608
Behavior Genes
index cases with bipolar disorder have
an excess of relatives both with bipolar
disorder and with unipolar depression.
In contrast with schizophrenia, the adop-
tion data on affective disorders are fairly
sparse, but the most carefully conducted
studies suggest that the familial clus-
tering of these disorders largely results
from genetic effects. This is supported
by twin research. In recent studies of
unipolar depression, there is consistently
higher concordance in monozygotic than
in dizygotic twins. The effect appears to be
stronger in more severe or recurrent de-
pression and model Ftting studies aimed
at estimating heritability suggest that this
is around 70% in studies where the index
cases have been ascertained via hospitals
or clinics. If the depressed index cases are
ascertained via population surveys, per-
haps consisting of milder disorder, the
heritability is typically around 30 to 40%.
The twin study model Ftting results also
suggest that whether the sample is ob-
tained clinically or is population-based,
the environmental effects are all of the
nonshared kind. There have been several
studies attempting to examine the inter-
play between genetic and environmental
risk factors in unipolar depression. There
is a great deal of evidence supporting the
common sense hypothesis that onsets of
depression are frequently associated with
unpleasant events. However, there is no
evidence that depression that is associ-
ated with adversity is any less familial
than depression that arises ‘‘out of the
blue.’’ There is also considerable evidence
from studies of families and twins that
life events are not completely randomly
distributed in the population. Some in-
dividuals are more prone to life events
than others and people who score high
on personality traits designed to mea-
sure dimensions such as extraversion or
novelty-seeking seem to have more event-
ful lives than those who score low on such
trait measures. These personality traits
have been found to show both positive
correlations in siblings and, in twin stud-
ies, to be heritable. Therefore, it is perhaps
not too surprising that at least some forms
of life events also appear to be familial. ±or
some events, the familial effect is purely
environmental. ±or example, the death of
a parent affects both members of a pair
of siblings or twins. Nevertheless, several
studies indicate that some events reflect
the influence of genetic factors rather than
shared environment.
Molecular
genetics:
searching
for
‘‘blue
genes’’
Bipolar disorder has been the
subject of quite a number of linkage stud-
ies including whole genome scans. The
data are less extensive than those for
schizophrenia and recent meta-analyses
would suggest that they are much less
clear-cut. Despite this, regions of inter-
est have been identiFed on chromosomes
13q and 22q, which overlap those that
have been implicated in schizophrenia.
Although the conventional view is that
schizophrenia and bipolar disorder are
largely different genetically, families have
frequently been described containing suf-
ferers from both conditions and occa-
sionally identical twins or triplets have
been described who have nonidentical
psychoses. A recent twin study using
multivariate structural equation modeling
looked at symptom patterns rather than
conventional diagnoses, that is, subjects
were classiFed as having a bipolar syn-
drome, schizophrenia, or schizoaffective
disorder, an intermediate form. Instead of
assigning an exclusive diagnosis, subjects
were classiFed according to the symptom
pattern as having one, two, or all three
disorders. The analyses suggested a large
previous page 608 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 610 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off