554 HUMAN BIOLOGY 



proportion of pure dominants, or recessives, in the second 

 hybrid generation will be changed. Thus, instead of one- 

 quarter showing the recessive condition, only one in i6 may 

 show it fully developed, or even one in 64. In these cases all 

 sorts of intermediate grades between presence and absence 

 will appear. 



The phenomenon of segregation appears in the second 

 hybrid generation, as stated in the last paragraph, for some 

 of the individuals show a trait and some do not show the 

 trait. Actually, in any hybrid population, we do not have the 

 experimental conditions set down in the foregoing paragraph 

 for first hybrid generation individuals do not exchisively, 

 or regularly, mate with each other. In most of the hybrid 

 populations that we know there are, besides the crossing of 

 first generation hybrids, back crossing with the parental 

 types, second hybrid generations mated with second hybrid 

 individuals, also with first hybrid generations, or the parental 

 types. After three or four generations, the hybrid generation 

 is a great mixture of all conceivable combinations of hybrid 

 generations of various degrees with each other and with the 

 parental stocks. In such a population, minghng by chance, we 

 expect no definite proportions of individuals possessing any 

 particular trait, or any particular blend, but rather a great 

 variabihty in respect to the trait in question, some individ- 

 uals being characterized by its presence, some by its absence 

 and others by various grades between these extremes. The 

 standard deviation of the trait in question is thus high in a 

 hybrid population. 



HETEROSIS 



When two races are crossed it sometimes happens that 

 the first generation hybrids show a character that is not 

 favorable in either of the parental races. This result is 

 ordinarily found in the case where a trait is dependent upon 

 two factors for its expression. One of these factors, a, may 

 be carried by the one race in which the trait does not appear 

 phenotypically, the other factor, b, may occur in the other 

 race which is also phenotypically without the factors. The 

 combination, a b, will bring together the two essential 

 factors and the trait is expressed phenotypically. To cite 



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