Behavioural Genetics

Behavioural Genetics Notes by Dr. Alyson E Blanchard

Introduction to Behavioural Genetics

Plomin et al. (2013) state that genes significantly contribute to individual differences in personality, while shared environment does not. Environmental influence on personality is almost entirely of the non-shared variety.

Evolution and Genetics

What is Evolution?

Evolution is driven by several key factors:

• Variation: Individuals within a population are similar but not identical.
• Heredity: Characteristics are passed from parents to offspring.
• Competition: Not all individuals survive to reproduce.
• Natural Selection: Characteristics that provide a survival and mating advantage are more likely to be passed on, becoming more common in future generations.

Genes and DNA

Genes

Genes are the set of instructions for running the body.

• Genes reside on strands of DNA.
• RNA enters the cell nucleus to transcribe instructions from the gene.
• The RNA then returns to the cell, gathering amino acids to form proteins.
• Proteins are the building blocks of life.

Central Dogma of Genetics

• Genotypes are transmitted across generations.
  • This explains why people can be carriers for a gene.
• Phenotype is the biological, physical, behavioural, and psychological trait caused by an individual’s genotype (entire set of genes).
• Changes to the phenotype during an individual’s lifetime are not passed on to offspring.
• Most cells in the body maintain phenotype (e.g., liver cells).
• Gametes (eggs in females and sperm in males) are concerned with making more genotypes.

Variation Between Individuals

Sexual Reproduction

• Humans are diploid organisms with two sets of 23 chromosomes (one from each parent), totaling 46.
• Gametes (sperm and egg) contain 23 chromosomes each.
• Meiosis is the random split of chromosomes during gamete formation.
• There are approximately 8.3 million possible gene configurations in terms of gene configurations. This means that you are one in 8.3 million!

Chromosomes

Chromosomes are tightly wound lengths of DNA. Key concepts include the homologous pair, paternal copy (from father), and maternal copy (from mother).

How Variation Emerges (1)

Recombination

• DNA is exchanged between paired chromosomes before meiosis.
• This occurs every time a gamete is formed, increasing variation.

Linkage

• Genes located close together on DNA strands tend to be inherited together.
• Sexual reproduction reshuffles genes, leading to considerable variation in everyone’s genome (blueprint/unique set of instructions).

How Variation Emerges (2)

Mutation

• Segmental Duplications
  • Chunks of DNA (including multiple genes) make extra copies during replication (also segment deletions).
  • Extra genes are subject to mutations, leading to new functions without disrupting the original gene's function.
  • Gene families are multiple genes descended from a common ancestor through duplication events.
• If a mutation leads to a survival or mating advantage, natural selection retains that trait.
• Most mutations have no phenotypic effect or are detrimental.

Alleles

• Different versions of the same gene can be dominant or recessive.
• Single-gene characteristics: One allele specifically causes one phenotype (e.g., cystic fibrosis, red hair).
• Polygenic characteristics: Multiple alleles contribute to a trait.
• The idea of a single "gene for something" is often incorrect; typically, one allele has been discovered to contribute to a particular phenotype.

Genes and Behaviour

• The link between genes and behaviour involves changes in neuronal or hormonal mechanisms.
• Behaviour is considered a phenotype.
• Monoamine oxidase A (MAOA) enzyme, regulated by the MAOA gene on the X chromosome, helps regulate serotonin.
• Brunner et al. (1993) studied a Dutch family with severe conduct disorder (violence, rape, arson) in male relatives. They found that these men had an absence of MAOA activity due to a gene sequencing error.

Quantitative Genetics

• Estimates the extent to which observed differences among individuals are due to genetic differences and environmental differences.
• Produces a "heritability estimate."

Heritability

• The proportion of phenotypic variance accounted for by genetic differences in individuals.
• It does not refer to the genetic contribution to a trait in one individual.
• Estimated from correlations between blood relatives (e.g., twins, parent-child).
• For example, correlations between psychopathy scores in twins or between parent and child are used.
• A zero correlation would mean zero heritability (no commonality).

How to Estimate Heritability (1)

Adoption Designs

• Genetic and environmental causation of behaviour is estimated by looking at behavioural similarities and differences between genetically related individuals growing up in different environments (shared and non-shared environments).
• Early research suggested genetics contributes around 50% to personality traits, challenging Environmentalism.
• Schizophrenia (Heston, 1966): Children of schizophrenic parents are equally likely to develop schizophrenia whether raised by birth or adoptive parents.

How to Estimate Heritability (2)

Twin Designs

• Comparison of monozygotic (genetically identical) and dizygotic twins (same relatedness as siblings).
• Monozygotic twins should be phenotypically more similar than dizygotic twins.

Interpreting Heritability

• Both genetic and environmental influences contribute to educational attainment.
• In a uniform environment, differences in educational attainment would be fully accounted for by genes (highly heritable).
• In contrast, differences in education quality and upbringing reduce heritability due to greater environmental contributions.
• Heritability describes what is in a population at a particular time and is indicative (an estimate).
• There is no “genetic determinism.”

Shared Environmental Influences

• Non-genetic influences that make family members similar to each other (e.g., parenting, schooling, neighbourhood).
• Appear to contribute little to personality traits and cognitive abilities.
• Family resemblance not explained by genetics.
• Resemblance amongst adoptive relatives.

Non-Shared Environmental Influence

• Non-genetic influences that are independent (un-correlated) for family members.
• Individual experiences growing up that are unique (e.g., differential parental treatment, family life differences, experiences outside the home).
• Anything left after removing genetic and shared-environmental influence.
• Twin studies examples: birth weight, parental negativity influencing behaviour problems and academic achievement, but also mediated by genes.

Genotype-Environment Correlations

\text{Genes} \rightarrow \text{Environment} \rightarrow \text{Phenotype}
\text{Phenotype = personality trait or behaviour}
\text{Genes x environment}

How Genetics Shape Environment

• Shared positive affect & responsiveness in 3-year-olds and their parents (Deater-Deckard & O’Connor, 2000).
• Family relations such as marital quality and parenting (Ganiban et al., 2009).
• Relationships & control of finances (multiple studies).
• Friends and peer groups (multiple studies).
• Perception of classroom environment (Walker & Plomin, 2006).
• Television (Plomin et al., 1990).
• Work environments (Hershberger et al., 1994).
• Social support (multiple studies).
• Exposure to drugs (Tsuang et al. 1992).

Implications of Genetic Influence on Environment

• Behaviors, like watching TV, are outcomes of genes associated with more fundamental factors.
• Examples:
  • Friends: High sensation-seeking individuals seek out similar friends.
  • TV: Sensitivity to stimulation and/or reward.
  • Relationships: Less empathy leads to more punitive behaviour in relationships and parenting.

Types of Genotype x Environment Correlation (Plomin et al., 1977)

• Passive: Children receive genotypes correlated with their family environment.
• Evocative: Individuals are reacted to based on their genetic propensities.
• Active: Individuals seek or create environments correlated with their genetic proclivities.
• Demonstrates that environmental influences can be over-estimated due to the role of genetics in exposure to the environment.

Gene x Environment Interaction

• Genetic sensitivity or susceptibility to environments.
• The effect of environment on a phenotype depends on the genotype, or vice versa (moderation).

\text{Phenotype } \leftrightarrow \text{ Genotype or environment } \leftrightarrow \text{ Environment or genotype}

Examples of Gene x Environment Interaction

• (Bohman, 1996; Brennan et al., 1996): Adoptees with criminal birth parents are at risk for criminal behaviour (genetic). Adoptees with criminal adoptive parents are also at risk (environmental). Criminal convictions of adoptive parents led to increased criminal convictions of their adopted children more so when the adoptee’s birth parents also had criminal convictions.
• (Leve et al., 2010): Adopted children whose birth parents had more psychopathology symptoms benefit from adopted mother’s structured interactions at 18 months old. Children whose adopted parents who had psychopathology symptoms were at risk for developing behavioural problems, but only when their birth parents were likewise high in psychopathology.
• Influence of adoptive marital hostility is moderated by birth parents’ temperament (Rhoades et al., 2011).

Further Examples of Gene x Environment Interaction

• Effect of stressful life events on depression is worse for individuals at genetic risk for depression (Kendler et al., 1995).
• Effect of physical maltreatment on conduct problems is greater for children at genetic risk for conduct problems (Jaffee et al., 2005).
• Socioeconomic status does not moderate the heritability of children’s intelligence (Hanscombe et al., 2012).
• High heritability of adolescent antisocial behaviour in economically advantaged families (Tuvblad et al., 2006).
• High heritability for adolescent antisocial behaviour when parenting is negative (Feinberg et al., 2007).

Gene x Environment Interaction and Cognitive Abilities

• (Plomin et al., 1977): Comparison of cognitive ability in children and birth parents (high or low education genotype) and children and adopted parents (high or low education environment).
  • Findings: Significant effect of birth parent education level on adopted child’s general cognitive ability.
  • No environmental effect was found for adoptive parent’s education on child’s general cognitive ability.
  • No genotype x interaction was found either.

DNA Studies of Gene x Environment Interactions

• MAOA gene (Caspi et al., 2002):
  • No difference in behavioural outcome for children with either low or high MAOA genotype in absence of child maltreatment.
  • Children with the low MAOA genotype developed antisocial behaviour when exposed to child maltreatment.
• 5-HTTLPR serotonin transporter gene (Caspi et al., 2003):
  • No difference in the relationship between the gene and depressive symptoms without stressful life events.
  • Presence of depression was significantly higher in the presence of the gene and increasing stressful life events.

Schizophrenia

• Characterised by delusions, hallucinations, disorganised speech and behaviour, low emotionality, & lack of motivation.
  • General population risk: 1%
  • Second-degree relatives: 4%
  • First-degree relatives: 9% (offspring: 13%)
  • Identical twin: 46%
• Shows that almost 50% is attributable to other influences
• Birth complications
• Neurological abnormalities (Mosher et al., 1971; Torrey et al., 1994)

Schizophrenia & Adoption Studies

• Risk of schizophrenia in adopted children whose birth mother is schizophrenic is 11% compared to adoptees whose birth parents do not have schizophrenia (0-1%) (Heston, 1966).
• 10% of adoptees with schizophrenic parents report some form of psychosis (rate of 1% in adoptees without schizophrenic parents) (Tienari et al., 2004).
• Schizophrenic-related symptoms were worse for adoptees whose birth parent(s) were schizophrenic and grew up in poorly functioning adoptive families.
• Schizophrenia is not influenced by the home environment fostered by schizophrenic parents (Kety et al., 1994).
• Severe forms are more heritable (Gottesman, 1991).
• Disorganised type (Cardno et al., 1999; Farmer at al., 1987).

Major Depressive Disorder (MDD) & Bipolar Disorder (BD)

Family Studies

• Increased risk for first-degree relatives: 9%
• MTHFR gene variant implicated.
• More severe and recurrent MDD potentially more heritable (Janzing et al., 2009; Milne et al., 2009; Sullivan et al., 2000).
• Early onset, drug use & suicide attempts, response to medication potentially more heritable in BD (Smoller & Finn, 2003; Schulze et al. 2006; grof et al., 2002).

Twin Studies

• MDD: 38% (no shared environmental influence; Kendler et al., 2006), maybe as much as 70% (McGuffin et al., 1996). Concordance rates for MZ /DZ twins – 43%/28%.
• BD: Concordance rates for MZ /DZ twins – 65/7%; 40/5%; 55/7%.
• Adoption studies and candidate genes – mixed findings.

Autism/ASDs

• Environmental factors were originally thought to be the cause (even vaccinations).
• Twin studies consistently show high concordance rates for MZ twins far beyond DZ twins (at least 60%).
• Now considered one of the most heritable conditions (Freitag, 2007; Ronald & Hoekstra, 2011).
• Genetic influence is the same irrespective of severity.
• The three impairments of autism: poor social interaction, language & communication problems, and restricted range of interests and activities are influenced by different genes.
• 100 genes are associated (but not consistently demonstrated).
• Some cases are due to rare mutations (only 10%), but molecular genetic research supports varieties of ASDs.

ADHD

Twin studies:

• First-degree relatives 5 times more likely to be diagnosed (Biederman et al., 1992).
• Research consistently demonstrated strong genetic influence for the hyperactivity component, perhaps up to 76% heritability.
• Heritability greater for ADHD which continues into adulthood.
• Substantial genetic overlap for the inattention and hyperactivity components of ADHD.
• Genes – various dopamine transporter genes implicated (DAT1, DRD4, & DRD5).
• 30 other candidate genes – but more research is needed, results remain inconclusive.

Co-occurrence of Disorders

• High levels of comorbidity.
• Genetic overlaps between MDD and BD, and BD and schizophrenia.
• Genetic overlap among anxiety disorders – differences caused by non-shared environmental factors.
• Anxiety and depression are caused by the same genetic factors (Kendler et al., 1992) – non-shared environmental influences appear to be the differentiating factor (Kendler 1996; Mddeldorp et al., 2005).

Two Over-Arching Categories of Disorders:

• Internalising:
  • Anxious/misery: Major depression/GAD/Panic disorder
  • Fear: Panic disorder/animal phobia/situational phobia
• Externalising: Alcohol dependence/other drug abuse or dependence/antisocial behaviour/CD
• Non-shared environmental influence causes heterogeneity.

Personality

• Loehlin & Nichols (1976):
  • Nearly all personality traits show moderate heritability (30 – 50%).
  • Environmental influence is not to do with parents but non-shared environment actually influences personality more.
  • A lot of other studies have supported these findings and from using self-report measures.

Personality - Twin Studies

Loehlin (1992)

Other FFM Traits

• Openness to experience – 45%
• Conscientiousness – 38%
• Agreeableness – 35%
• (Loehlin, 1992; Jang et al., 1996)
• Different results when looking at the sub-factors – unique genetic variance:
• Sensation seeking (60%) (Fulker et al., 1980; Koopmans et al., 1995)

Genes:

• Openness to experience regions identified on chromosome 5
• Conscientiousness regions identified on chromosome 18
• But – much more research in this area is needed – specific genes/set of genes yet to be identified

Other Contexts

Situational Change:

• In a laboratory task, identical twins changed similarly in adapting to different tasks than non-identical twins (Mateny & Dolan, 1975).
• Genetic influence in the stability of shyness in different situations (Cherny et al., 1994).
• Personality trait changes across different situations (Dworkin, 1979).
• Genetic influence in how people respond to personality questionnaires (Eaves & Eysenck, 1976; Hershberger et al., 1995).

Development:

• Genetics influence the stability of personality traits from childhood into adulthood.

Other Contexts - Relationships

• Parent-offspring:
  • Attachment – moderate heritability, but substantial shared environmental influence (i.e., parenting).
• Romantic:
  • Quality of relationship (15 – 35%).
  • About 50% of relationship quality is affected by personality – so indirectly by genetics.
• Sexual orientation:
  • 34-39% for men (+ non-shared environments).
  • 20% for women (+ modest shared environmental influence) (Langstrom et al., 2010).
• Self-esteem:
  • Modest genetic influence, but no influence for shared family environment.

Other Contexts - Attitudes & Interests

• Traditionalism, sexual & religious attitudes – 50% genetic, shared environment – 15% (Eaves et al., 1989; Olson et al., 2001) (lower for taxes, the military and politics)

Personality Disorders

• Different from psychopathology as people affected by the disorder consider it as part of their personality, rather than a condition that they are affected by
• Disorders at the quantitative extreme end of the normal genetic and environmental factors that contribute to personality

Schizotypal PD

• Less intense version of schizophrenia
• Runs in families
• 33% concordance rate for MZ twins, and 4% for DZ twins (Torgersen et al., 2000); other studies suggest 20 – 80% heritability
• Risks of schizotypal personality disorder are 11% for first-degree relatives
• Twin study suggests that the negative and positive aspects of schizotypy differ genetically (Linney et al., 2003)
• Schizotypy is genetically related to schizophrenia (Jang et al., 2005)

ASPD

• Risk for ASPD is increased by five times for first-degree relatives of ASPD males; for females, this goes up to ten-fold

Antisocial behaviour:

• Genetics – 40%; shared environmental influence – 15%; non-shared – 16% (Rhee & Waldman, 2002)
• Meta-analysis – 50-60% (Burt, 2009)
• Non-shared environment contributes to change as individual gets older (Burt et al., 2010)
• Increased rate of ASPD in the adopted children of birth parents with criminal records (Cadoret & Stewart, 1991; Crowe, 1974)

Criminal Behaviour

• Av. concordance rates of 52% in MZ twins; 21% in DZ twins
• Adopted children at greater risk for criminal behaviour when birth parents have criminal convictions (Mednick et al., 1984)
• Appears to be for property crimes and not violent crimes
• G x E interaction – adopted children were at greater risk if both their adopted and birth parents engage in criminal activity (Beaver, 2011)
• Twin studies might over-emphasise genetic effects – partners in crime (genetic effects seen less in adoption studies)

Genetic influence contributes to both ASPD and substance use disorders:

• Adoptees (male & female) at increased biological risk for ASPD – increased aggressiveness, CP, ASPD and substance dependence (Cadoret et al., 1995; Cadoret et al., 1996)

Conclusion

• Genetics plays a major role in how we think and behave.
• Adoption and twin studies can demonstrate the extent to which genetics has this influence (although there is some current debate about this).
• Environmental influence can be misattributed because the genetic causation is hidden.
• Provides an effective explanation for how we can be nothing like our siblings and parents at all, or how we can be very similar!
• We don’t have to resort to claims of genetic determinism – it’s not nature vs nurture, but nature AND nurture.