An Introduction to a Biology 



second hybrid generation is made up of 25 per cent, of plants 

 like one parent, the pure tall ; 25 per cent, of plants like the 

 other parent, the pure dwarf ; and 50 per cent, of plants 

 like the hybrid of the first hybrid generation. When I say 

 " plants like," I mean " like " both in their appearance and 

 in their breeding properties. 



The third point to be noticed is that individuals, in the 

 second hybrid generation, bearing the recessive character 

 ( dwarf ness in this case) breed true ; whilst indi\'iduals bear- 

 ing the dominant character (tallness) do not all breed true : 

 one out of every three, on the average, does ; but the re- 

 maining two produce some dwarf (one in every four plants) 

 as well. The general rule, which is derived from such a case 

 as this, is that an individual in the second hybrid genera- 

 tion which bears the recessive character may be counted 

 upon to breed absolutely true to that character, at once, i.e. 

 before breeding from it. But in the case of an individual 

 bearing a dominant character, w^e do not know whether it 

 will breed true to that character or not until we have bred 

 from it. The breeding properties of an individual bearing a 

 recessive character are branded on it in unmistakable letters 

 directly it appears in the second hybrid generation — PURE. 



It is not so with an individual bearing a dominant cha- 

 racter, which brings us to the last point to be noticed. It 

 is this. Two individuals (in the case before us, two plants 

 bearing the tall character) may be absolutely identical in 

 their outward appearance and yet differ profoundly in respect 

 of the kind of offspring which the}^ will produce. One tall 

 plant will produce nothing but tall plants. Another tall 

 plant, indistinguishable from the first, will produce 25 per 

 cent, dwarfs amongst its progeny. A more striking instance, 

 which will be explained later, is that of two pairs of white 

 rose-combed fowls. One pair will produce nothing but white 

 rose-combed birds. Another pair, identical in appearance 

 with the first pair, will produce, besides white rose-combed 

 birds, white single-combed, black single-combed, and black 



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