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TEXTBOOK OF BOTANY 



One chromosome in a sperm from 

 the tall smooth plant 



One chromosome in an egg from 

 the dwarf hairy plant 



Chromosome complement in the Fj generation 



Chromosomes in the gametes of the Fi generation 

 Sperms and Eggs 



Chromosome complements in the 



various plants of the 



F2 generation 



Fig. 213. A diagrammatic representation of the transmission of two hereditary 

 factors located in the same chromosome. 



smooth plants completely dominated the effects of the genes that condi- 

 tion dwarfness and hairiness.' In the F2 generation 1/4 of the plants 

 were pure-line tall, smooth plants; 2/4 were hybrid tall, smooth plants; 

 and 1/4 were pure-hne dwarf, hairy plants. All the genes in one chromo- 

 some go with the chromosome. They appear, therefore, to be linked 

 together. In this example all the dominant genes considered are in the 

 same chromosome, but this condition should not be construed to be the 

 general rule. The reader may be interested in solving a similar problem 

 when the genes represented by T and h are in one chromosome, and 

 those represented by d and S are in its synaptic mate.^ 



" Because of this fact we followed the convention of capitalizing the symbols for the 

 dominant genes, hut not those for the recessive genes. As symbols for the genes condition- 

 ing height we might also have chosen T and t, or D and d. The latter is preferred by many 

 geneticists. 



* It is important to remember that chromosomes and genes are in the cells of the plant, 

 and that the letters used as symbols for the genes are only on the printed page. Progress 

 in understanding is made only when there is an effort to \isualize the chromosomes and 

 genes. Moreo\er, when each factor in which one is interested is in a different chromosome, 

 as in Fig. 212, the factors may be represented in diagrams as letters without indicating 

 their relation to chromosomes. But when Uvo or more of these factors are in the same 

 chromosome, the fact must be indicated in some way, as suggested in Fig. 213. That is, 

 the letter symbols for the genes in any one chromosome should be kept together in the 

 diagrams. 



