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Behavior. Chapter 12. Central Points . 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 Twin studies are an important part of behavioral genetics. 12.1 Behavior.

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Behavior l.jpg


Chapter 12

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Central Points

  • 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

  • Twin studies are an important part of behavioral genetics

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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?

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Animation: Communication among neurons

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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

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Drugs Mimic Neurotransmitters

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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

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Normal Brain and One with HD

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Chromosomal Abnormalities

  • Example: fragile X syndrome mutation on X chromosome

  • Causes autistic typebehaviors as well as problems with aggression

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Schizophrenia: A Multifactorial Condition

  • Many symptoms including hallucinations, delusions, disordered thinking, changed behavior

  • Genes associated with schizophrenia found on X chromosome and other autosomes

  • Also possible environmental component

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12.4 Studying the Genetics of Behavior

  • Search to find the genetic influences

  • Multiple backgrounds:





    molecular geneticists

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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

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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

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Concordance in Selected Traits

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Pedigree of Schizophrenia

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Pedigree of Alcoholism

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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

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Animal Studies (2)

  • Human gene for a protein associated with memory inserted into mice

  • Transgenic mice improved maze running and other tests

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Pedigree of Homosexuality

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Genetic Causes of Sexual Orientation

  • Used twin studies, chromosomal analysis, and pedigree analysis

  • Suggest genetic component to homosexual behavior

  • Used linkage and pedigree studies to ID area on X chromosome associated with homosexuality

  • Foundation for further work on genetics and homosexuality

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Concordance of Homosexuality

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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

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Pedigree of Family with Violent Behavior

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Knock out Mice for Serotonin Receptors

  • Failure to rapidly break down neurotransmitters such as serotonin

  • Can cause abnormal behavior

  • Researchers deactivate (“knock out”) serotonin receptor gene in mouse

    • Knockout mouse aggressive to unfamiliar mouse

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Knockout Mice

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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

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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

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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

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Video: ABC News: Heart healthy

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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

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Case B: Important Conference on Hold

  • “Genetic Factors in Crime: Findings, Uses, and Implications”

  • Protests caused meeting to be cancelled

  • What should the scientists do?

  • What are the important issues?

  • See the textbook for further questions on this case

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1966: Richard Speck

  • Tried and convicted of killing eight student nurses

  • Attorney prepared defense trying to use XYY karyotype as argument against death penalty

  • Speck did not have the XYY karyotype

  • Many unanswered questions about genetics and behavior remain

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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

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Adult and Teenage Brains

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Areas that control impulsive behavior



Fig. 12-1, p. 199

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Ethical and Legal Issues

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Ethical and Legal Issues (cont.)

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Spotlight on Law: Mobley v. Georgia

  • Stephen Mobley on trial

    • 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

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Mobley’s and MAOA Deficiency Pedigrees

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Spotlight on Law: Questions

  • Women in Mobley’s family are identified with trait. Could this be MAOA deficiency?

  • Would you have allowed the testing?

  • How does this relate to a genetic cause of crime?

  • See the textbook for further questions on this case

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