296 PLANT HYBRIDIZATION BEFORE MENDEL 



The matter may be simply outlined by the usual familiar dia- 

 gram. Supposing the case of plants bearing round, and plants 

 bearing wrinkled seeds; the dominant parent (round) being ex- 

 pressed by R and the recessive parent (wrinkled) by W, then: 



(1) R X W Parents 



In the above diagram it is assumed (and so it occurred in Men- 

 del's case) that the R's and RW's in (3) will all look alike — i.e., 

 that they will all appear round, and hence will make up 75 per 

 cent of the total, while the other 25 per cent will be pure W's 

 or wrinkled's, and will appear as such. Now by growing self- 

 fertilized plants of the combined R's and RW's in (3) it will 

 come out in the F3 generation (4) that one-third of the combined 

 lot of R's and RW's of (3), i.e., 25 per cent of the whole number, 

 will breed true as R's, while the other two-thirds (50 per cent of 

 the entire number) will turn out to split up again in the ratio of 

 three R's, or rather of apparent R's, to one W — in other words, 

 the ratio that indicates their hybrid composition. 



We can say, therefore, that in any case of simple Mendelian 

 hybrids, i.e., where one character-pair only is concerned, the 

 hybrid or F^ generation has always internally the following in- 

 visible composition, which can be revealed by breeding, 



25% 50% 25% 



pure dominant-recessive pure 



dominant or hybrid (appearing recessive 



dominant) 



All these are bound together in the F^ generation under the 

 apparent uniformity which the dominant character imposes. Gen- 

 eralizing, and expressing the dominant by D and recessive by R, 

 we have : 



D X R = DR 



DR X DR=lDD :2DR: IRR 



Thus far we have followed, in considerable detail, Mendel's 

 original experiments themselves. It is plain that these exhaustive 



