SELF FERTILIZATION 323 



this strain by brother and sister mating no further improvement occurs, and 

 very often deterioration finally takes place as a result of close inbreeding. 



East and Jones explained the results of inbreeding in accordance with the 

 rules of Mendelian heredity. The parents belonging to two different inbred 

 strains possess genetic constituents which differ from each other. In fertilized 

 ova, giving rise to the F x generation, a large number of the dominant factors 

 from both parents are brought together, and this combination may cause 

 an increase in size, fertility and strength of the F 1 hybrids. This is per- 

 haps due to the fact that mutations are mostly recessive and injurious and 

 that these injurious effects become manifest if two recessive alleles are com- 

 bined in the offspring. In the F 1 generation, the chances that these injurious 

 recessive factors become manifest are slight as compared to the genetic 

 constitution in inbred strains. If the F x hybrids are inbred the advantages 

 gained in F x disappear again. In the F 2 and following generations, these 

 dominant factors again become segregated in the large majority of the in- 

 dividuals and a loss of the advantages gained in the F x generation may take 

 place, until, as the result of continued inbreeding, the individuals have again 

 reached a condition in which they all have acquired essentially the same, or 

 at least a very similar genetic composition, and then no further deterioration 

 needs to take place. The increased vigor In the F 2 generation of hybrids, due 

 to the bringing together in the same individual of factors which are derived 

 from different lines, and especially of dominant favorable genes which pre- 

 vent the injurious effects of recessive mutants from becoming manifest, is a 

 condition called "heterosis". In accordance with what we have stated above, 

 inasmuch as in inbreeding in the beginning two individuals are united, belong- 

 ing not to two different species but to the same species, although to two 

 different lines or perhaps to different varieties, it would be preferable to 

 designate this stimulated state in the F x hybrids as homoiosis, depending on 

 a "homoiozygous" in contrast to an "autozygous" condition of the gene sets. 

 Such a state of homoiosis would then, in the course of further inbreeding 

 be followed by a state of syngeniosis and ultimately by one approaching 

 autosis. In some instances, even a certain stimulation may result from fertili- 

 zation, when the two germ cells belong to different though closely related 

 species. In the latter case we would have to deal with a true heterosis. How- 

 ever, according to common usage the term "heterosis" is meant to signify 

 the beneficial effects derived from the fact that unlike genes derived from 

 unlike parents are combined in the same individual, and furthermore, it 

 attributes to this unlikeness of the genes certain effects without regard as to 

 whether the relationship of the genes is a heterogenous or a homoiogenous 

 one. If it is desired to express merely the mutual strangeness of the genes 

 derived from the two different parents, we might apply the term "allosis" to 

 the condition usually designated as heterosis. 



According to East and Jones, the injurious effects of inbreeding are then 

 due to combinations of certain recessive allelomorph genes in the same in- 

 dividual; injurious conditions which had existed previously in a potential 



