An Introduction to a Biology 



hornless, one will be pure breeding and two will be hybrids. 

 But how are we to tell which are which ? In the peas it was 

 easy enough ; peas are hermaphrodite, both sexes being 

 present in a single flower, so that we could know that in allow- 

 ing them to self-fertilise we were mating pure with pure or 

 hybrid with hybrid. But if we took a hornless cow of the 

 second hybrid generation and a hornless bull, we should not 

 know with regard to either of them whether the animal was 

 pure or hybrid. There is, however, a very simple way of 

 finding out that is, by mating the hornless animal in ques- 

 tion with a horned one. If the hornless animal is pure, all its 

 offspring by a horned animal will be hornless ; if it is hybrid, 

 half the offspring will be hornless and half horned. The 

 general rule is : To find out whether an individual bearing 

 a dominant character is pure for that dominant character 

 (i.e. will produce exclusively individuals bearing that domin- 

 ant character when mated with a similar pure), or is hybrid 

 for that dominant character (i.e. will produce three in- 

 dividuals bearing that dominant character, and one indivi- 

 dual bearing the recessive character in every four on the 

 average), mate it with an individual bearing the correspond- 

 ing recessive character. If the individual was pure, all of 

 the offspring of such a cross will be individuals bearing the 

 dominant character ; if it was hybrid, half the offspring 

 will bear the dominant and the remaining half the recessive 

 character. 



A case amongst cattle which is parallel with that of the 

 Andalusian fowl is afforded by the inheritance of colour 

 amongst shorthorn cattle. The roan colour has been shown 

 to be produced by crossing the red with the white. In the 

 two cases, therefore the colour of shorthorn cattle and 

 that of the Andalusian fowl the red corresponds with the 

 black, the white with the white, and the roan with the blue 

 Andalusian. An even closer parallel amongst cattle to the 

 blue Andalusian is the blue roan, which is produced by cross- 

 ing black with white. There is a difference, however, be- 



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