Transcript Human Genetics

Chapter 8
Genetics of
Behavior
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Learning Outcomes
• Identify the physical basis of behavioral traits in
the brain
• Explain how genes can affect behavior
• Discuss the genetic and environmental influences
on sleep, intelligence, addiction, mood, and risk
of developing schizophrenia or autism
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Behavior
• Complex continuum of emotions, moods,
intelligence, and personality
• Occurs in response to environmental factors, but
how we respond has genetic underpinnings
• Behavioral genetics considers nervous system
function and variation
• Including mood and mind
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The Human Brain
• Weighs about three pounds
• Consists of 100 billion neurons and at least a
trillion other supportive and nurturing cells called
neuroglia
• Neurons communicate across synapses using
neurotransmitters
• Genes control the production and distribution of
these chemical signals
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Neurotransmission
Figure 8.1
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Behavioral Genetics
• Uses empirical risk, twin studies, and adoption
studies
• Association studies with SNPs and analysis of
specific mutations that are present in individuals
with the behavior
• Genetic studies of behavioral disorders are
challenging traditional psychiatric classification
• Disorders may lie on a continuum with many
genes having input
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Sleep
• Vital behavior of unknown function
• Without sleep animals die
• Twin studies indicate 4 of the 5 stages of sleep
have a hereditary component
• Fifth stage, REM sleep, is associated with
dreaming and so reflects input of experience more
than genes
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Narcolepsy with Cataplexy
• Daytime sleepiness with tendency to rapidly fall
asleep (narcolepsy) and periods of muscle
weakness (cataplexy)
•
Genetic basis was first
identified in dogs,
then humans
Figure 8.2
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Familial Advanced Sleep
Phase Syndrome
• Disorder characterized by a very unusual sleepwake cycle
• Affected members of a large family enabled
researchers to identify the first clock gene in
humans
• Period gene enables a person to respond to day
and night environmental cues
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Familial Advanced Sleep
Phase Syndrome
• Pedigree of the autosomal dominant form of the
disease
Figure 8.3
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Intelligence
• Complex and variable trait subject to multiple
genetic, environmental influences, and intense
subjectivity
• Refers to the ability to reason, learn, remember,
synthesize, deduce, and create
• Intelligence quotient (IQ) test was first developed
in France in 1904
• Predicts academic success of developmentally
disabled children
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IQ Test
• Later modified at Stanford University to assess
white, middle-class Americans
• IQ is normally distributed around a mean of 100
•
•
•
•
Below 50 - Severe mental retardation
50-70 - Mild mental retardation
85-115 - Average intelligence
Above 115 - Above average intelligence
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• IQ has been a fairly accurate predictor of success
in school and work
Figure 8.4
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IQ Test
• Tests verbal fluency, mathematical reasoning,
memory, and spatial visualization ability
• g value measures a general intelligence factor that
represents the inherited portion of IQ
• Environment has less of an influence on IQ as a
person ages
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Drug Addiction
• Compulsively seeking and taking a drug despite
knowing its adverse effects
• Characteristics
• Tolerance - Need to take more of a drug to
achieve the same effect
• Dependence - Onset of withdrawal symptoms
with cessation of drug
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Drug Addiction
• Heritability is 0.4-0.6
• Twin and adoption studies support role of genes in
drug addiction
• Produces long-lasting changes in the brain
• Brain changes that contribute to addiction are in
the limbic system
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Events of Addiction
Figure 8.5
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Figure 8.6
Nicotine's effects at the cellular level. (1) Binding of nicotine to nicotinic receptors, which also bind the
neurotransmitter acetylcholine, triggers release of dopamine (2) from vesicles into the synapse. Some dopamine
binds receptors on the postsynaptic (receiving) neuron (3) and some dopamine re-enters the presynaptic neuron
through a protein called the dopamine transporter (4). Uptake of dopamine into the postsynaptic cell triggers the
pleasurable feelings associated with smoking. The inset illustrates the mechanism of a smoking cessation drug,
which blocks the nicotinic receptors.
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Proteins Involved in Drug Addiction
• Enzymes involved in biosynthetic pathways of
neurotransmitters
• Neurotransmitter-reuptake transporters
• Cell-surface receptors
• Members of signal transduction pathways in
postsynaptic neuron
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Drugs of Abuse
• Abused drugs are often derived from plants
• Cocaine, opium, and tetrahydrocannabinol (THC),
the main ingredient in marijuana
• Chemicals bind receptors in human neurons
• Endorphins and enkephalins are the human
equivalents of opiates
• Relieve pain
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Candidate Genes for Drug Addiction
• Nicotine binds a receptor that normally binds
acetylcholine, causing dopamine release and
pleasure
• Candidate genes for addiction include those that
encode the:
• Dopamine D(2) receptor
• Nicotine receptor parts
• Protein neurexin-1
• Ferries nicotinic receptors to neuron’s surface
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Mood Disorders
• Represent the extremes of normal behavior
• Types
• Major depressive disorder - Marked by
unexplained lethargy, sadness, and chronic
depression
• Bipolar disorder - Marked by depression
interspersed with mania
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Major Depressive Disorder
• Affects 6% of the US population
• Likely cause is a deficiency of the
neurotransmitter serotonin
• Affects mood, emotion, appetite, and sleep
• Many antidepressant drugs are selective serotonin
reuptake inhibitors (SSRIs)
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Major Depressive Disorder
Figure 8.7
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Bipolar Disorder
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•
•
•
Also called manic-depression
Affects 1% of the population
Associated with several chromosome sites
Genetic roots are difficult to isolate
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Schizophrenia
• Loss of ability to organize thoughts and
perceptions - withdrawal from reality
• Worldwide - 1% affected
• Typically early adult onset
• Progression
• Difficulty in paying attention, memory and
learning difficulties, psychosis (delusions and
hallucinations)
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Schizophrenia
• Disjointed drawings by schizophrenic patients
display the characteristic fragmentation of the
mind
Figure 8.8
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Schizophrenia
• Heritability of 0.8 and empiric risk values indicate
a strong genetic component for schizophrenia
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Schizophrenia
• Dozens of genes may interact with environmental
influences to cause this disease
• One powerful candidate is infection during
pregnancy
• Prenatal exposure to the influenza or herpes
viruses
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Autism
• Spectrum of disorders
• Characterized by loss of language,
communication, and social skills beginning in
early childhood
• Seizures and mental retardation may occur
• Affects 3-6 children out of every 1,000
• Strikes four times as many boys as girls
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Autism
• More than 30 genes so far have been associated
with autism
• Two genes in particular may finally explain how
autism develops
• Encode the cell adhesion proteins neurexins and
neuroligins
• Proteins strengthen synaptic connections in
neurons associated with learning and memory
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Understanding Autism
•
Autism may arise from failure of synapses to
form that enable a child to integrate experiences
Figure 8.10
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