Explanation Of Gene Action As Related To Physiological

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genes have manyeffects, although that may be going too far. _en a gene
has more than one effect, the technical term for that is "pleiotropy".
The well-known gene for yellow in the mouse._is one of the earliest
examples found in genetics. This gene has at least three effects; it
makes the mouse yellow and this effect is dominant. Secondly, it makes
him unusually fat and this, likewise, is dominant . Thirdly, it kills him
and this effect is recessive, else we would never have found the gene. It
is not obvious (to me) why yellow color and fatness, which are dominant,
and lethality which is recessive should be caused by a single gene. Perhaps
all three of these effects result from one primary thing which the
gene does and they wouldn't seem so unrelated, to each other if I understood
all the chemistry and physiology of how that gene does its work.
Students of genetics have sometimes distinguished between "primary"
and "secondary" pleiotropy. The primary action occurs if the gene does |
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two or more different things at the very beginning , perhaps produces three\
different catalysts which work on entirely different processes or organs \ \
of the body. "Secondary" pleiotropy occurs if the gene does Just one 1
thing primarily but the product of this action itself affects two or more 1
characters or other actions and these in turn may each affect more than !
one character. This would give a many-branching chain of many final
effects, some of them not obviously related, but all resulting from a
single primary effect. The evidence for secondary plelotropy seems more
convincing but, for practical purposes, it is simply not pertinent whether
plelotropy is primary or secondary. The practical results are the same.
Ofcourse, the distinction is of interest scientifically and finding the
facts in each case would aid our understanding of gene action and might
suggest some therapeutic ways of magnifying some effects while diminishing
others.