CHAPTER I I 



ADJUSTMENT AND BALANCE 



Levels of Adjustment — Haldane's Rule — Genotypic and Segregational 

 Sterility — Inbreeding Depression and the Hyhridity Optimum 



Levels of Adjustment 



Heredity, as we have now come to see it, depends on an organiza- 

 tion of chromosomes, genes and cytoplasmic determinants which 

 are adjusted to one another in the development and reproduction 

 of each individual. We have recognized this adjustment at three 

 levels; the organismal adjustment of cells with one another; the 

 cellular adjustment of the parts of single cells, in particular of 

 nucleus and cytoplasm; and the nuclear adjustment, or balance as 

 we have called it, of the genes within the nucleus. There is, of course, 

 a further adjustment of the whole system with the environment. 



Each level of adjustment has its own rules. The cells which work 

 together as parts of the same body or soma normally have identical 

 nuclear genotypes. This relation may, as we saw, be altered by gene 

 or chromosome mutation in the body, with consequences ranging 

 from a mere mosaicism of colour to a mixing of tissues normally 

 confmed to distinct individuals, as in gynandromorphs. The mixture 

 of unlike cells may also lead to intersexuality and sterility. 



The disparity of genotypes that can be associated in one soma has 

 been investigated by grafting experiments. In mammals and birds, 

 such grafts are successful only when the genotypes are very closely 

 related. Otherwise antigenic differences come into play and the 

 transplant is sloughed off. In other animals the tissues seem to be 

 more nearly autonomous in their behaviour, for transplantation 

 between genetically dissimilar individuals is more widely successful 

 and in the insects, as we have seen, it has even led us to a clearer 

 understanding of how genes act. 



In plants grafting can be accomplished between still more widely 

 separated forms, even between genera, to give more or less stable 

 graft-hybrids. We can put the aerial parts of one species onto the 



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