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MCDB 1041 Class 33
Genes and Behavior
Learning goals
•  Work with numbers to calculate concordance, heritability and
relatedness when considering multifactorial traits.
•  Describe the basics of neurotransmission and its connection to
behavior.
•  Interpret the possible effects of different mutations on behavior
What are the genetic factors that contribute
to a given behavior and how do they interact
with the environment?
First: need to figure out the relative fraction
of a trait that is due to genetics
This is called Heritability, and can be
calculated by looking at:
-relatedness
-concordance
Concordance= % of pairs
of individuals that share a
particular trait
Example:
20 pairs of identical twins are being studied to look
at factors that influence heart disease
Of the 20 pairs, 10 sets of twins both have heart
disease.
The concordance in this example is 50%, or .5
Heritability
A measure of the % of phenotype due to genes
Estimated from the proportion of people sharing a trait
compared to the proportion predicted to share the trait.
Heritability = concordance/ relatedness
Concordance= % (or fraction) of pairs of related
individuals that share a trait
Relatedness= % (or fraction) of genetic relatedness (on
average) between people
Heritability for a
variety of traits
H=1
H=0
genes only
environment only
Let’s practice
how this
correlation is
calculated
Trait
Heritability
Clubfoot
0.8
Height
0.8
Blood pressure
0.6
Body mass
0.5
Verbal aptitude
0.7
Math aptitude
0.3
Spelling aptitude
0.5
Fingerprint ridge
count
0.9
Intelligence
0.5-0.8
Total serum
cholesterol
0.6
You have calculated the heritability of depression in a large
scale fraternal (non-identical) twin study to be 0.4.
Heritability = concordance/ relatedness
What is the concordance in your population of twins?
a. 0.2
b. 0.25
c. 0.4
d. 0.5
e. 0.8
Heritability = concordance/ relatedness
If you have a fraternal twin study in
which the concordance is 20%, what is
the heritability of the trait you are
studying?
a.  20%
b.  40%
c.  60%
d.  10%
What will happen if environment
is more important than genetics?
To figure this out, scientists
compare phenotypes of:
siblings raised together
siblings raised apart
Example: Tobacco addiction
What is most likely with tobacco addiction from this data?
a. Genetics plays a strong role
b. Genetics plays a minor role
c.  Genetics plays no role
You think genetics is the primary contributor to the abuse of a new drug called
“Rage”.
You compare abusers of Rage to their monozygotic twins, dizygotic twins, siblings,
adopted siblings, or siblings raised apart. Which of the following graphs matches
your prediction that genetics plays the primary role in the abuse of Rage?
B.
concordance
D.
concordance
(concordance = fraction
of times that for each
user, there was a
sibling user in the
category shown)
concordance
Adoptive siblings
Siblings raised apart
concordance
Monozygotic twins
Dizygotic twins
Siblings
C.
A.
Most behaviors involve the brain
Drug addiction
Thrill Seeking Behavior
Eating Disorders
Depression
Bipolar disorder
Schizophrenia
Anxiety
Attention Deficit Disorder (ADD)
Attention deficit hyperactivity disorder (ADHD)
Autism
What genes could be important for
behaviors?
- genes that code for Neurotransmitter
receptors
- genes that code for proteins that
affects how neurons connect to each
other
- lots of other possibilities!
How does the brain work?
•  Neurons bring information to the brain and send information
away from the brain (mostly to muscles)
•  Information is transmitted along AXONS to the SYNAPSE
Transmitting neuron
(Presynaptic neuron)
Axon
Receiving neuron
(Postsynaptic neuron)
Synapse
Direction of action potential
Neuron Animation:
http://www.pbs.org/wnet/closetohome/animation/neuron-main.html
Close-up of a Synapse
Reuptake of the neurotransmitter dopamine
After a neurotransmitter is released, it binds to
receptors on the post-synaptic cell. This causes a
change in that cell.
Neurotransmitters are removed from synaptic cleft
by:
•  enzymes that break them down
•  transporters that take the neurotransmitter back in
to the presynaptic cell. This is called “re-uptake”.
Once back in the presynaptic cell, they are broken
down by enzymes and re-used.
Which of the following would decrease the
amount of neurotransmitter binding to its
receptors?
a.  Higher production of neurotransmitters
b.  A non-functional reuptake protein
c.  A mutation that makes the enzyme that
breaks down the neurotransmitter
nonfunctional
d.  A mutation that makes the receptor
nonfunctional
Mood disorders
When F.F discovered his wife in bed with another man, he shot and
killed them both.
F.F.’s defense ordered a psychiatric workup that included
sequencing of two genes:
•  The gene that codes for the enzyme Monoamine oxidase A
(MAO-A), which breaks down serotonin and dopamine
•  The gene that codes for the serotonin re-uptake transporter
What were they trying to
establish?
Is this a valid approach for the
defense?
Depression: biochemical basis
• Serotonin is known to affect mood, emotion, appetite and
sleep. Usually, low levels of serotonin cause these
disorders
Clinical depressions can be treated with anti-depressant
drugs like Prozac, Paxil or Zoloft. How might these work?
A.  break down serotonin
B.  decrease the uptake of serotonin into the postsynaptic
neuron
C.  decrease the uptake of serotonin into the presynaptic
neuron
D.  increase uptake of serotonin into the presynaptic
neuron
Antidepressants change the way the way serotonin is made
available to neurons:
– prevent serotonin from being broken down
– prevent serotonin from being taken back up into neurons
Both of these increase the level of serotonin, and thus
(usually) improve mood
Mood disorders are often triggered or exacerbated by
environment—stress, life events
Thus, scientists also look for genes that might impact
response to stress as well
Other mood disorders
(bipolar, schizophrenia)
are more challenging than
treating clinical
depression alone.
Many genes have been
associated with these
disorders, but we still
don’t understand enough!
Interestingly, both
bipolar disorder and
schizophrenia have a
higher H than depression
Areas associated with bipolar disorder
Although genetics
alone may not
explain mood
disorders, we
know that
serotonin and
dopamine levels
effect mood
AND how well
people respond
to drug therapy
What’s the connection to genes?
•  Neurotransmitter receptors, re-uptake proteins and
enzymes are coded for by genes in your DNA.
•  Neurotransmitters are themselves not coded for by genes,
but the enzymes that help to synthesize them from
precursors are
•  Alleles of these genes can affect how well these proteins
work
•  These differences impact brain function (drug use or
addiction, mood disorders, etc) and thus affect behavior