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Behavior
Chapter 12
Central Points (1)
 Behavior is a reaction to environment
 Animals and humans have similar behaviors
 Brain chemicals important in human behavior
 Single gene or groups of genes can affect
behavior
Central Points (2)
 Twin studies are an important part of behavioral
genetics
 Courts are unclear on how to address the issue
of genetics and behavior
Case A: Twins Found Strangely Alike
 University of Minnesota Twin Study Group,
studying twins reared apart
 “Two Jims”: amazing number of physical and
other similarities
 Twin studies assist in the study of the genetics
of behavior
12.1 Behavior
 A reaction to stimuli
 Human behavior similarities to animal behavior,
response more variable, difficult to study
 Behavioral genetics: study of the influence of
genes on behavior
 Which more important: genetics or environment?
Animation: Communication among
neurons
12.2 Neurotransmitters
 Brain chemicals that can change our moods and
actions
 Neurotransmission: Neurotransmitters released
when certain neurons in the brain fire
 Impulse travels to synapse, neurotransmitter is
released
 Increases or decreases in the release of the
neurotransmitter, may change behavior
Synapse
Drugs Mimic Neurotransmitters
12.3 Changes in Genes Changes Behavior
 Chromosomal abnormalities, single-gene defects,
and multigenic conditions can alter behavior
Example of single-gene defect: Huntington disease
(HD)
 Codes for protein huntingtin, necessary for the
survival of certain brain cells
 Symptoms of HD: involuntary movements and
progressive personality changes
Normal Brain and One with HD
Chromosomal Abnormalities
 Example: fragile X
syndrome mutation
on X chromosome
 Causes autistic type
behaviors as well as
problems with
aggression
Schizophrenia: A Multifactorial Condition
 Collection of mental disorders
 Many symptoms including hallucinations,
delusions, disordered thinking, changed behavior
 Genes associated with schizophrenia found on X
chromosome and other autosomes
 Also possible environmental component
12.4 Studying the Genetics of Behavior
 Search to find the genetic influences, also
studied by psychologists and psychiatrists
 Medical geneticists study genetic basis for
medical illnesses
 Differences between medical and behavioral
geneticists blurring
 Now treat depression as a physical illness
Identical Twins (Monozygotic)
 Identical twins have identical genomes
 If a specific trait or behavior is same in both
twins, most likely significant genetic component
 If twins raised in separate environments and
have different behavioral traits, traits have a
significant environmental component
Concordance in Monozygotic Twins
 If one identical twin is alcoholic, 55% chance other
twin will be alcoholic regardless of environment
 Concordance: how often a trait occurs in both
members of a pair of twins
 Genetic trait, cystic fibrosis concordance = 1.0
 Concordance helps establish whether or not trait
has genetic basis
Concordance in Selected Traits
Pedigree of Huntington Disease
Animal Studies (1)
 Mice reproduce quickly, behaviors are observable
 May use recombinant DNA techniques
 Insert a human gene
 Mutate a mouse gene
 Change gene’s pattern of expression
Animal Studies (2)
 Human gene for a protein associated with
memory inserted into mice
 Transgenic mice improved maze running and
other tests
Case A Questions
 Why were these two men a perfect set of twins
to study behavior?
 Do you think the evidence shows the behaviors
listed in the case are 100% genetic?
 Can you see any problems studying twins raised
apart?
 See the textbook for further questions on this
case
12.5 Single-Gene Defect and Aggressive
Behavior
 Large family with aggressive and violent behavior
 Only in men, many committed violent offenses
 Mapped to short arm of X chromosome
 Gene encodes monoamine oxidase type A
(MAOA) that breaks down neurotransmitters
 Mutated form of gene, MAOA deficiency
Pedigree of Family with Violent Behavior
Knock out Mice for Serotonin Receptors
 Failure to rapidly break down neurotransmitters
such as serotonin
 Does not allow the normal transmission, disrupts
normal functions in the nervous system
 Can cause abnormal behavior
 Researchers deactivate (“knock out”) serotonin
receptor gene in mouse
• Knockout mouse aggressive to unfamiliar mouse
Knockout Mice
12.6 Legal and Ethical Issues
 Not sure what causes aspects of behavior
 Courts finding it difficult to deal with topic
 Experts do not agree on findings, individual
courts cannot decide how it will be used
 If certain genes or groups of genes cause
someone to commit a crime, motive no longer
relevant
XYY Syndrome
 Prisoners incarcerated for violent crimes showed
higher percentage of men with XYY syndrome
 Studies done on relationship between criminality
and XYY individuals
 Generally taller and perhaps more aggressive,
but this does not mean they are criminals
 Information incomplete
Other Questions
 If criminal or antisocial behavior is genetically
controlled, how can someone be rehabilitated?
 Roper v. Simmons: an amicus curiae brief
presented
• Area of brain controlling impulsive behavior
underdeveloped in teens
• Therefore unable to realize actions
 Most judges and juries considered genetic
predisposition to crime junk science
Adult and Teenage Brains
Spotlight on Law: Mobley v. Georgia
 Stephen Mobley on trial
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Violent murder and armed robbery at Domino’s
History of rape, robbery, assault, and burglary
Aggressive behavior prevalent in his family
Wanted to be tested for mutation of MAOA gene
 Court denied, genetic connection not at level of
scientific acceptance to justify its admission