36 The Third and Fourth Generation 



Mendel's experiment with the yellow-green 

 cross, as have others, and obtained 105,145 

 yellow peas to 34,792 green in the F, generation, 

 a proportion of 3 .02 to i. 



When the recessive peas of the second 

 generation (the grandchildren) are interbred, 

 they are found to give nothing but recessives; 

 green peas of this generation gave Mendel 

 nothing but greens when planted generation 

 after generation; short plants produced only 

 shorts. A third of the dominants were found 

 to give nothing but dominants, but the other 

 two-thirds, when interbred, gave offspring that 

 were again of the two types, dominant and 

 recessive, and again in the proportion of 3 to i . 

 Using the letters D and R to stand for dominant 

 and recessive, respectively, Mendel's law for 

 the crossing of two plants or animals differing 

 in only this one particular may be diagram- 

 atically shown on opposite page. The plants 

 of the Fi generation (children) are designated 

 D (R) rather than just D, although they show 

 only the dominant character, because the reces- 

 sive character shows up in the next genera- 



