126 PLANT GROWTH 



Thus Mendel showed that even when thousands of plants 

 were used, the F2 had a ratio of approximately 3 dominant 

 plants to one recessive plant. There must be some reason 

 for this behavior which is consistent enough to be a basic law. 

 Mendel did not know about genes, and the chromosomes 

 hadn't been discovered. He did know that there were two 

 sex cells necessary for fertilization. Since these sex cells 

 unite to form a new individual, they must contain character- 

 istics of their respective parents. Suppose we were to take 

 pollen from a tall plant and put it on the stigma of a dwarf 

 plant. The pollen contributes some element or factor for 

 tallness; the egg contributes an element or factor for dwarf- 

 ness. Since all of the Fi were tall it is evident that when 

 both factors are present the factor for tallness dominates the 

 factor for dwarfness. But, dwarf plants appeared in the F2. 

 This means that the factor for dwarfness must separate from 

 the factor for tallness and appear by itself in a pollen cell or 

 an egg cell. This can be represented diagrammatically as 

 follows. Let T represent the factor for tallness and t the 

 factor for dwarfness, then: 



F, F2 



TT-*TALL PLANT 

 Tl —TALL PLANT 

 Tt -TALL PLANT 



t -DWARF PLANT 

 ZYGOTE ZYGOTES 



POLLEN Tv ^POLLEN 



EGG 



Tv ^^POLLEN t TSt 



Nti-TALL PLANT<^ /^^ 



As soon as we conceive of the idea of the factor for tallness 

 separating from the factor for dwarfness in the formation of 

 pollen and egg cells, we see the reason for the 3 : 1 ratio in F2. 

 Those plants with two elements for tallness are tall; those 

 plants with one tall factor and one dwarf factor are tall, like 

 the Fi; and only those with two recessive factors are dwarf. 

 Now, if this is the case, it should be possible to show, by 

 selfing the F2 plants, that some of the tall plants are pure 

 and will breed true while some are like the Fi and will pro- 

 duce some dwarf plants. This was perceived by Mendel, and 



