588
Behavior Genes
(a)
(b)
(c)
Fig. 2
Distribution of traits influenced
by additive genetic effects. (a) A single
biallelic gene gives rise to three
phenotypes; (b) two biallelic genes give
rise to Fve different phenotypes;
(c) numerous additive genetic and
environmental factors give rise to
continuously distributed phenotypes.
Liability
Non affected
Affected
Fig. 3
A liability threshold model in
which it is assumed that multiple genes
and environmental factors contribute to
the underlying liability to a produce a
normally distributed liability towards a
disorder. A person who falls below the
predeFned threshold is not affected by
the disorder whilst all those who fall
above the threshold are affected.
distributed hereditary traits are due to
poly-
genic
mechanisms of this kind, with each
individual gene still being transmitted ac-
cording to Mendel’s laws. The multiple
genes that contribute to the continuous
variation of a multifactorial phenotype
are referred to as quantitative trait loci
(QTL).
Some behavioral traits such as psychi-
atric disorders display discontinuous char-
acteristics, for example, when an individ-
ual is categorized as unaffected or affected
by schizophrenia, or depression. Although
some comparatively rare disorders such as
Huntington’s disease and early-onset vari-
eties of familial Alzheimer’s disease show
Mendelian segregation, most psychiatric
disorders do not conform to Mendelian
patterns of inheritance. For such pheno-
types, we can consider that there is an
underlying continuum of liability to the
disorder. It is usually assumed that liability
has an approximately normal distribution
resulting from the combined effects of
multiple QTLs and environmental effects.
Individuals who fall below a prede±ned
threshold are unaffected whilst those who
at some point exceed it are affected. This is
referred to as the liability threshold model
(see Fig. 3).
2
Quantitative Genetics
Phenotypic variance results from both ge-
netic and environmental influences, and
one of the concerns of quantitative genet-
ics is the partitioning of the phenotypic
variance into its environmental and ge-
netic components. This is discussed in
more detail in Sect. 2.4. The phenotypic
covariance between family members re-
su
l
t
sf
rombo
thth
e
i
rsh
a
r
edg
en
e
sand
shared environments. By taking advan-
tage of the different genetic and en-
vironmental relationships between fam-
ily members, we are able to quantify
the extent to which genes and environ-
ment influence a phenotype. To do this,
we employ family, twin, and adoption
studies.
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