Behavioral Genetics
Ernst Mayr, a leading figure in twentieth century evolutionary thought, saw
behavior as a continuum ranging from completely closed, or fixed by the
genotype, to completely open, extremely flexible and dependent on the
environment. Behavioral genetics has become an increasingly important field as
modern technology has allowed researchers to locate specific genes and
alleles responsible for some behaviors. The field has also stirred
controversy: people are hesitant to attribute behavior to genetics in the age-
old nature versus nurture debate. But nature versus nurture is a false
dichotomy. The truth is that both can play a key role in determining a behavior.
The genotype determines the potential for a behavior to exist, while nature
determines what phenotypic behavior actually results.
Behavioral genetics are more complicated than Mendelian
genetics because while Mendel's pea plants showed
clearly distinctive characteristics, animal behavior does not always fit into
such hard and fast categories. Successful identification of behavioral genes is
most likely when the gene shows a high penetrance, that is, the genotype
is expressed in the phenotype. Degree of expression is also important for
success in determining behavioral genes. Behaviors that can be readily observed
and quantified or categorized are easier to work with.
Heritability
While some behaviors are indeed attributable to a single gene, most behavioral
traits are polygenic, or influenced by several genes. Further, more often than
not, the environment will mediate the affects of those genes. Such a complex
situation can be troublesome for researchers seeking to isolate the genetic
mechanisms of behavior. In order to create some standard to measure specific
behaviors in relation to genes and environment, scientists have devised a scale
of heritability. Measured on a scale between 0 and 1, heritability is a
measure of how much of the variance seen in a trait is due to its inherited
components rather than its environmental components. A heritability value of 0
means that a certain behavior is completely independent of genetic makeup; a
heritability value of 1 means that a behavior is totally dependent on genes.
The actual calculation of the variance due to heredity can be extremely
complicated, and we need not be concerned with that calculation here. The
important thing to remember is that a heritability value that is above 0 and
less than one indicates that neither nature nor nurture is the sole factor in
determining a behavior.
Important Techniques and Strategies of Behavioral Genetics
Behavioral genetics as an experimental field can be extremely technical and
complicated. Here we will examine some of the important techniques and
approaches to the study of behavioral genetics.
Inbreeding
In order to tease apart the genetic and environmental factors contributing to a
behavior, it is useful to be able to hold one factor constant. Inbreeding
animals over many generations will produce a population that is homozygous
and genetically identical. Once experimental subjects are genetically
identical, variations in a behavior due to environmental differences can be
identified and their relative importance analyzed. Mice are commonly used for
such inbreeding experiments. After about thirty generations, the population is
about 98 to 100 percent homozygous.
To examine the relative importance of genetic components of behavior while
holding environmental components constant, two or more inbred strains may be
used. Two different inbred strains that are different from each other can be
assessed while holding the environment constant across both strains.
Evolution changes species through time by
means of
natural selection. But breeds of pets, plants, and livestock have been
rapidly changed by human intervention for at least centuries; humans breed
animals and plants for a purpose: beauty, docility, meat production, etc. Such
breeding techniques are called artificial selection. Inbreeding is really a
form of artificial selection, although the traits selected for do not need to be
specified. Instead, the population is being bred to be identical, a trend
opposite of that observed in nature.
Twin Studies
Twin studies are extremely important in studying human subjects, because humans
cannot be bred for certain traits and are hard to manipulate in an experimental
fashion. Monozygotic twins, commonly known as identical twins, are
genetically identical. Dizygotic twins, or fraternal twins, are no more
genetically similar than non-twin siblings are. Therefore, if monozygotic twins
share a behavioral trait significantly more frequently than dizygotic twins do,
it can be assumed that behavior has a genetic component. It is important to
study the shared traits between monozygotic and dizygotic twins, as well as
nontwin siblings, because each set of siblings can yield information in
comparison with the others. Monozygotic twins and dizygotic twins differ in the
amount of genetic material shared, but should have the same prenatal
environment, and so behavioral comparisons can isolate genetic components for
assessment. Dizygotic twins are no more genetically similar than nontwin
siblings, but non-twin siblings do not share their prenatal environment, and so
comparison can yield opportunities to asses environmental components.
Knockout Studies
An important technique in molecular analyses of behavioral genetics is the
knockout study. Mice are literally designed to express or fail to express
certain traits by inserting or subtracting genes from embryonic cells and then
reinserting them into a female to gestate. In most knockout studies, a
mechanism is designed so that researchers can turn on and off the gene, usually
by treatment with an antibiotic. This is accomplished by combining the inserted
or deleted gene with another gene susceptible to antibiotics.
