BG5.ppt

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BG5.ppt

  1. 1. BEHAVIORAL GENETICS Topic #5 Twin and Adoption Studies
  2. 2. Inferring the Existence of Genetic Influences <ul><li>Comparing similarity in pairs of relatives who differ in degree of genetic but not environmental relatedness: </li></ul><ul><li>MZ vs. DZ twins </li></ul><ul><li>Assessing similarity in relative pairs where there is either no genetic or no environmental basis for similarity </li></ul><ul><li>Adoption Studies </li></ul>
  3. 3. Outline <ul><li>Twin Studies </li></ul><ul><ul><li>Rationale </li></ul></ul><ul><ul><li>Representativeness </li></ul></ul><ul><ul><li>Equal Environmental Similarity </li></ul></ul><ul><ul><li>Biometric Formulation </li></ul></ul><ul><li>Adoption Studies </li></ul><ul><ul><li>Rationale </li></ul></ul><ul><ul><li>Representativeness </li></ul></ul><ul><ul><li>Selective Placement </li></ul></ul>
  4. 4. Types of Twins <ul><li>Monozygotic (MZ; “identical”): result from the fertilization of a single egg by a single sperm; share 100% of their genetic material. </li></ul><ul><li>Dizygotic (DZ, “fraternal” or “non-identical”): Result from the independent fertilization of two eggs by two sperm; share on average 50% of their genes. </li></ul>
  5. 5. MZ Twinning <ul><li>w/i first 3-5 days after fertilization, they are dichorionic and diamnionic </li></ul><ul><li>from 5 - 10 days post-fertilization, they share a single chorion but separate amnions </li></ul><ul><li>from 10-13 days, they are monochorionic and monoamnionic </li></ul>
  6. 6. Mono- vs Di-chorionic Twins 100% of DZ twins are dichorionic Approx 1/3 of MZ twins are dichorionic and 2/3 are monochorionic
  7. 7. Chorion Type and Twin Correlation (N=451 pairs) * * Jacobs et al. (2001). Heritability estimates of intelligence in twins: Effect of chorion type. Behavior Genetics , 31: 209-217.
  8. 8. Zygosity Determination <ul><li>Physical similarity </li></ul><ul><li>DNA based: Protein markers (1950s)  DNA markers (1990s) </li></ul><ul><li>Questionnaire Methods </li></ul>
  9. 12. Rates of Twinning
  10. 13. Representativeness of Twins <ul><li>Congenital disorders/Prenatal Exposure </li></ul><ul><li>Personality and psychopathology </li></ul><ul><li>Cognitive abilities </li></ul>
  11. 14. Twin-Singleton Differences (in SD units) in Personality Johnson et al. (2002)
  12. 15. Equal Environmental Similarity Assumption <ul><li>A key assumption of the twin method is that MZ twins are not more similar on trait-relevant environmental factors than DZ twins. </li></ul>
  13. 16. Equal Environmental Similarity <ul><li>Do MZ twins have more similar environments than DZ twins? </li></ul><ul><li>Does greater MZ than DZ twin environmental similarity account for greater phenotypic similarity? I.e., is it trait relevant ? </li></ul><ul><li>Why do MZ twins have greater environmental similarity? </li></ul>
  14. 17. Loehlin & Nichols (1976) <ul><li>If greater environmental differences in DZ than MZ twins helps explain their greater behavioral dis-similarity, </li></ul><ul><li>then behavioral dissimilarity among MZ twins (which must owe to environmental factors) should be related to dissimilarity on these environmental factors </li></ul>
  15. 19. Biometrical Formulation <ul><li>r MZ = a 2 + d 2 + i 2 + c 2 </li></ul><ul><li>r DZ = 1/2a 2 + 1/4d 2 + (k)i 2 + c 2 </li></ul><ul><li>1.0 = a 2 + d 2 + i 2 + c 2 + e 2 </li></ul><ul><li>where k is unknown but ~0 </li></ul>
  16. 20. Falconer Model <ul><li>r MZ = a 2 + c 2 </li></ul><ul><li>r DZ = 1/2a 2 + c 2 </li></ul><ul><li>1.0 = a 2 + c 2 + e 2 </li></ul>
  17. 21. Falconer Estimates <ul><li>a 2 = 2(r MZ - r DZ ) </li></ul><ul><li>c 2 = 2r DZ - r MZ </li></ul><ul><li>e 2 = 1 - r MZ </li></ul>
  18. 22. Adoption Study Bio Parents Adoptive Parents Genetic Rearing Env. Offspring Phenotype <ul><li>Adopt parent-offspring </li></ul><ul><li>resemblance reflects </li></ul><ul><li>environmental factors </li></ul><ul><li>Bio parent-offspring </li></ul><ul><li>resemblance reflects </li></ul><ul><li>genetic factors </li></ul>
  19. 23. Issues with Adoption Studies <ul><li>Feasibility </li></ul><ul><li>Representativeness </li></ul><ul><li>Selective Placement </li></ul>
  20. 24. Feasibility <ul><li>Estimated 1.5% to 2.5% of U.S. population is adopted </li></ul><ul><li>Adoption records are confidential </li></ul><ul><li>Rare to have information on biological background </li></ul><ul><li>Scandinavia </li></ul>
  21. 25. Representativeness <ul><li>Environmental background (adoptive parents) </li></ul><ul><li>Biological background (birth parents) </li></ul><ul><li>Adoptee functioning </li></ul>
  22. 26. Intellectual Abilities of Adoptive Parents <ul><li>Mean (SD) IQ of adoptive parents in MN Transracial Adoption Project was 120.7 (10.6) for dads and 118.3 (10.4) for moms (Scarr & Weinberg, 1977). </li></ul><ul><li>Mean (SD) IQ of adoptive parents in Texas Adoption Project was 115.2 (11.1) for dads and 112.6 (10.4) for moms (Horn et al., 1979) </li></ul>
  23. 27. Comparison of Minnesota Adoptive and Nonadoptive Parents (McGue et al., 2001) %
  24. 28. Birth Background <ul><li>Cognitive Ability </li></ul><ul><li>Psychopathology – Scandinavian data </li></ul><ul><li>Personality – Texas data </li></ul>
  25. 29. MMPI Scale Scores for Birth Mothers in Texas Adoption Project (after Horn)
  26. 30. Adoptee Functioning <ul><li>Cognitive Abilities </li></ul><ul><li>Psychological adjustment and psychopathology </li></ul>
  27. 31. Rates (%) of problems in adopted and non-adopted adolescents (Miller et al., 2001) Above 95%
  28. 32. Factors Affecting Psychological Adjustment of Adopted Individuals <ul><li>Referral bias ++ </li></ul><ul><li>Biological background + </li></ul><ul><li>Preplacement history ++ </li></ul><ul><li>Psychological effects of adoption ?? </li></ul>
  29. 33. Selective Placement Mother 1 Ed Mother 2 Ed Twin 1 IQ Twin 2 IQ r = -.001 r = -.001 r = .41 Two conditions must be met:
  30. 34. Summary <ul><li>Twin Study: </li></ul><ul><ul><li>Rationale & Biometric Formulation </li></ul></ul><ul><ul><li>Epidemiology </li></ul></ul><ul><ul><li>Representativeness </li></ul></ul><ul><ul><li>Equal Environmental Similarity </li></ul></ul><ul><ul><li>Zygosity Determination </li></ul></ul><ul><li>Adoption Study: </li></ul><ul><ul><li>Rationale </li></ul></ul><ul><ul><li>Representativeness </li></ul></ul><ul><ul><li>Selective Placement </li></ul></ul>

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