Explanation Of Gene Action As Related To Physiological

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willing to test and say if it is low then I have a high threshold and not
bother to make this cross?
Lush: Well, you might want to make it still higher. If your mortality
is low, say 15 per cent, you mean that you would be content there?
Wyatt: No, not particularly.
Lush: We have to allow for a certain irreducible amount of wholly accidental
death loss. Your ability to push mortality any lower goes almost (hut
not quite) to zero by the time you get the fraction who are dying much under
twenty or fifteen per cent, even with progeny testing on stock of their own
kind.
If you progeny test them then on your own stock, so few would die that
you can hardly tell the difference between those with a little excess vitality
and those with a big reserve of that. But if you had a weaker stock, let us
say a stock in which sixty per cent of them would die, then if you progeny
test your own males on these, you would expect their offspring to be near
enough the threshold, where the test is sensitive, that you could discriminate
between males with a small and males with a large reserve or "factor of safety"
against high mortality.
You might need to allow also for some heterosis if your tester stocks were
distinctly unrelated to your own flock.
Your first approximation in these things is always to suppose that the
offspring will average about halfway between the parental races, plus or minus
whatever heterosis there is in the character concerned.
Warren: May I ask, Dr. Lush, in view of what you have said Just here
then, probably the best thing to do from the practical point of view if you
get mortality below ten is just to forget it_ is that not right, not have
these
tests?
Lush: Well, I suppose if you have other troubles that are more important
practically, I would go along with you. But if that is still the most serious