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[♪ dramatic percussive music ♪]
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>>[Narrator] This double helix is DNA,
the genetic code that contains
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instructions for
all living things,
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but how much of the way
we think and behave is stored
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in this sequence of genetic information,
and how much do we receive
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from the environment
around us?
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Most of our traits are
influenced by many genes.
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How tall you are, for example,
reflects the size of your face,
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vertebrae, leg bones, and so forth,
each of which may be influenced
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by different genes interacting
with your environment.
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Complex traits, such as intelligence,
happiness, and aggressiveness
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are similarly influenced
by groups of genes.
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Thus our genetic predispositions,
our genetically influenced traits,
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help explain both our shared
human nature and our human diversity.
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>>[David Myers] Heredity is
what we get when our parents
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shuffle their gene decks
and deliver a hand to us,
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and it turns out, in many, many ways
to be very important.
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So our temperament,
our personality, our intelligence
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are all powerfully
influenced by our genes.
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>>[Narrator] It is the pursuit
of behavioral genetics
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to tease apart what is
genetically programmed
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from what is influenced by the environment
around us, and how the two interact.
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One of the classic techniques
to scientifically tease apart
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the influences of environment and heredity
is to study identical and fraternal twins.
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>>[Myers] Twin studies are a valuable
tool of behavior geneticists,
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and what they enable us to do is
compare genetic clones, identical twins,
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to see how similar they are
compared to fraternal twins,
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who also are born at the same
time and raised in the family.
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And it turns out that identical twins are
more alike in so many different ways
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than are fraternal twins.
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>>Studies of thousands of twin pairs
in the USA, Sweden, Finland, and Australia
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provide a consistent answer.
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Measuring traits such as
extroversion and neuroticism,
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identical twins are much more
similar than fraternal twins.
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In a seminal experiment beginning
in 1981 by Thomas Bouchard
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at the University of Minnesota,
researchers located and studied
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80 pairs of identical
twins reared apart.
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Participants were given tests
to measure personality traits,
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intelligence, and abilities, occupational
and leisure interests, and social attitudes.
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>>Despite their different rearing,
they're often strikingly alike,
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and that's a powerful illustration
of the power of genetic influences.
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>>Another way to study behavior
genetics is through adoption studies.
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>>Studies of adopted children,
comparing them to their biological parents,
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with whom they share genes,
and their adopted parents,
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with whom they share
a nurturing environment,
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also reveal the power of genes.
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>>[Michael Lyons] The idea is to see,
does this offspring resemble
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the biological parents,
or resemble the adoptive parents?
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>>Twin studies and adoptive family studies
do seem to throw the focus on genetics,
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but to what extent is
our behavior genetic?
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To try to figure this out,
behavioral geneticists
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can mathematically estimate
the heritability of a trait.
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Heritability refers to the extent
to which variations among people
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within a group are
influenced by genes.
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In an example looking at the
heritability of intelligence,
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the slope of this graph shows
the correlation between the
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average intelligence of parents
and their offspring.
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If we have calculated that
the heritibility of intelligence
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in this case is 50 percent,
this does not mean that
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your intelligence is
50 percent genetic.
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Rather it means that genetic
influence explains 50 percent
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of the observed
variation among people.
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>>[Lyons] So take a concrete example,
I think that's the clearest way
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to try to explain it, if we wanted
to look at height and see to what extent
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do genes and the environment influence
individual differences among height.
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>>If the heritability of
height is 90 percent,
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this does not mean that a 60 inch tall
woman can credit her genes entirely
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for 54 inches and her
environment for the other 6 inches.
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Whether it is height, personality,
or intelligence, we can never say
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what percentage of an individual's
traits are accounted for by their genes.
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>>Another related kind of
phenomenon is what's called
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the 'gene environment interaction,'
and that is cases in which our genes
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determine how sensitive
we are to environmental influences.
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>>Today, behavior geneticists
have progressed beyond
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asking if genes
influence behavior.
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Fundamentally, we
know they do.
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So the question is, what are the
specific genes that influence behavior,
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and how can we
identify them in populations?
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The molecular approach to
behavioral genetics
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addresses this question
from the bottom up,
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and has the power to reveal
at-risk populations
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for some of the most
prevalent diseases today.
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In labs worldwide, geneticists
team up with psychologists
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to pinpoint genes that put people at risk
for such genetically-influenced disorders
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as learning disabilities, depression,
schizophrenia, and alcohol dependence.
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It is the study of chromosomes
using the powerful tools of
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DNA scanning and
genome sequencing.
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The most powerful potential for
DNA sequencing in this capacity
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is to predict risk so that steps can be taken
to prevent problems before they happen.
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The molecular approach also opens up
the door to a realm of ethical dilemmas.
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As we begin to identify
more specifically the genes
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that influence a range of faculties,
could future parents have the opportunity
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to screen a developing fetus
for these genetic traces?
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Should we enable parents
to screen their fertilized eggs
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for health, and for brains, or beauty?
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>>Bipolar disorder, if you could
wave a magic wand and eliminate
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the risk genes for bipolar disorder,
would you do that?
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Well bipolar disorder, for example,
is associated in many interesting ways
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with creativity. So many of our greatest
works of art might not exist
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if these genetic variants that were
associated for risk of bipolar disorder
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didn't exist in the gene pool.
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>>The pursuits of molecular
behavioral genetics have the potential
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to relieve a great deal of human suffering
and to give us insights into the
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delicate relationship between
nature and nurture,
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but we must keep an open dialogue about the
ethical implications of what is possible.