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TEXTBOOK OF BOTANY 



Anyone who is familiar with the facts discussed in the section on the 

 pairing and segregation of chromosomes in the previous chapter can use 

 the foregoing facts about peas and predict what Mendel must have ob- 

 tained in each of his experiments. Before going further, the reader will 

 find it profitable to predict what Mendel must have obtained in the Fi, 

 F2, and F3 generations after crossing (1) a pure-line tall plant with a 

 pure-line dwarf plant, ( 2 ) a pure-line red-flowered plant with a pure-line 

 white-flowered plant, (3) a pure-line yellow-seeded plant with a pure- 

 line green-seeded plant; and so on for each of the other pairs of con- 

 trasting factors. 



Such predictions may be shovni diagrammatically by means of con- 

 ventional symbols for the different hereditary factors as described in the 

 previous chapter. Any kind of symbols may be chosen for the factors if 

 one indicates what factors they represent.^ It is important to remember 

 that these symbols indicate only the genes, or hereditary factors, in the 

 chromosomes. Words should always be used to describe the visible char- 

 acters which develop as a result of the presence of these genes. Hence 

 the symbols TT and Td each represent a pair of genes either of which, 

 when present in the cells of a plant, influences its growth in height. 

 They represent the hereditary make-up, or genotype, of the plants with 

 respect to height. If these plants are peas, both of them will be tall plants. 



^ For the hereditary factors in the garden pea discussed in this chapter we ha\ e chosen 

 the following letter symbols. The dominant factors are indicated by capital letters. 



