Discussion 55 



DISCUSSION 



Rockstein: Perhaps the small but significant difference obtained 

 for longer-lived animals from the older parents might have resulted 

 because you started with a fairly long-lived strain, and you were 

 dealing with inbred animals. 



Comfort: This may be so; it has been said of at least one other 

 strain of horses that the longevity depends on the proportion of 

 English thoroughbred blood they have. 



Rockstein: You said that among thoroughbreds there is a tendency 

 to breed for longevity as well as for racing ability. 



Comfort: This is not deliberate. It happens because short-lived 

 animals contribute surprisingly little progeny to the total. Mares are 

 not usually covered until they are taken out of training. As you 

 know, horses in flat racing very rarely run over the age of four, 

 whereas horses which are raced under National Hunt rules may go 

 on being ridden up to quite high ages. Most of these animals here, 

 if they had been raced, would have been raced before they were used 

 as brood mares. The successful ones would then have been valued as 

 brood mares; likewise the stallions — as soon as a stallion has made 

 its name as a promising racehorse its value goes up enormously and 

 it will be used to sire just as many foals as can be got out of it during 

 the rest of its life. 



Maynard Smith: I should like to make a few comments about the 

 genetics of longevity. The consideration that was at the back of my 

 mind in suggesting earlier that there might not be very much cor- 

 relation between parents and offspring was as follows : if a character 

 has been influenced by natural selection for a long time — if there 

 has been natural selection tending to move it in one direction — then 

 most of the genetic variability that is left will not be additive in the 

 genetic sense, and will not give a positive correlation between father 

 and child or mother and child, though it will, of course, do so between 

 brother and sister. Resemblances of the kind that Prof. Jalavisto 

 showed are very similar to the ones which I shall show you later on in 

 Drosophila; that is, there are resemblances between parents and 

 ofl'spring of the same sex, but not of different sexes. This pattern is 

 what one would call sex-limited, and would be expected if the causes 

 of death were to some extent different in the two sexes. On the 

 other hand, if you get a resemblance between mother and son, and 

 father and daughter, as you might in some cases, then this is what 

 would be expected in sex-linked inheritance. 



