Behavior Genes
50% for MZ twins, 10 to 12% for DZ twins
or full siblings, but only 2 to 3% for second-
degree relatives such as uncles/aunts or
grandparents. One explanation of why
concordance declines more steeply than
the proportion of shared genes (see Fig. 6)
would be an interactive effect of multiple
loci. For example, Risch has suggested
on mathematical grounds that just two
or three loci with epistatic effects would
be suf±cient to explain the inheritance of
schizophrenia. However, recent molecular
studies appear to indicate that a larger
number of loci is involved.
As we also discussed earlier, part of
the environmental variance is common
or shared environment (
), which is de-
±ned as any environment that is common
to a pair of relatives and makes them more
similar. In contrast, individual-speci±c, or
nonshared environmental variance results
from any environmental effects that pro-
duce differences between relatives (
Phenotypic variance may also be af-
tion and gene–environment correlation.
when speci±c genotypes confer suscep-
tibility to speci±c environments. For in-
stance, it has been demonstrated that
adoptees who faced adversity during their
upbringing were at increased risk for crim-
inality if at least one biological parent had
antisocial behavior. More recently, Caspi
and his colleagues found more speci±c
evidence of gene–environment interac-
tion. Young men who had experienced
early parental maltreatment had an in-
creased risk of antisocial behavior if they
had inherited the X chromosomal gene
that results in low activity of an enzyme
involved in brain signaling, monoamine
oxidase A (MAOA). Men who had the al-
ternative, high-activity form of the MAOA
gene and early maltreatment showed no
signi±cant increase in antisocial behav-
ior when compared with men who had
not been maltreated. Gene–environment
correlation may result from the covaria-
tion genotype and environmental factors.
Three types of gene–environment (G-E)
correlation have been described: passive,
active, and evocative. Passive G-E correla-
tion arises because children inherit both
genetic risk factors and environment fac-
tors from their parents. For example, a
musically talented parent could transmit
both predisposing genes and a predis-
posing environment to their offspring.
Active correlation occurs because people
search out or modify environments to suit
their genetic predisposition. An example
of this would be a person of high musi-
cal ability seeking out environments where
they could make best use of their talents.
Fig. 6
The genetic concordances
between family members and
corresponding concordances for
schizophrenia. The concordances
decline more steeply than the
percentage of shared genes.
MZ twins
DZ twins
and full sibs
2nd degree
Percentage of shared
Concordance for
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