Explanation Of Gene Action As Related To Physiological

-30-
f per cent ofwhat is left in these selected lines is specific.
Now, I do not know whether poultry breeders have been breeding
in the
same direction so long and so diligently that they have nearly used up the
additive effects or not; neither do I know that for pigs, cattle or sheep,
but
it would not be surprising if some of them have gone further in that
direction than others. Again, some evidence would be welcome.
Heisdorf:
Dr. Lush, suppose a poultry breeder has a commercial cross
which
he thinks is fairly good, but in which he knows considerable improvement
can be made.
However, he is afraid that if he continues to select for better
vitality he might actually lose ground in other respects and end up with a
product which is not as desirable as what he has at present. What can he do
to protect himself, whereby he can divide his population and practice additional
selection with one portion, what can he do with the other to hold what he has
already gained so far?
Lush:
Offhand I do not see why he would expect to lose ground unless
the flock Is small enough that the inevitable inbreeding gets out of hand
and he might have some undesirable things happen that way.
Other than that,
I do not see why he should get any worse.
We have run on to one
or two cases in genetics where the correlation
between parent and offspring was negative.
They are very rare and unlikely
conditions.
They involved some prior selection of parents.
If the flock is small enough that the inbreeding is likely to run away
and you fix some things you did not want to, while you were not looking, and
they turn out to be undesirable
later, I do not know any good remedy for that.
Keeping the flock larger might not be financially possible.
It might be better
even to split it in two or three different stocks.
I suppose you have some
other practical reasons for splitting a flock in two or more parts, such as
against disease or disaster wiping
out the flock.