454 TEXTBOOK OF BOTANY 



in the flower; and its homolog, or synaptic mate, in the gametes of the 

 white-flowered plant lacked this gene or contained a corresponding gene 

 that lacked this particular potentiality. The zygotes formed by cross- 

 fertilization contained both of these chromosomes and the plants that 

 grew from them were pink-flowered. 



We may for the present ignore all the other genes in these two 

 chromosomes and also all the other chromosomes in these two kinds of 

 plants, and fix our attention on these two chromosomes through two 

 complete life cycles of snapdragon. This we may do by means of a 

 diagram. We shall use the letter R as a symbol for the gene, or the po- 

 tentiality, in the chromosome that conditions the formation of red pig- 

 ment in the flower; and the letter r to indicate the potentiality in the 

 homologous chromosome that conditions colorless flowers (Fig. 212).'^ 



Owing to the behavior of chromosomes in reduction division, two 

 kinds of sperms and two kinds of eggs with respect to flower color will 

 be formed in the pink-flowered hybrid as shown in the diagram. The 

 plants of the F2 generation that obtain the chromosome containing the 

 gene represented by R from both sperm and egg have red flowers. If 

 they obtain this chromosome only from the egg or from the sperm and 

 obtain its synaptic mate from the other gamete, the flowers are pink as 

 in the Fi generation. If the chromosomes from both sperm and egg lack 

 the gene represented by R, the flowers are white. The different kinds of 

 sperms and eggs unite at random. That is, their union is influenced not 

 by the kinds of chromosomes and hereditary potentialities they contain, 

 but by the chance of a sperm coming in contact with the egg.^ Hence, if 

 a large number of them unite, the progeny ( F2 generation ) would con- 

 sist of plants of which approximately 1/4 are red-flowered, 2/4 pink- 

 flowered, and 1/4 white-flowered. Both the red- and the white-flowered 

 plants, if selfed, would continue to breed true. 



In order to check some of the inferences one might draw from this 

 example, it will be necessary to consider another example of cross-fertili- 

 zation in which one of the chromosomes in the sperm from a tall, smooth 

 plant has a gene (T) that conditions height of plant and another gene 

 (S) that conditions smooth epidermis; while one of the chromosomes in 



^ In preference to letters, small dots may be used to represent the genes in the chromo- 

 somes. 



" In seed plants the kind of sperm that may come in contact with an egg depends upon 

 the kinds of pollen grains that become attached to the pistil, and upon the subsequent 

 growth of pollen tubes. 



