Explanation Of Gene Action As Related To Physiological

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tlon were then relaxed, the population would soon return to its former
condition.
What can we do about it if overdominance is widespread? There might
be other genes which, in the homozygous condition, would produce the
effect we want. If not, I see nothing better than to breed so that as
large a fraction as possible of the animals in the commercial flocks shall
be heterozygous for these genes whlch sho_ overdomlnance. This means some
kind of a crossbreeding or rotational crossbreeding system patterned some_
what after the hybridcorn plan. The method might even be improved over
the hybrid corn plan if reciprocal recurrent selection actually is effective
in gathering together into contrasting strains the genes which, when
thestrains are crossed together, will produce the maximum amount of
heterosis. This is a vastly different breeding system from the classical
idea of continually improving the relatively pure stock until it becomes
better and better and purer and purer, with the ultimate end of making a
purebred stock which will be perfect. Because this is so different from
those traditional methods, it is of the utmost importance to know whether
overdominance really is abundant enough to justify these more complicated
methods.
TESTS FOR PLEIOTROPY
The test least capable of being interpreted wrongly is to select one
line for largeness in X and the other for smallness in X and then to
observe whether the two lines become different in other characteristics.
Such a change in characters not intentionally selected is often called a
"correlated response". Dr. Falconer, who is here today, Dr. John W.
MacArthur, and many others, have observed such correlated responses.
Experiments involving selecting a high llne and a low line are difficult
to do for more than one or two characters at a time. Also, they need to