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.
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