4 



THE INFLUENCE OF GENETIC 



CONSTITUTION ON SENESCENCE AND 



LONGEVITY 



4-1 Inheritance of life-span 



411 GENERAL 



It is evident in any comparison of laboratory stocks that differ- 

 ences of specific age are to some degree 'inherited' (Pearl and 

 Parker, 1922; Gonzales, 1923, Gruneberg, 1951, Fig. 31), but 

 detailed genetic knowledge of the manner of their inheritance 

 is not plentiful. Much variation in life-span occurs between 

 inbred lines. This variation is often related to a single heritable 

 predisposition to die of cancer, renal disease, or some other 

 single cause: in these cases it is often short life, not long life, 

 which is capable of genetic selection in the homozygote. Bittner 

 (1937) showed that in some cases it is possible to transpose the 

 longevities of strains of mice by cross-suckling. In other cases, 

 secondary causes, such as restricted capacity for activity in 

 deformed stocks, affect the life-span. In a stock of mice bred 

 by Strong (Strong, 1936; Strong and Smith, 1936) longevity 

 increased the apparent incidence of disease by allowing animals 

 to reach the cancer age. Two factors appear at first sight to be 

 involved in inherited longevity — absence of genetic predis- 

 position to specific causes of death, and a less definite quantity 

 ('vigour') which contributes to Darwinian fitness because it is 

 usually expressed both in fertility and in longevity. It is by no 

 means certain that these factors are distinct. 'Vigour' itself may 

 in fact represent either the covering-up of deleterious recessives 

 by heterozygosis, or a state of over-dominance, in which the he- 

 terozygote is inherently more vigorous than either homozygote. 



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