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Biopsych RM - Behavioral Genetics (Y2) - Coggle Diagram
Biopsych RM - Behavioral Genetics (Y2)
What is behavioural genetics?
Partioning variance - Behaviour genetics aim to explain individual differences
What causes human variation -
-> Sexual reproduction - one copy from father and one from mother
-> Each individual has 2 copies of each chromosome and 2 copies of each gene, which may be similar or different
Methods - gene studies, family studies, twin studies, adoption studies and comparative psychology
Laws of heredity
The law of segregation -
Observed traits are phenotypes - the allele activated
Allele forms are genotypes
Dominant and recessive genes - heterozygous and homozygous
Huntington's disease - comes from a dominant allele, which you only need one of for the trait to be activated - personality changes, forgetfulness - issue in the indirect route (thalamus inhibition) of nigrostriatal pathway - involuntary movements are not suppressed
Phenylketonuria - recessive disease, requires two copies of allele to be activated as a trait - intellectual disability and caused by disturbance in metabolism of phenylalanine
Inheritance of one gene is independent of the inheritance of another
Why is it important?
Inform further research designs to identify specific causes of physical and psychological traits and disorders
Improve public understanding and reduce stigma concerning physical and psychological traits and disorders
Direct efforts to prevent or reduce risk of disorders
Genetic counselling services
Enhance understanding of human brain development and behaviour
Understand why two or more traits or conditions often co-occur
Quantitative genetics
Theory and set of methods (e.g. twin and adoption designs) for studying inheritance of complex, quantitative traits) - R.A. Fisher
Polygenic traits -
Multiple genes that contribute to a disorder - 1 gene has 3 genotypes and 3 phenotypes for example, 2 genes have 9 genotypes and 5 phenotypes
Having 4 allele sets can produce a range of combinations
Central limit theorem -
Normal distribution is to be expected whenever variation is produced by the addition of a large number of effects, non-predominant
This plausibly holds quite often
Galton - questioned whether heredity affects human behaviour
Proposed twin, family and adoption study designs
Student eminence (high intelligence) in families
Complex traits -
Schizophrenia - risk increases with genetic relatedness
Cognitive ability - resemblance for cognitive ability increases with genetic relatedness
Genes and environment - With behaviour there needs to be both an organism and an environment - interaction between the two
Research designs
Adoption studies, family studies, twin studies
Sources of variance - additive genetics (A), interaction between alleles of the same dominance or different locus (epistasis), common environmental influences, non-shared environmental influences unique to the individual and measurement error
Environmental causes - Any influences other than the actual DNA code that contribute to individual differences on a particular phenotype
Shared environment - influences that make children growing up in the same family similar - E.g. socioeconomic status, parenting, characteristics of home - distal and proximal types
Non-shared environment - Influences that make children growing up in the same family different, Includes variance attributable to measurement error
Family design
Family studies involve studying the resemblance between relatives from shared genes, common environments and differences between relatives caused by nonshared genes and unique environments
Sibling recurrence risk ratio λS
-> Estimate of familial aggregation of disease
-> Ratio of disease manifestation, given that if one’s sibling is affected, compared with disease prevalence in rest of population
-> λ = risk of disease in the relatives of an individual with disease / population prevalence of disease
Advantages -
Much more accessible than twins or adoptees
Representative of general population
Useful for very rare conditions
Allows investigation of many relationships including parent-offspring and older-younger siblings, cousins, etc
Disadvantages -
Cannot disentangle genetic influences from family environmental influences
Need a carefully matched control group
Family members vary in age
Paternity not always established
Adoption studies
Adoption studies - look at genetic and environmental relatives to determine if phenotypes are from shared environments or genetics, or if it is an interaction of the two
Loehlin (1989) - family resemblance for cognitive ability is both genetic and environmental
Mednick et al, 1984 - criminality is increased if both the adoptive and biological parents are convicted of crimes
Advantages -
Powerful design - directly splits up nature and nurture
Adoption common in 1960s - as many as 1% of children were adopted
Employs singletons, therefore does not have issues specific to twins
Disadvantages -
Sample unlikely to be representative of population
Adoption less common now than it used to be
Children are often matched to their adoptive parents - selective placement - common environment because parents have to match standards
Age at time of adoption varies
Does not control for effects of the prenatal environment
Twin design
Classic twin design -
Involves monozygotic and dizygotic twins - based on comparison between the two - 100% v 50% of genetic code; two sperm for two eggs v one sperm for two eggs
Twin pairs reared together
Demonstrates relative role of genes, shared environment and non-shared environment
Assumptions -
Findings from twins are generalisable to singletons
Assume no gene-environment correlation or gene-environment interaction
No effect of chorion or amnion differences - the prenatal positions of the children and range from twins who share amnions and chorions, placentas and wombs, too twins who have separate wombs and placentas
Equal environments assumption - assumption does tend to hold however
-> Posits that environmental influences are equivalent for MZ and DZ twin pairs, and that any excess similarity is therefore due to genetic factors - not always case, as DZ twins are sometimes treated differently because they are different - different interests for example or different personalities leads to different treatment, or over emphasis on difference from parents
-> Evidence - mistaken zygosity, effect of physical similarity and effect of environmental similarity
Identical twin design - Focuses on monozygotic - identical - twins
Twin pairs reared apart design demonstrates influence of genes
Differences within identical twins reared together demonstrates influence of non-shared environment
Shared environment - what else makes them similar
Non-shared - what is causing the difference
Additive principle - genetics, shared and non-shared environment
Advantages -
Can partition variance
Reasonably representative of general population
Willing participants
Twin design assumptions mostly supported by data
Don’t need a ‘control’ group
All participants are the same age
Disadvantages -
Require large samples to have the power to detect effect
Design has assumptions - but so do all study designs
Some twin-specific characteristics are not generalisable to singletons
Twin's Early Development Study (TEDS) twins: Falconer’s rules -
Rmz - Similarity between monozygotic twins
Rdz - similarity between dizygotic twins
Heritability is the proportion of variance of a phenotype explained by genetic influences in a population of individuals
A2 = heritability
C2 = shared environment estimate
E2 = nonshared environment estimate
Correspondence predicted by a2 + c2 for MZ, and .5 a2 + c2
A2 = 2 (rmz - rdz)
C2 = rmz - a2
E2 = 1 - (a2 + c2)
Meta-analysis of twin studies (Polderman et al, 2015)
Distribution of twin studies between 1958 and 2012 - heavily America, Australia, China and Europe (Scandinavia and UK)
MZ and DZ twin correlation estimates across the 2748 twin studies published in this time shows strong results that MZ twins had more genetic similarity on traits than DZ twins
Meta-analysis of all twin studies on infant psychologically-relevant traits
-> Systematically identified 139 papers using the classical twin design in infants up to the age of 2 years
-> Findings on 377 psychological and developmental phenotypes measured in pooled sample of 79,044 twin pairs, 52 twin cohorts in 21 countries
-> Found that most traits intersected as being due to shared and nonshared environments, with language being more shared environment, and attention being nonshared environments - mixture of environment and heritability
Twin model fitting - Formal statistical approach to estimate genetic and environmental parameters
Mapping - univariate twin model
Zavos et al, 2014 - most traits in psychotic experiences show that traits are a mixture of being explained either due to nonshared environments or genetics - both can therefore impact traits
Twins reared apart design
Advantages -
Demonstrates effects of genes independent of a shared environment
No control group needed
Useful as case studies or anecdotal evidence to complement statistically stronger research
Identification of specific nonshared environmental influences
Disadvantages for scientific utility -
Relies on rare occasions of twin separation
Unethical to do purposefully
Small samples, typically individual case studies - limits analysis and generalisability
Identical twins reared together design - Vitaro et al, 2009
Advantages -
Excellent natural design for demonstrating effects of nonshared environment
Does not require a control group
Statistical analyses are possible if samples are large enough
Disadvantages -
Nonshared environment component includes a measurement error
Relies on generalisability of identical twins to singletons
Need large twin samples to identify even just a small number of discordant identical twins
Univariate and bivariate twin correlations
Bivariate traits -
Are the genetic influences on trait A shared with trait B?
Are there genetic influences unique to trait A?
Is the phenotypic correlation explained by genetic influences?
What is the genetic correlation?
The same questions apply to shared and nonshared environmental influences
How twin 1’s score on phenotype 1 correlates with twin 2’s score on phenotype 2
Genetic correlation -
Rg is a statistic derived from bivariate twin models - Independent of - heritability estimates of either trait and the strength of the phenotypic relationship between two traits
falls at 0.5
Re - non shared environment correlation - 0.5
Psychotic experiences as traits in the community - multidmiensional and quantiative
Common in population - Paranoia, negative symptoms, cognitive disorganisation, grandiosity, hallucinations
Psychotic experiences in depression - significant phenotypic correlations
Psychotic experiences in sleep disturbances - significant phenotypic correlations
What causes overlap between cognitive skills?
Path diagram in which multiple latent factors can contribute to a cognitive skill
Genetic correlations between diverse cognitive skills
Generalist genes - hypothesis (Plomin and Deary, 2015)