598
Behavior Genes
a genetic influence on behavior. Inbreed-
ing, the mating together of close relatives
such as siblings, when repeated over many
generations, creates animals that are vir-
tually identical. Consequently, differences
within strains are largely or entirely due
nongenetic effects, whilst differences be-
tween strains kept in similar environments
are due to genetic effects. Inbreeding has
been used to demonstrate a genetic ba-
sis to numerous mouse behaviors such as
fearfulness, held to be a model of human
anxiety, and ability to solve problems such
as mazes, which provides analogies for
components of human intelligence. Cur-
rently, there are over 100 inbred strains of
mice commercially available. Mouse mod-
els are, of course, limited when it comes to
more complex human behaviors and can-
not shed direct light on behaviors involving
speech and language. Another limitation
is that inbred strains may not represent the
behavioral repertoire of naturally segregat-
ing genetic populations. Another practical
problem is that inbreeding depression may
result in reduced infertility, interfering
with breeding of laboratory strains.
The ability to select for a trait is only
possible when there is an underlying
genetic contribution. Thus, if we are able
to selectively breed for a trait, we can infer a
genetic component. For example, selection
studies have been carried out with regard
to so-called open ±eld activity in rodents,
where the rodents are introduced to an
enclosure that is open at the top, often
in the form of a cage with beams of
light transmitted near the floor to record
activity. A rat or mouse that displays
low
levels
of
activity
and
frequently
‘‘freezes’’ and defecates in an open ±eld
is thought to display the equivalent of
high anxiety, whilst one that is highly
active in an open ±eld and shows less
frequent defecation is thought to have
low anxiety. By selectively mating together
animals that show high activity, and
mating together those that display low
activity it has been possible to develop
behaviorally distinct lines, the best known
of which are the ‘‘Maudsley reactive’’ and
‘‘Maudsley nonreactive’’ rats.
There are a number of methods used to
identify genes using animal models. These
include studies of transgenic, knockout,
and recombinant inbred strains. In trans-
genic mice, genes from another species,
such as humans, are inserted into the
mouse genome, and the effects of the
gene can be observed. Genes such as the
Dopamine D1 receptor gene has been im-
plicated in anxiety and hyperactivity using
transgenic mice.
Knockout mice result from eliminating
activity of genes thought to be influential
and observing the effect this has on the
animal’s behavior. This technique has
been used to show that the monoamine
oxidase A gene and the nitric oxide
synthase gene both play a role in mouse
aggression as knockout mice for each of
these displayed higher rates of aggression
compared to the wild-type mice.
By mating together a single male and
female from two different inbred strains
and
then carrying out brother
sister
matings for 20 or more generations, it is
possible to develop a recombinant inbred
(RI) strain. Like any form of inbred strain,
RIs are essentially identical and each
contains only two alleles for each locus,
one from each of the original inbred strains
from which they were derived. It is possible
to identify which of the progenitor strains
each of the alleles at a locus originate
from. When two alleles at different loci
originate from the same progenitor strain,
they are said to be concordant, and when
they are from different progenitors, they
are said to be discordant. The greater
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