Behavior Genes
587
genes and environmental factors? How do these genes and environmental factors
act, coact, and interact to produce the phenotype? To what extent is the covariation of
phenotypes the result of the same genes and environmental factors acting on both?
Does the extent to which genes affect a phenotype change over the life span and
are different genes important at different stages of development? Behavioral traits
and disorders, with some rare exceptions, show complex patterns of inheritance
involving gene–environment interplay and therefore quantitative genetic methods
assume a particularly important role alongside molecular techniques.
1
Complex Phenotypes
Behavioral and psychiatric phenotypes
usually exhibit complex, or multifactorial,
inheritance, resulting from a combination
of multiple genes and environmental
contributors, and most normal behavioral
traits
are
continuously
distributed
in
the
population.
For
readers
used
to
dealing with classic Mendelian traits, these
featuresmayat±rstbepuzzling.However,
if we start by considering a single gene with
two alternative forms or alleles,
A
and
a
,as
weseeinF
ig
.1
,wehavethreegeno
types
aa, Aa
,and
AA
, whose phenotypic values
on some particular scale are
x
1
,
x
2
,and
x
3
respectively. Depending on the value of
x
2
, we can have three possible situations.
When
x
2
is equal to
x
3
, we have classical
dominance of
A
with respect to
a
,whereas
if
x
2
is equal to
x
1
,wehaverecessivity.
If the value of
x
2
is exactly midway be-
tween
x
1
and
x
3
,wehaveapurelyadditive
gene effect. Suppose the two alleles have
frequencies in the population of
p
and
q
(equal to 1
p
) and assuming there is no
migration, mutation, or selection against
a genotype, the genotype would be dis-
tributed in the population as follows:
Genotypes:
aa Aa AA
Frequency:
p
2
2
pqq
2
that is, in
Hardy–Weinberg equilibrium
.
If, for the sake of simplicity, we take
p
=
q
=
0
.
5, and suppose that we have
variations about the mean value in each
genotype due to environmental factors, we
might see three phenotypic distributions
as in Fig. 2(a), occurring in the propor-
tions 1:2:1. For a trait controlled by two
loci with each having two alleles of equal
frequency and additive effect, we would
see, as in Fig. 2(b), ±ve phenotypic dis-
tributions occurring in the proportions
1:4:6:4:1. In general then, the rela-
tive proportions of phenotypes controlled
by N loci will be given by (
p
+
q
)
2
N
.
As the size of
N
increases, the overall
phenotypic distribution more closely ap-
proximates a normal distribution, which
is the limiting case when
N
becomes very
large. It is thought that most continuously
Fig. 1
Genotypes and
corresponding phenotypic
values.
aa
Aa
AA
Genotypes
Phenotypic values
x
1
x
2
x
3
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