312 GENETICS 



reproduction. New combinations of genes are produced, 

 and most of these give origin to commonplace plants like 

 the original varieties from which the showy types were 

 produced. 



Organisms in which uniparental reproduction does not 

 occur: There are many organisms in which it is not possible 

 by vegetative reproduction to perpetuate and multiply the 

 new combinations produced in biparental reproduction, for 

 vegetative reproduction does not occur. This is notably the 

 case in higher animals and man, as also in some lower 

 animals, and in some plants. In most such organisms there 

 occur sporadic cases of the production of identical twins 

 or the like, which illustrate in principle the multiplication 

 of given gene combinations by vegetative reproduction. But 

 such processes are not continued for successive generations, 

 so that they cannot give origin to vegetative races. 



In such organisms there is a slow, difficult and imperfect 

 method of perpetuating some of the new combinations of 

 genes and of characteristics that result from biparental re- 

 production. This is by inbreeding: the mating of individuals 

 that have come from common ancestors, the mating of 

 "close relatives." Inbreeding is an unsatisfactory and un- 

 certain method compared with vegetative reproduction, but 

 if carried on for many generations it may yield permanent 

 stocks having new character combinations. 



The principles on which inbreeding acts are the follow- 

 ing: 



/. By inbreeding for successive generations homozygous 

 stocks are produced. 



2. Homozygous stocks breed true; that is, when they 

 reproduce sexually they do not produce new gene combina- 

 tions. The offspring have the same gene combinations as 

 the parents. 



3. Thus if in this way a homozygous stock is produced 



