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Gene expression occurs
only after modification by the environment. A good example is
the recessively inherited disease called galactosemia, in which
the enzyme necessary for the metabolism of galactose-a component
of milk sugar-is defective. The sole source of galactose
in the infant's diet is milk, which in this instance is toxic.
The treatment of this most serious disease in the neonate is to
remove all natural forms of milk from the diet (environmental
manipulation) and to substitute a synthetic milk lacking galactose.
The infant will then develop normally but will never be able to
tolerate foods containing lactose. If milk were not a major part
of the infant's diet, however, the mutant gene would never be
able to express itself, and galactosemia would be unknown.
Another
way of saying this is that no trait can exist or become actual
without an environmental contribution. Thus, the old question
of which is more important, heredity or environment, is without
meaning. Both nature (heredity) and nurture (environment) are
always important for every human attribute.
But this
is not to say that the separate contributions of heredity and
environment are equivalent for each characteristic. Dark 
pigmentation
of the iris of the eye, for example, is under hereditary control
in that one or more genes specify the synthesis and deposition
in the iris of the pigment (melanin). This is one character that
is relatively independent of such environmental factors as diet
or climate; thus, individual differences in eye colour tend to
be largely attributable to hereditary factors rather than to ordinary
environmental change.
On the other
hand, it is unwarranted to assume that other traits (such as height,
weight, or intelligence) are as little affected by environment
as is eye colour. It is very easy to gather information that tall
parents tend, on the average, to have tall children (and that
short parents tend to produce short children), properly indicating
a hereditary contribution to height. Nevertheless, it is equally
manifest that growth can be stunted in the environmental absence
of adequate nutrition. The dilemma arises that only the combined,
final result of this nature-nurture interaction can be directly
observed. There is no accurate way (in the case of a single individual)
to gauge the separate contributions of heredity and environment
to such a characteristic as height. An inferential way out of
this dilemma is provided by studies of twins.
Rick
Weiss
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