374 Inbreeding, Selection, and Heterosis 



of a family to produce a given type of monstrosity and a decline 

 in vigor. The isolation of biotypes showed that apparently there 

 was no inheritance of general vigor, for in many families there 

 was extreme vigor in certain respects and extreme weakness in 

 others, and there were those in which all kinds of vigor and all 

 kinds of weakness were combined. This study illustrates clearly 

 that one of the most important results of inbreeding is the bring- 

 ing to light and fixing of characters in a family. Very similar 

 results have been reported for various herbage grasses by Nilsson, 

 who emphasizes that all the effects of inbreeding may be ex- 

 plained by the segregation of genes. Inbreeding does not create 

 anything new but merely (and very effectively) sorts out what 

 was already present and fixes it. 



In Chapter 4 we pointed out that some of the higher plants 

 are capable of reproduction from vegetative organs. If vegeta- 

 tive reproduction occurs in this manner for a number of genera- 

 tions from one original plant, all the various plants will be di- 

 rect asexual descendants of the original plants or, more strictly 

 speaking, will be pieces of that original plant. All these indi- 

 viduals form a clone. Since all the individuals of a clone are 

 really pieces of one plant, they will be genotypically alike ex- 

 cept for somatic mutations. Being alike genetically and being 

 the lineal descendants of one original plant, they resemble the 

 pure lines that segregate out by successive self-fertilizations 

 among the descendants of one original homozygous plant. They 

 differ from such pure lines, however, in two fundamental ways. 

 First, the various plants of a sexually produced pure line are 

 genotypically identical except for mutations and are homozy- 

 gous. The members of a clone are also genotypically identical 

 except for mutations, but they may or may not be homozygous 

 and, unless the original individual of this clone was homozygous, 

 are not homozygous. Second, repeated self-fertilizations from 

 a heterozygous plant result in the establishment of a number of 

 different pure lines, but only one clone is produced by repeated 

 vegetative reproduction from one original heterozygous plant, 

 apart from somatic mutations. 



Selection 



Let us carry our hypothetical problem in plant heights a little 

 further. Let us suppose that we had two strains whose geno- 



