An Introduction to a Biology 



have no dominant and recessive characters here. In the 

 case of the peas, the hybrid and one of the parents — the 

 tall — were identical in appearance. Here the hybrid has a 

 character of its own, the blue plumage ; so that in the second 

 hybrid generation we have none of the difficulty which we 

 had in the peas, of distinguishing between pure tails and 

 hybrid tails ; we see at once which are the parent forms 

 (the blacks and the whites) and which are the hybrids (the 

 blues). The hybrid is always blue, and as there are no 

 blues that are not hybrids, the blues are always hybrids ; 

 so that it is running one's head against a stone wall to try 

 to breed a pure race of blues by continuing to breed from 

 blues for a great many generations. Yet that is the prin- 

 ciple on which we should have acted if Mendel had not made 

 his experiments with peas, or somebody had not discovered 

 the same thing with other or the same material. 



One more point before we proceed to the next case. The 

 two cases shown respectively in Figs. 1 and 2 differ from 

 one another in the fact that there is dominance in the first, 

 and blending in the second. In all other respects they are 

 identical ; in the reappearance of the parent and hybrid 

 forms in the second hybrid generation ; in the proportions in 

 which these forms appear ; and in their breeding properties. 

 And the conclusion we draw from this fact is that domin- 

 ance is not an essential feature of Mendelian inheritance. 



A case amongst cattle, exactly comparable to that of 

 tallness and dwarfness in peas, is that of the absence or 

 presence of horns. If a cow of a horned breed, such as the 

 Highland, is served by a bull of a hornless breed, such as 

 the Aberdeen Angus, all the first crosses will be hornless — 

 hornlessness being dominant. When these hybrids are 

 mated together, they will, if there are plenty of them and 

 they are given time, produce a generation consisting of 

 three hornless and one horned in every four, on the average. 

 The horned, since they bear the recessive character, will 

 breed true at once when mated together ; but of the three 



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