DISCUSSION 
mind, all these studies should be repeated on a much larger 
scale. 
Bronowskt: Ona purely theoretical point: Professor Parkes 
asked in his paper if there is any significance in the fact that 
the sex-ratio is roughly 1:1. There are two parts to this 
question. One part, of course, is answered by remarking that 
if you start with a population with any sex-ratio at all, then 
because males contribute X and Y chromosomes in equal 
numbers, you will move to an equalization of the sex-ratio. 
That is a pure question of mechanics. 
Lederberg: ‘This is begging the question. The Mendelian 
expectation is a sex-ratio of 1:1 at conception. This appears to 
be very far from true. Instead some selective process causes the 
sex-ratio to approach 1:1 at maturity. The problem is the 
physiology in the evolutionary mechanism of this adjustment. 
Professor J. F. Grow? has pointed out a very simple formula- 
tion of the population dynamics of the sex-ratio. If at the 
reproductive age one sex is less numerous, individuals of that 
sex will have a greater average number of progeny than indi- 
viduals of the other sex. Then an individual who produces 
more offspring of the scarcer sex will transmit the corresponding 
genes to more grandchildren than the individual who produces 
the average ratio. Thus the genes controlling sex-ratio will be 
selected to the point of equality for the sex-ratio at reproductive 
age. 
Bronowski: Secondly, there is a more subtle part to the 
question, which Professor Parkes was also asking: Is there any 
advantage conferred on a species with a sex-ratio near to 1? 
Parkes: Is there any advantage, that is, when there is such 
a large difference in reproductive potential between the two 
sexes ? 
Bronowski: ‘There is an advantage, if you call it such, an 
advantage in variability. Mutation doesn’t produce variability 
in itself, it only produces the potential to variability. Now if 
you have equal mutation rates in males and females, then you 
take the highest advantage of this, if variability is what you 
want, by having an equal sex-ratio. (In physics this principle 
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