3o8 GENETICS 



set of genes is taken apart and recombined in new ways, 

 giving a new set of characteristics. The very process that 

 produced the new characteristics does away with them 

 again. If by crossing two weak races carrying many re- 

 cessive gene pairs, a large and strong Fi generation is pro- 

 duced, in the way above described, in the F2 generation the 

 combination is broken; weak races are again produced. If, 

 by crossing of plants having flowers of diverse colors, 

 flowers of new colors are produced, in later generations 

 these new colors may disappear, since the combination of 

 genes that produced them is taken apart. In general, while 

 biparental reproduction may produce good combinations 

 from poor ones, it likewise produces poor combinations 

 from good ones. In the succeeding generation the combina- 

 tions of genes and characteristics are different from those 

 of the earlier generation. 



But in some organisms there are methods by which the 

 new combinations made in biparental reproduction may be 

 perpetuated, and may be multiplied, so that they become 

 characteristic of an entire race. 



The most important method by which this is accom- 

 plished is through reproduction from a single parent: fis- 

 sion, reproduction by budding, by bulbs, offsets, runners, 

 cuttings; in general by what is called vegetative reproduc- 

 tion. In such reproduction the genes are not recombined. At 

 the basis of such reproduction is ordinary cell division, in 

 which each gene divides, so that all the offspring produced 

 have the same set of genes. 



In many organisms biparental reproduction and vegeta- 

 tive reproduction alternate. By biparental reproduction a 

 great number of new combinations of genes are produced, 

 represented by different individuals. Then follows a period 

 of vegetative reproduction, in which each of the different 

 combinations is multiplied. In this way from each combina- 

 tion there is produced a large number of individuals, a 



