206 University of California FuTilications in Agricultural Sciences [Vol. 2 



having long ago been segregated out of the race and having perished 

 in the competition with their more hardy and vigorous sibs. 



Maize is a naturally cross-fertilized species, and heterozygosity is 

 therefore the general condition of the germinal material instead of 

 homozygosity, as is the case in self-fertilized species. In this hetero- 

 zygous condition the genes of recessive harmful characters may be 

 carried along in the germ-plasm under the protection of desirable 

 dominant characters and appear only when the latter are absent. 

 Inbreeding furnishes conditions favorable to the accumulation of these 

 recessive genes in the germ-plasm and for the appearance of the 

 recessive characters in some of the individuals. 



The increase in size and vigor observed in the progeny when two 

 different inbred strains, or inbred strains and unrelated non-inbred 

 strains, are crossed is due to the establishment of a heterozygous germ- 

 plasm containing more dominant factors influencing size and vigor 

 than were present in either of the parents. Linkage of such genes 

 to recessive or to dominant genes Avhich influence vigor and size ad- 

 versely prevents the production of homozygous dominant races. For 

 this reason the vigor noticed in the Fj is less marked in the F^ and 

 subsequent generations where segregation and recombination have 

 taken place. 



Most of the knowledge of the effects of continued inbreeding and 

 the results obtained from crossing inbred strains have come from 

 experiments on plants and animals under domestication. Such species 

 have been the subject of conscious selection for particular types, which 

 often preserves in the species characters desirable from an agricul- 

 tural point of view, but so detrimental that the race could not exist 

 except under the conditions of domestication. It has been questioned 

 whether the germinal material of such races is comparable to that 

 of wild species in which natural selection may have largely eliminated 

 from the germ-plasm genes which produce characters detrimental to 

 the natural existence of the species. In view of this possibility the 

 question has arisen as to whether or not the results of continued 

 inbreeding would be the same if wild species, in place of domesticated 

 ones, were the subject of such experimentation. It is in this connection 

 that this report on inbreeding in Crepis is of interest. 



