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Mental activities,
expressed in human behaviour, are intimately related to physical
activities in the brain and nervous system. In 1929 Garrod emphasized
this when he wrote: "Each one of us differs from his fellows,
not only in bodily structure and the proteins which enter into
his composite, but apart from, or rather in consequence of, such
individualities, men differ in mental outlook, character and ability."
Since that time, many investigators have sought to analyze the
molecular and cellular components of behaviour in order to relate
genes to such abstractions as intellect, temperament, and the
emotions. Because ultimately the brain is responsible for behavioral
development, neurobiologists attempt to understand the unusual
precision by which the brain's various regions are interconnected
and how it organizes its complicated nerve fibre circuits.
The effects
of environment and heredity are nowhere more difficult to disentangle
than in the field of normal and disturbed behaviour (mental illness).
Although patterns of behavioral development are consistent and
"programmed" for species and subgroups of species, the uniqueness
of the individual is strikingly evident in behavioral variation.
As stated earlier, much of the information attempting to quantify
the genetic component (heritability) of various behaviours has
come from twin studies in which the concordance for a specific
trait in identical twins is compared to that for a similar group
of fraternal, like-sexed twins. These studies are based on the
assumption that differences in measurements between identical
twins are nonhereditary, since these twins are genetically the
same. Particularly impressive are studies that involve identical
twins reared apart from birth; extraordinary similarities have
been found between identical twins who were unaware of each other's
existence for about 40 years.
A large variety
of genetic diseases are associated with diminished IQ as well
as other behavioral abnormalities, pleiotropic
manifestations of a biochemical error produced by a genetic defect.
These can be single-gene diseases such as phenylketonuria,
Huntington's chorea,
or
muscular dystrophy. Chromosomal abnormalities involving
the autosomes (e.g., Down syndrome) are almost always associated
with lowered IQ and other behavioral manifestations. As mentioned
before, abnormalities involving the sex-chromosome number are
not as lethal as those of the autosomes and produce less serious
manifestations. Mental retardation is not characteristic of aberrations
in the number of sex chromosomes, and, in general, only minor
behavioral abnormalities associated with slow language development,
reading difficulties, and general immaturity are evident. There
is, however, evidence that mental illness occurs more frequently
among affected adults. The majority of mildly retarded ("feebleminded")
individuals are at the lower end of the curve of continuous variation
for intelligence, and hence their intellectual deficits fit a
polygenic interpretation.
The genetic
components of the psychoses (i.e., the major forms of mental illness)
have been analyzed for schizophrenia and manic-depressive psychosis.
Schizophrenia is a category of psychosis characterized by a combination
of disorders affecting thought processes and mood. Little is known
about its causes, and it may well be a heterogeneous mixture of
diseases. Consequently, the mode of inheritance is unknown, although,
as discussed carlier, family and twin studies point strongly to
a genetic predisposition. In the absence of a clear mode of inheritance,
risk factors are based on empirical studies.
Manic-depressive
psychosis occurs in two forms: bipolar, which is characterized
by alternating moods of depression and elation interspersed with
relatively normal periods, and unipolar, in which either the depressive
or manic state predominates. Twin studies have shown concordance
of 70 percent for identical twins and only 13 percent for fraternal
twins, evidence of a strong genetic factor in this disease. In
general, siblings of persons with the bipolar type have about
a 15 percent risk of developing the disorder, whereas siblings
of those with the unipolar type have about a 7 percent risk of
being affected.
It should
be stressed that although these and other behavioral diseases
(e.g., alcoholism) are familial, they have strong environmental
components. Many of them respond well to a combination of psychotherapy
and drug administration.
Lee
M. Silver
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