288 General Botany 



are two kinds of tall plants: (A) those that breed true and 

 (B) those that, like the hybrid tails of the Fi generation, have the 

 dwarfness latent in them. These latter plants produce three 

 tall and one dwarf in the F3 generation. The recessive dwarfs 

 that come out in the F2 generation always breed true. Hence, 

 in the F2 generation one fourth of the plants are pure tails, two 

 fourths are hybrid tails, and one fourth are pure dwarfs. 



A plant that breeds true for any character when self-pollinated 

 is said by breeders to be homozygous with respect to that character. 

 This implies that it carries only one kind of factor out of any pair 

 of contrasting characters. In the A-group of tall plants above, 

 both the pollen and the ovule (or more definitely the sperm and 

 the egg) carry only the factor for tallness. Hence all the off- 

 spring are tall. The other two tall plants, those of the B-group, 

 that appear among every four in the F2 generation, carry two 

 different factors and are called heterozygous. Half of the number 

 of their eggs and sperms will carry the factor for tallness, the 

 other half carry the factor for dwarfness. They produce, when 

 self -pollinated, plants with the dominant character and plants 

 with the recessive character in the proportion of 3 to i. 



With these facts in mind we can rewrite the above diagram, 

 using T for the dominant character, tallness, and d for the reces- 

 sive character, dwarfness : 



Sperm (or egg) Egg (or sperm) 

 T X d 



Fi generation = Td Td 



Selfing Td X Td 



/ / \ \ 



F2 generation = TT Td — Td dd 



Selfing TT X TT d X Td dd X dd 



I I Y 



F3 gensration = AU TT TT— Td— Td— dd All dd 



