, this instance may be one of 

 latency. In this study attention will be paid to these phenomena. 



What determines dominance in any case? This is a disputed point. 

 De Vries (1905, pp. 278, 280) suggests "that hybrids between a species and 

 its retrograde variety will bear the aspect of the species," and "that the 

 older character dominates the younger one." However, he says it is not 

 the systematic relation of the two parents of a cross that is decisive, but only 

 the occurrence of the same quality, in the one in an active, and in the other 

 in an inactive condition. Hence, whenever this relation occurs between the 

 parents of a cross the active quality prevails in the hybrid, even when the 

 parents differ from each other in other respects so as to be distinguished as 

 systematic species. Correns (1905) also cites cases in which the active 

 allelomorph dominates. In my studies constant attention is directed toward 

 this matter. 



To recapitulate : This study has been undertaken to determine the differ- 

 ent forms of inheritance (alternative, particulate, blending) occurring in 

 poultry, and to study especially the phenomena of alternative inheritance as 

 exhibited in this group in order to see in how far they accord with Mendel's 

 law and in how far the accessory phenomena of dominance, latency, and 

 reversion occur. 



