Behavior Genes
589
2.1
Family Studies
Family studies are used to establish the
extent to which behavioral phenotypes
cluster in families. For example, a familial
influence would result in relatives cor-
relating positively for a continuous trait
or, when studying a disorder, relatives of
probands (individuals affected by the dis-
order, also referred to as an index case)
having a higher prevalence than the rela-
tives of controls (individuals unaffected by
a disorder, often matched to the proband
for characteristics such as age, sex, and
ethnicity). We can also compare correla-
tions and concordances between family
members with differing degrees of ge-
netic relatedness to the proband. The main
sources of genetic correlation or covari-
ance between relatives are called additive
variance and dominance variance. Essen-
tially, additive variance reflects the extent
to which the QTLs contributing to a trait
show simple additive effects and domi-
nance variance reflects the extent to which
heterozygotes deviate from the mid val-
ues between homozygotes (Fig. 1). The
genetic covariance between pairs of rel-
atives is given by the simple expression
β
V
A
+
α
V
D
,where
β
is the (average) pro-
portion of genes held in common,
α
is
the probability that at any given locus the
pair of relatives share two alleles that are
identical by descent (IBD) from common
ancestors,
V
A
is the additive genetic vari-
ance, and
V
D
is the nonadditive genetic,
or dominance variance. In addition to ge-
netic covariance, family members share
the home environment in which they are
raised and this contributes a proportion
of variance
V
C
.Thesourcesofcovar
iance
between different family members can be
seen in Table 1.
One of the major limitations associ-
ated with family studies is the inability
to distinguish between genetic and envi-
ronmental effects. So-called competition
effects between family members may re-
duce resemblance between relatives and
make a trait appear less genetically influ-
enced than it actually is. More commonly
shared environment contributes positively
to resemblance between family mem-
bers and therefore familial clustering of
a phenotype is not suf±cient evidence of
genetic causation.
2.2
Twin Studies
Identical, or monozygotic (MZ) twins are
t
h
ep
r
o
d
u
c
to
fas
i
n
g
l
ef
e
r
t
i
l
i
z
e
de
g
g
Tab. 1
The sources of covariance between pairs of relatives.
Relationship to proband
Genetic relationship
Phenotypic covariance
MZ twins
V
A
+
V
D
V
A
+
V
D
+
V
C
DZ twins
0
.
5
V
A
+
0
.
25
V
D
0
.
5
V
A
+
0
.
25
V
D
+
V
C
Full sibs
0
.
5
V
A
+
0
.
25
V
D
0
.
5
V
A
+
0
.
25
V
D
+
V
C
Parent or offspring
0
.
5
V
A
0
.
5
V
A
Half sibs raised together
0
.
25
V
A
0
.
25
V
A
+
V
C
Raised apart
0
.
25
V
A
Uncles/aunts/nephews/nieces
0
.
25
V
A
0
.
25
V
A
Cousins
0
.
125
V
A
0
.
125
V
A
Grandparents
0
.
25
V
A
0
.
25
V
A
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