ANIMAL AND PLANT BREEDING 317 



In normally cross-fertilized plants, pure lines are not obtained so 

 easily. They can be prepared by artificial selfing. The efficiency of this 

 process in bringing about homozygosis is best seen if we compare the 

 results of selection in cross-fertilized and self-fertilized lines. Thus five 

 years of selection for high protein content in self-fertilized maize gave 

 a greater rise in average protein content than twenty-three years of 

 mass selection, while plants from the sixteenth generation of mass 

 selection could still be rapidly improved by self-fertilization and 

 selection.^ 



Comparatively few varieties which are themselves valuable have been 

 isolated by self-fertilization of normally cross-bred plants. This is 

 because it is commonly found that the inbreeding of such plants leads 

 to a general loss of vigour. In an outbreeding population, rare recessive 

 genes with adverse effeas on vigour can spread fairly widely through 

 the species in a hidden, heterozygous condition, and many, if not most, 

 individuals may contain one or more such factors (p. 286). Inbreeding, 

 and particularly selfing, will tend to produce individuals homozygous 

 for such factors, which will then be expressed in a lowering of the 

 vitality of the plant, since any dominant factors there may be for 

 normal health will produce no more effect when homozygous that they 

 did when heterozygous. The loss of vigour on inbreeding is found to be 

 fairly general in practice, though it may be absent in particular cases ; 

 for instance, Collins^ has described an inbred strain of Saxton June 

 maize which was extremely vigorous and presimiably originated in a 

 plant which happened not to contain any deleterious genes. 



On crossing two different varieties or inbred strains, the effects of the 

 recessive genes of one plant are hidden by the dominant allelomorphs 

 of the other, and perfectly healthy plants result. The phenomenon is 

 known as heterosis,^ or hybrid vigour. The effect usually disappears in 

 generations later than the Fi ; the reason for this is supposed to be that 

 the beneficial factors in one strain are often hnked to deleterious factors, 

 so that the segregates in F2 and later will be homozygous both for 

 beneficial and deleterious genes and will therefore have only a mediocre 

 appearance. 



Ashby^ has attempted to explain the physiological processes under- 

 lying heterosis. He suggests, from observation on maize, that the 

 growth rate of the hybrid is actually always that of the faster growing 

 of its inbred parents and that its greater vigour is entirely due to an 



* East and Jones 1920. ^ Cf Babcock and Clausen 1927. 



' Rev. East 1936. * Cf. Ashby 1937. 



