Explanation Of Gene Action As Related To Physiological

-16o
As another example of what
is probably plelotropy, one of you, working
with a dominant gene but back-crosslng to the recessive, found that
this gene had an otherwise unnoticed effect on growth rate.
Because the
gene was dominant and the breeding plan was to backcross
continually to
the recessive, the chickens could be sorted clearly into two groups; those
which had the gene but were heterozygous for it and those which didn't
have the dominant gene at all.
Then it became clear that those which did
not have the dominant gene exceeded the others by a full tenth of a pound
in weight.
Now this difference is big enough to be important practically
but small enough that it would not have been noticed had not the other
effect of the gene been so conspicuous that it was possible to separate
the chickens into a group which had the gene and another group which did
not.
Cases like this make one wonder how many other effects, too small
to be noticed, are exerted by genes which we know by and name for their
major effects only.
Present knowledge of genetics seems (to me) to
indicate that pleiotropy is very common, --- perhaps universal.
One reason for thinking that plelotropy is exceedingly common is the
complex nature of the machine which is the living animal or plant.
In
the much
simpler machines, such as watches and automobiles, the anatomy
and functioning of which we know fairly well, Itls
rare indeed that we
can make a major change in an important part of that machine without having
several different changes results in other aspects of the way that
machine functions.
It seems to me almost certain that this must be true
ofthe
vastly more complicated machines which our living animals are.
Such reasoning seems to me validand
plausible, although it is far less
convincing than actual detailed evidence in each case would be.
The practical consequences of pleiotropy are all in our favor if we
like all of the effects which the gene has. Then if we select for one