INBREEDINC: DEPRLSSION AND TUC IIYBRIDITY Ol'TIMUM 



between them. By the same token, crossing plants within the same 

 inbred line has no effect in restoring vigour. The vigour is hybrid 

 vigour, or Jieterosis as it is sometimes called. 



In maize, therefore, it is possible to show that full vigour and 

 fertility are the result of a hybrid condition : loss of hybridity leads 

 to loss of vigour and fertility. The same principle applies in rye and 

 sugar beet, in pigs and flies, indeed, as we shall sec later, in nearly 

 all cross-fertilizing species. In this way we are led to see that both 

 undue hybridity, that is unaccustomed hybridity, and unaccustomed 

 lack of it, may lead to genotypic unbalance. Hybridity is dangerous 

 only where its kind or its degree is unaccustomed. Each species has, 

 it seems, an optimum degree of hybridity to which it is accustomed. 

 Any departure from this optimum, no matter in which direction, 

 is followed by unbalance. 



Now, with rare exceptions, species can maintain hybridity only 

 by the crossing of different individuals in every generation. Equally, 

 of course, excessively wide crossing will lead to undue hybridity. 

 Thus the adjustment of the genes within the nucleus, the ultimate 

 one of the three adjustments we have been led to recognize, itself 

 demands a fourth, and one not between parts of the cell or even 

 of cell with cell, but of individual with individual. There must be 

 a relationship between the hereditary materials, and their behaviour, 

 throughout the whole group or species; and it is on this relationship 

 that the genetic system depends for its character. Hybridity optimum, 

 segregation, and recombination must all be related throughout the 

 group. Furthermore, they must be related not merely amongst 

 themselves, but also to the mating habits of the individuals which 

 compose the group, and to the barriers which make that group by 

 separating or isolating it from others. 



If we are to understand the nature of heredity and variation, all 

 these relations require our consideration. We shall commence with 

 the breeding system, the habit of mating within the group, in its 

 simplest and most general characteristics. 



REFERENCES 



BRIDGES, c. B. 1922. The Origin of variation in sexual and sex-limited characters. 



Am. Nat., $6: 51-63. 

 DOBZHANSKY, T. T941. Genetics and the Origin of Specief. 2nd cd. New York. 



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