Twin Data on the Genetics of Ageing 



141 



similar in causes of death as two-egg pairs of the same or 

 opposite sex. 



The gene-specific basis of longevity and general health 

 potentials in old age is confirmed by a comparison of the life- 

 spans of the sibs of senescent twin index cases with those of 

 their parents (Table II). The most striking result of this 

 analysis is the evidence for a direct relationship between 

 parental age and the lifespan of the offspring. The effect 



Table II 



Effect of parental age on mean lifespans of senescent twins and 



their siblings 



Life expectancy of the gener 

 Moles. 48.2 years 

 Females. 51. 1 



population born ir 



of parental age is independent of sex differences and 

 expresses itself in the lifespans of both sons and daughters, 

 irrespective of whether the ages reached at death by the 

 parents are considered individually or together. The com- 

 paratively highest mean age levels were attained by twin 

 index sibships when an age of 70 years and over was reached 

 by the mother alone, or by the father alone, or by both parents. 

 There is no evidence for the operation of a sex-linked factor in 

 the inheritance of general longevity potentials. 



Based on these findings, the effects of genetic factors in 

 ageing may be summarized as follows : 



Genetic variations in adjustment to ageing may arise from 



