BEHAVIORAL GENETICS

PSYCH 3102 Dr Hewitt

HOMEWORK # 6 AND PRACTICE EXAM 4.

NAME ___________________________

DATE _____________________

1. Matching

The proportion of phenotypic variance due to all effects if genes.   broad heritability

Genes that have been associated with the risk for alcoholism in some recent studies.  ALDH DRD2

The proportion of a population having a disorder.                          prevalence

Commonly employed measures of substance abuse.               quantity and frequency

A measure of the resemblance between 2 people for a dichotomous trait.    concordance

risk                     prevalence              quantity and frequency                  ALDH and DRD2

concordance       correlation                 Type I alcoholism                      Type II alcoholism

DSM-IV    alcohol dependence              factor analysis                 heterozygosity

additive genetic variance                      broad heritability                    narrow heritability

anticipation                                   non-additive genetic variance

Twin, family and adoption studies

The following table gives the reported correlations for same sex twins raised together for two aspects of personality. Assume that sample sizes are large.

TRAIT               MZ CORRELATION                   DZ CORRELATION

Neuroticism                    0.48                                                 0.24

Extraversion                    0.50                                                0.13

Basing your answers on these correlations and on evidence from other studies mentioned in class:

a. What is the narrow heritability for neuroticism? 48%

Show how you got this MZ correlation

since there is no evidence for shared environment (DZ correlation is not more than ½ MZ correlation) heritability is given by MZ correlation

   b.Which trait shows the most evidence for non-additive genetic influences? extraversion

Justify your answer.  DZ correlation is less than ½ MZ correlation

c.What evidence leads us to conclude that there is no assortative mating for these personality traits?  

absence of spousal correlations

d.Why do we conclude that there are genetic influences on personality?

Give evidence for your answer.  MZ correlations are greater than DZ correlations, reflecting genetic similarity

family resemblance shown by P/O and sib correlations (although this could include shared e)

biological correlations >> adoptive correlations, indicating genetic similarity produces resemblance but shared e does not

e.What is the evidence that the shared family-rearing environment has little or no influence on adult personality?  

adoptive P/O and adoptive sibs show correlations close to zero

DZ correlations are no greater than expected on genetic model for reared together pairs.

f.Why do we conclude that the non-shared environment has important consequences for personality development?

Justify your answer.

MZ correlations are much less than 1.0 so identical genotypes do not produce identical personalities

First degree relative correlations are much less than 0.5, highest expected under genetic model

3. Brief account

Write a brief account of the behavior genetics of alcohol use and abuse. Include in your account:

a. a definition of alcoholism Overuse of alcohol resulting in 3 or more DSMIV criteria met in a 12 month period, eg arrest record, failure to fulfill major obligations, continued use despite health risks etc.

physiological addiction indicated by tolerance and withdrawal symptoms

b. What have animal studies contributed to our knowledge on the genetic influences on the effects of alcohol?

All aspects of alcohol that have been studied have shown genetic influences – genes influence both use and effects of alcohol

- successful selection for response to alcohol in mice (long and short sleep mice)

- strain differences for preference to alcohol ( non-preferrers and preferrers)

- strain differences for withdrawal symptom response, tolerance and so on

c. What have twin and family studies contributed?

For alcoholism (dependence and abuse)

family studies: risk for alcoholism in proband families is 2-3 times prevalence

risk remains on adoption indicating genes not shared e

twin studies indicate heritabilities of around 50-60% for both males and females for community-based studies involving adult twins. Younger cohorts give lower heritabilities and more shared e as influences.

Early-onset alcoholism and Type 2 alcoholism both show higher heritabilities than late-onset and Type!

For abstinence, initiation of use and early use

genetic influences are less important than environmental, with shared e being at least as important as non-shared. Especially influential is the e shared with same sex sibs that are close in age

any genetic influences seem to act through genetic tendency towards externalizing disorders – as a general manifestation of behavioral disinhibition

d. Are there differences between men and women? Give evidence for your answer.

Yes male prevalence rate is much higher, males tend to show higher genetic influences and higher rates of early-onset and alcohol-related aggressive (Type II) alcoholism

females show lower prevalence but greater risks of health effects, lower heritabilities in some studies and more shared environmental influences, high rate of assortative mating towards heavy-drinking partners resulting in partner abuse

e. Are there differences between early and late onset alcoholism?

Yes early onset shows higher heritabilities (70%) and is more common in males

late onset shows lower heritabilities (30%) more shared and non-shared

f. What progress is being made towards identifying individual genes that might contribute to the risk for alcoholism?

good progress several genetic loci seem to influence risk for alcoholism

ALDH gene strong association, one allele is protective

ADH, COMT, DRD2, DRD4 genes some studies on each of these genes show association with alcoholism, some alleles may confer risk some may be protective some studies do not replicate