What is behavioural genetics?
- The interdisciplinary scientific effort to establish causal links between genes, behavioural traits and neural mechanisms.
- Aims to distinguish the genetic and environmental contribution to behaviour, and the interactions between the two.
- Just as much focused on clarifying nature of environmental contribution.
- Originated by Francis Galton, the 'parent' of behavioural genetics
- Identified a normal distribution across a population for intelligence.
- Developed the statistical concepts of regression and correlation.
- Designed early twin studies.
- But championed eugenics - the active promotion of selective mating to promote what are judged to be desired characteristics.
- Behavioural genetics is not eugenics.
What does 'behaviours' refer to?
- Cognitive abilities and disabilities.
- Psychopathology - symptoms, abnormal behaviours, mental disorders.
What is a genotype?
- An individual's combination of alleles at a particular locus.
What is Heritability?
Proportion of variance in behaviour (i.e. you either have a disorder or you haven't) or it can be variance across a continuous variable (like IQ) that can be accounted for by genetic differences amongst individuals.
- If you say that there is .5 or 50% heritability, that means the remaining 50% is environmental.
What is polygenic inheritance?
A large number of genes contribute cumulitively to small effects towards significant heritability of a behavioural phenotype, with a normal distribution.
What is the signiciance of the large scale twin and adoption studies?
- Shift from gene to behaviour.
- These studies found a largely polygenic inheritance of behaviour
- Refined estimates of heritability and environmental constirbution
- Genome wide association studies linking smaller units than genes (SNPs) and copy number variants (CNVs) to behavioural phenotypes.
What does this diagram show?
- This shows how you can go from a single gene inheritance (top bit shows a single gene affecting phenotype)
- Move on to a disorder where you have 2 genes contributing to that disorder or phenotype.
- Move onto 3 genes and you can start to see that there is a curve.
- And going on beyond that, there is a normal distrubutary curve.
- It could be 5, 50, 500 genes.
Expain the significance of shared and non-shared environments.
- Shared environment - the non-genetic factors experienced by all siblings or both twins.
- Non-shared environment - the non-genetic factors which do not correlate between siblings or twins, including any error of measurement in the behavioural trait concerned (which can lead to overestimates of non shared environmental contribution)
- Accounts for all differences between identifcal twins brought up in the same family household.
- For most behavioural traits, the impact of non-shared environmet is greater than the shared environment.
- In large scale genetic studies, particularly twin studies, the scale of genetic error can't be shared with non-shared environment.
- Maybe we need to accept that proportion of non-shared environment traits are exaggerated.
- However, always, it seems that the impact of non-shared environment is bigger than shared environment.
What research methods are used in quantitative genetics?
- Adoption studies - compared those reared together and apart
- Twin studies - comparing identical and fraternal, in long-term cohorts.
- Combination studies e.g. twins adopted apart.
What research methods are used in molecular genetics?
- Candidate gene studies
- Genome-wide association studies-using single nucleotide polymorphisms and copy number variants in large samples.
- Multivariate genetic analysis to examine genetic mediation of behavioural associations within individual twin pairs.
Describe the nature and limitations of adoption studies.
- Direct comparison of a behavioural phenotype in genetic relatives can evidence the genetic contribution, and comparison in environmental relatives can evidence the environmental contribution.
- First adoption study of schizophrenia - Heston 1966.
- Compared incidence in adopted offspring of women with schizophrenia with that of matches adoptees whose parents had no known mental illness: 10% incidence vs zero.
- Adoptee study vs adoptee family methods.
- Comparing parent and offspring, or siblings.
Limitations of Adoption Studies
- Gradual decline in adoption in high-income countries.
- Adoptive parents and adoptees may be substantially different from normal population.
- Prenatal environment may make a contribution before adoption.
- Open adoption.
- Selective placement.
What are twin studies?
- The comparison of behavioural trait concordance in monozygotic twins vs dizygotic twins - 1.0 or 100% concordance means a wholly determined genetic variation.
- 'Equal environments' - assumption holds reasonably well.
- Results expressed as concordances for dichotomous outcomes such as mental disorders, and as correlations for continous variables like cognitive ability.
What is the significance of the Twins Early Development Study?
- Large population-based twin cohort study examining language, cognition and behaviour.
- Whole population birth cohort over 2 year period in the UK.
- 1994 to present day.
- 20700 twins now in 32 countries.
- 1/3 identical.
- Initial focus on early child development, now on emerging adulthood.
- Allows comparison between twins and their families for range of behavioural traits.
- DNA analysis now complementing the quantitative genetic approach.
Give an example of what TEDS has identified
Genetic and environmental influences on food preferences in adolescence
- Used TEDS cohort to examine the genetic and environmental contribution to food preferences.
- TEDS twins invited to self-report on food preferences.
- Advantage of large size - n=2865
- Food preferences have a moderate heritability in adolescence.
- But shared environments (family, school) have no effect.
- All the non-genetic contribution is non-shared environment - the unique environment as experienced by that individual has a substantial effect.
What are genome wise association studies?
- Large scale molecular genetic studies of unrelated individuals, including comparisons of cases vs controls.
- Single nucleotide polymorphisms (SNPs) and copy number variants 10 million SNPs in our DNA, most with no known effect on function, but some are biomarkers for genes and with known functions.
- DNA microarrays can examins 100s or 1000s of SNP's in a single study and compare their frequencies in those with and without a phenotype, or their correlation with a continuous behavioural variable.
- Large sample sizes, small individual effects.
- Missing 'heritability' problem
- Increasing international collaboration between cohorts achieved enormous samples.
- For example, Holmans 2017 - Meta-analysis of GWAS blah blah.
What can DNA methylation do?
DNA methylation can silence a particular gene and hence change its contribution to behavioural traits through the life course or in response to environment.
- e.g. Increase SERT DNA methylation is associated with bullying victimisation and blunted cortisol response to stress in childhood.
- MicroRNA sequences in introns regulate gene expression by degrading or silencing mRNA
- e.g. microRNA markers in autism spectrum disorders.