Chapter J 



The Individuality Differentials of Closely 



Inbred Animals 



Closely inbred animals are those which have been bred by brother and 

 sister matings in a sufficiently large number of successive generations. 

 As the result of this procedure, these animals have a genetic composi- 

 tion which has become even more similar than that of ordinary brothers and 

 sisters; they exemplify an intensified brother and sister relationship. This 

 close relationship should exist even between animals which do not belong to 

 the same litter, but which have common ancestors in not far distant genera- 

 tions, and after very long-continued inbreeding, also between animals whose 

 common ancestors are somewhat farther removed. 



Theoretically, after from eight to ten consecutive brother-sister matings, the 

 genetic composition of different individuals should be about the same (Sewall 

 Wright) ; their individuality differentials should then be almost as nearly 

 related as are those of different parts of' the same organism or of identical 

 twins. However, our transplantation experiments have shown that such an 

 identity of individuality differentials among different members of the same 

 closely inbred strain or family is approached with very much greater diffi- 

 culty than would have been anticipated. As factors which might prevent or 

 delay a homozygous state, we have, in the first place, to consider mutations in 

 the germ cells, which may be expected to take place spontaneously and with a 

 frequency which is not yet known. In the second place, a selection of the ani- 

 mals to be mated might influence the results. Thus Dr. Helen D. King, in her 

 inbreeding experiments, selected in every case the most vigorous rats for 

 breeding; this might imply a selection of the most heterozygous individuals, 

 those which differ most in their genetic constitution and in which the in- 

 dividuality differentials are most dissimilar from those of brothers and sisters. 

 Such a process of selection might delay the attainment of perfect homozygosity 

 in the closely inbred strains, but this retardation would probably not be of 

 very great consequence. A third factor involves the relationship between the 

 animals in the first brother-sister mating; if these two individuals are very 

 different in their genetic constitution, a greater number of consecutive genera- 

 tions of brother-sister matings will be required to produce homozygosity than 

 if they are very similar to each other, and lastly there exists the possibility 

 that a difference in the individuality differentials between host and donor of a 

 transplant will be found if a branching-off from the common line of descent 

 has taken place at a certain point and if the two individuals whose individuality 

 differentials we wish to compare belong to different branches ; the difference 

 thus developed should be greater the further back the branching-off from the 

 common line of descent occurred. 



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