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Behavioral Genetics Jeffrey Clothier, M.D.
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Behavioral Genetics Jeffrey Clothier, M.D.

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  • 1. Behavioral Genetics Jeffrey Clothier, M.D.
  • 2. Objectives
    • Describe the genetic methods applied to behavior
    • Describe role of genetics and environment in conditions such as MR, IQ, Substance abuse, schizophrenia, affective disorders, and dementing illnesses
    • Describe the value of twin studies and adoption studies in behavioral genetic studies
    • Define heritability
  • 3. Methods of Genetic investigations
    • Transmission studies
      • Family
      • Twin
      • Adoption
    • Pedigree analysis and linkage studies
      • Pedigree and genetic markers
      • Molecular studies
    • Point is to try and predict who will get sick
  • 4. Family studies
    • Describe risk of occurrence of disease in two related at risk persons
      • concordance rate is related to shared characteristics
        • genetic and environmental
    • Heritability -A measure of the relative influence of genetics to overall family transmission
  • 5. Twin studies
    • rare but precious resource for understanding genetic influences
      • Monozygote twins are 100% genetic and 100% environmental “ideally”
      • Dizygote twins are 50% genetic and 100% environmental “ideally”
      • Difference b/w DZ and MZ concordance rates is a measure of genetic influence
  • 6. Adoption studies-control environmental factors
    • Adoptee’s family method(index case is the child)
        • calculates concordance in parents of index case
          • needs information about adoptive and biologic parents
        • greater risk in biologic parents than adoptive parents suggest genetic factors
  • 7. Adoption studies (cont)
    • Adoptees study method (index case is the parent)
      • calculates risk to children of index case
        • increased risk to adopted away children of index case over adopted children of controls suggest genetic factors
    • Crossfostering method
      • Compares risk to adoptees whose biologic parents were ill and adoptive parents were normal with adoptees whose biologic parents were normal and adoptive parents were ill.
  • 8. cross fostering method
  • 9. Problems with twin studies
    • Ascertainment-diagnostic criteria
    • adequacy of control
    • phenotypic classification
    • determination of zygosity
  • 10. Pedigree analysis
    • address questions of penetrance and transmission mechanisms
      • single major locus
      • multifactorial-polygenetic
    • phenotype vs. genotype
      • effect of variable penetrance
        • assumption of dominance
      • endophenotypic expression
  • 11. Single major locus transmission
    • Autosomal
      • recessive
        • Wilson’s disease (ceruloplasmin and copper)
          • damage to liver, cornea and basal ganglia
        • Phenylketonuria (phenylalanine hydroxylase)
          • build up of abnormal metabolite (phenylpyruvic acid
          • carrier state is common (1 in 50)
          • endophenotypic expression
  • 12. Single major locus (cont)
    • Autosomal dominant
      • Huntington’s disease (atrophy of the caudate nucleus)
        • chorea and mental disorders
        • appears around mid 30’s
        • Chromosome 4
  • 13. Single Major locus (cont)
    • Sex linked disorders
      • Usually x linked recessive (ex:Lesch-Nyhan syndrome)
        • self mutilation, MR, Uric aciduria
        • mother is the carrier
        • 1/2 of sons have the disease
        • 1/2 of daughter are carriers
          • carrier state can be detected
          • prenatal diagnosis is possible
  • 14. Single major locus (cont)
    • Disorders of gene and chromosome numbers
      • Trisomy 21 (Down’s)
      • XYY more common in jails than society
        • not related to an elevation of testosterone and aggression
        • probably due to a lower intellectual function of XYY patients
  • 15. Personality
    • Cloninger’s 3 dimensional model
      • novelty seeking
      • harm avoidance
      • reward dependence
    • Some traits have a high inheritance
      • antisocial is greater than rheumatoid arthritis
    • Clear role for socialization and environment
  • 16. Intelligence
    • MZ twins scores on IQ tests have a correlation of 80-90%
      • whether reared together or separately
    • DZ twins have correlation of about 50%
    • heritability factor of about 50-70%
    • Inverse relationship of MR offspring and parents
  • 17. Schizophrenia
    • lifetime population risk of 1%
    • risk to sibs and children of index case is 8-12%
    • risk to parents is 5%
      • may reflect fact that sicker patients don’t marry
  • 18. Schizophrenia (cont)
    • MZ concordance of 40-70%
    • DZ concordance of 8-20%
    • second trimester hypothesis
      • time of neural specialization
    • frontal injury hypothesis
  • 19. Bipolar disorder
    • Lifetime risk about 1%
    • sexes are equally affected
    • risk to 1st degree relative is 8-20%
    • MZ risk-65-70%
    • DZ risk-15%
    • linkage studies suggest chromosome 11 and x-linked
  • 20. Unipolar
    • lifetime risk is 6-10%
    • 2-3 times more common is women
    • risk to first degree relatives is 10-20%
    • MZ risk-40%
    • DZ risk-11%
    • phenotypic expression-undiagnosed bipolar disorder for ex.
  • 21. Alcoholism
    • lifetime risk-
      • 3-5% for men
      • 0.5 - 1% for women
    • 25% of male first degree relatives
    • 3-10% of female first degree relatives
  • 22. Alcoholism
    • twin risk depends on definitions
      • MZ risk-
        • 60% (Swedish studies)
        • 25% (US)
      • DZ risk
        • 30% (Swedish)
        • 12% (US)
    • runs with affective disorders in families
  • 23. Alcoholism
    • RFLP
      • dopamine 2 receptor alleles
        • Dopamine receptor D2 receptor allele known as Taql B1 is reported as a marker for severe familial alcoholism
    • Aldehyde dehydrogenase
      • enzyme which metabolizes alcohol is found in varying degrees of activity in various cultures
  • 24. Alzheimer’s Disease
    • Difficult to study due to death before expression of gene for SDAT
    • 19% risk for 1st degree relatives
    • increases with survival age
      • 50% if you live to 80 y/o
  • 25. SDAT
    • two forms
      • early onset (prior to 65)
        • linked to chromosome 21
      • Late onset (after 60)
        • linked to chromosome 19