nature, nurture, and behaviour behavioural genetics

Nature, Nurture, and Behaviour

Behavioural Genetics

• Genetic contribution to behaviour

• But also environmental role

• Interaction

• Could just as well say BG studies the role of environment on behaviour as that of genetics…

Polygenic Traits

• Single gene effects (relatively) easy to study and understand

• Polygenic effects

• Complex behaviour traits

• Environmental interactions for both single and polygenic effects, but much more complex for the latter

Heredity

• Obviously important (and easily seen) in single gene effects

• More difficult to assess heredity’s role in complex behaviour traits

Synthesis

• Quantitative genetics– Estimates degree to which differences among

individuals due to genetic and environmental differences

– Doesn’t specify specific genes or environmental factors

• Molecular genetics– Identifies specific genes in behaviours

Humans

• Limited experimental designs

• Ethics

• Twin studies, adoption studies, naturally existing polymorphisms

Non-human Animal Studies

• Much more empirical control

• Can induce polymorphisms, transgenics, etc.

• Even more simply, directed selective breeding studies and inbred strain studies

Selected Breeding

• Variability in individuals’ behaviour due to genetic variance

• Variability can be exploited to show role of genetics on behaviour

• Pick specific trait, cross-breed individuals with that trait for multiple generations

• Can also study environmental effects this way– E.g., cross-fostering, different nutrition levels, training

protocols, etc.

Scott & Fuller (1965)

• 13 year study of hundreds of crossbreed and purebreed dogs– Beagle, Shetland sheep dog, African basenji, wire-

haired fox terrier, American cocker spaniel

• Behaviour patterns• Dogs perhaps the oldest domesticated species• High chromosomal similarity across breeds, but

high behavioural differences; easily picked out by selective breeding (Borgaonkar et al., 1968)

Critical Developmental Periods

• Heightened sensitivity to specific environmental stimuli

• Time critical

• Gene-environmental interaction

Puppy Toileting

• Born blind and deaf, but excellent sense of smell

• Mother needs to stimulate them to pass feces and urine– Mother consumes puppies’ waste; genetic

programming to keep nest clean and free of smells that would attract predators

– Activated by having litter; otherwise dogs not normally coprophagic

• After 4 weeks, pups mobile

• Nursing mother trains pups not to toilet near nest– Disciplining behaviour can appear harsh

• Pups separated from mother too early don’t learn proper toileting– Subsequently have difficulties learning to be

housebroken; critical period missed

First Fear Period

• Fear/hazard avoidance starts in pups at about 5 weeks, peaks between 8 to 10 weeks– Typically when people take puppy home

• Easier to condition fear responses at this time

• Fear of cars, fear of vet clinic, etc.

• (Rawlinson, 2005)

Farm-Fox Experiment

• Domestication– “Self-domestication” from

colonization of new ecological niches? (Morey 1994)

– Intentional decisions by human beings?

• Trut (1999): 40 year selective breeding study of 45,000 Russian silver foxes

<http://i.pbase.com/o4/29/336929/1/58820261.T1333SilverFoxFaceOn.jpg>

Patterns of Domestication

• Darwin recognized similar changes in appearance and behaviour across species

Appearance of dwarf and giant varieties AllPiebald coat colour AllWavy or curly hair Sheep, poodles, donkeys, horses,

pigs, goats, mice, guinea pigsRolled tails Dogs, pigsShortened tails, fewer vertebrae Dogs, cats, sheepFloppy ears Dogs, cats, pigs, horses, sheep,

goats, cattleChanges in reproductive cycle All except sheep

Selection

• Dmitry Belyaev• Believed behaviour, not size or

reproduction, was key factor in changes from domestication

• Specifically, selection for amenability (tamability)

• In 1959 started selected breeding of Vulpes vulpes for tamability

One Change to Many

• Balance of hormones and neurotransmitters at level of whole organism

• Slight changes to regulatory genes for some trait (e.g., amenability to domestication) can have wide-ranging effects on endocrine system

• Cascading effects on animal development, physiologically and behaviourally

Belyaev’s Hypothesis

• Two conditions required

• Variations in tamability must be partly determined by genes

• Domestication places animal under strong selective pressure

<http://sybilsden.com/images/imgfox/darla-2006/07-2006/08darla14.jpg>

Cross-Breeding Studies

• Cross-bred foxes of different behaviour

• Cross-fostered newborns

• Transplanted embryos between donor and host mothers with different tamability levels

• About 35% of variation in foxes’ defense response to people is genetically determined (i.e., heritability)

Selective Pressure?

• Attempted domestication of other wild-type animals

• River otters (Lutra lutra) and gray rats (Rattus norvegicus)

• Only 14-16% of wild-caught individuals showing weak defensive reactions produce offspring that survive to adulthood

• Domestication puts animals under extreme stress and selective pressure

The Experiment

• 30 male and 100 female foxes, mostly from commercial fur farms in Estonia– So, already tamer than wild relatives

• Selected for tameness alone– Only about 4-5% of male and 20% of female

offspring allowed to breed

• No training of foxes, minimal human contact– Ensures tameness due to genetic selection

Behavioural Classes• Class III

– Flee from experimenters or try to bite when handled

• Class II– Let themselves be handled but show no emotionally friendly

response

• Class I– Friendly to experimenters, wagging tails and whining

• Class IE– Domesticated “elite”

– Eager for human contact, whimper to attract attention, sniff & lick experimenters like dogs

– Appeared in sixth generation; by 10th gen. 18% of pups; by 20th gen., 35%; by 35th gen., 75%

Physical Changes• Domesticated fox pups respond to sounds two days

earlier and open eyes one day earlier than wild type• Fear response starts at 9 weeks in domesticated foxes

(instead of 6 weeks in wild type)– More time to become incorporated into human social

environment

• Appearance:– From 10th gen., changes in coat colour– From 10th - 15th gen., floppy ears seen– From 15th - 20th gen., tails and legs shortening– These are still unusual traits, but more common than in

wild-type

Traits

Days Weeks11 12 13 14 15 16 17 18 19 4 6 8 10 12

FarmedFoxes

DomesticatedFoxes

Dogs

Response to sound Eyes fully open Fear of unknown

Window of socialization

Ontogenetic Changes

• Changes in mechanisms regulating development

• Effects on normal timing of embryonic and post-embryonic development

Incidence of Change Animals per 100,000 with trait Increase

Characteristic Domesticated Nondomesticated in frequency Population Population (percent)

Depigmentation (Star) 12,400 710 +1,646

Brown mottling 450 86 +423

Gray hairs 500 100 +400

Floppy ears 230 170 +35

Short tail 140 2 +6,900

Tail curled up 9,400 830 +1,033

Inbreeding?

• Maybe changes due to recessive alleles– Would manifest from long-term inbreeding

• Study designed to prevent this– Outbreeding with commercial fox farm animals

– Inbreeding coefficients of 0.02-0.07

• Some of new traits are dominant, not recessive– Couldn’t have been hidden carriers in original

population

Star Gene• White patch on head• Affects migration rate of melanoblasts

(embryonic precursors of pigment cells)– Melanoblasts form in embryonic neural crest,

normally moving to embryo’s epidermis between 28-31 days after conception

• Foxes with single Star gene– Melanoblasts’ movement delayed two days– Leads to death of “tardy” melanoblasts– Alters pigment pattern, producing star pattern

Corticosteroids• Changes in plasma levels of corticosteroids,

hormones linked to adaptation to stress• Adrenal glands producing less adrenalin

– Hence, lowered stress response, and greater tamability

• Adrenalin also on biochemical pathway, linked to, among other things, melanin (pigment)– Changes in coat colour

• Different hormone levels have far-reaching effects• Cascade of changes to behaviour and physiology• Polygenic and environmental interaction• Video

Inbred Strain Studies

• Brothers and sisters mated for at least 20 generations

• Result is individuals that are virtual clones of all other members of strain

• Any differences therefore due to environmental effects

• Prenatal and postnatal environmental factors– Also, genetic factors, of course

Techniques

• Cross different strains to look at changes in behaviour across subsequent generations

• Diallel design– Compares several inbred strains and all possible F1

crosses

• Cross fostering– F1 crosses to mother from one strain or the other– Assess role of postnatal care

• Environmental manipulation– Alter specific environmental factors to examine genotype-

environmental interactions

Human Behaviour Studies

• Essentially “observational” in nature

• Limited to naturally occurring genetics and environmental variation

• Adoption designs

• Twin designs

• Combination designs

Adoption Designs

• Traits running in families can be due to nature and/or nurture

• Adoption studies can separate heredity and environmental factors

Parents and Offspring

• Genetically related family– Shared genes and environment

• Adoption produces “genetic” (i.e., birth) and “environmental” (i.e., adoptive) parents– Comparing genetic parents and offspring assesses

genetic contribution to parent-offspring similarity

– Comparing environmental parents and offspring assesses post-natal environmental contribution to parent-offspring similarity

Siblings

• Similarly, genetic and environmental siblings

• Again, comparisons made to assess genetic and post-natal environmental effects

Designs• Adoptees’ study method

– Examine incidence of trait in adopted away offspring of parents who possess the trait

• Adoptees’ family method– Compare adopted offspring who possess trait and lack the

trait of interest (control offspring)– Assess incidence of trait in the biological and adoptive

families– Genetic influence if trait greater for biological relatives

of affected adoptees than biological relatives of controls– Environmental influence if incidence greater for adoptive

relatives of affected adoptees than biological relatives of controls

Representative?

• Are adopted parents and adopted children representative of general population?

• Could be biases related to those parents who adopt and children who are adoptees

• Some studies suggest no bias with respect to population, others raise questions

Prenatal Environment

• Can be difficult to assess this• Sometimes simply hard to get access to

information about prenatal conditions of adopted children post-hoc

• Various environmental factors could be different between mothers (and fathers) who keep, as opposed to give up, their offspring

Selective Placement

• Placement of adoptive children may not be random– E.g., certainly isn’t based on appearance

• If preference for like characteristics between adoptive parents and offspring, this could inflate the correlation between biological parents and adopted-away children– Inflates effect of genes on behaviour

Selective Placement (Adjusted)

• Matches biological and adoptive parents• Genetic influences can increase correlations

b/t adoptive parents and their adopted children

• Environmental influence can increase correlations b/t biological parents and their adopted-away children

Twin Designs

• Monozygotic (MZ)– From single fertilized egg– Genotypically identical– Can be environmental differences, though

• Dizygotic (DZ) twins– From separately fertilized eggs– Just siblings born at the same time– Genotypically different; share 50% genes

• Allows examination of genetic and environmental variance of a behaviour (a phenotype)

Relations

• First degree relatives– 50% genetically related, r=0.5 (“coefficient of

relatedness”)– E.g., parents and children, full non-MZ siblings

• Second degree relatives– 25% genetically related, r=0.25– E.g., grandparents and grandchildren, half-siblings

• Third degree relatives– 12.5% genetically related, r=0.125– E.g., cousins

Equal Environment Assumption

• Common to twin design methods• Assumes environmentally caused similarity

about the same for both MZ and DZ twins• If MZ twins experienced more similar

environments than DZ twins, then assumption would be violated

• Studies show the assumption probably holds for most traits

Prenatal Environment

• Interestingly, MZ twins experience greater prenatal environmental differences than DZ twins

• Competition for resources hypothesis– MZ twins identical, so require exactly same

resources– Limited prenatal resources; competition

produces prenatal stress

Are Twins Representative?

• Twins not representative of population• Often born prematurely, uterine environment more

adverse– E.g., twins usually 30% lower in birth weight than

singletons, slower language development, poorer performance on early verbal and IQ tests

• However, most differences typically disappear by midchildhood– Shared and unique postnatal environmental effects

Combination Designs• Combine family, adoptive, and twin methods• Twin and family methods

– MZ twins have r=1.0– Their children (i.e., cousins) will be r=0.25, not typical

r=0.125 of cousins– Nephews of MZ male twins share same r-value (0.5)

with their father as their uncle, but there are different environmental factors in place

• Step-family analysis– Get half-siblings (r=0.25) and full siblings (r=0.5) in

same family– Different environmental effects for different children

Heritability

• Proportion of phenotypic variation in a population attributable to genetic variation among individuals

• Variation among individuals may be due to genetic and/or environmental factors

A Warning

• Heritability measures always refer to degree of variation between individuals in a population

• Can not be applied at level of individual• E.g., if heritability index of trait is 0.4 it is

incorrect to say that 40% of this trait is from the parents and 60% is from the environment– Consider, if all individuals were clones the heritability

index for all traits would be zero (all variability between individuals would have to be due to environment).

Statistical Significance

• Is similarity between genetically related individuals significantly greater than random?

• Depends on sample size

• Note that some significant effects really have very little “real” effect– I.e., account for little variance in a trait

Effect Size

• Extent to which individual differences for a trait in the population can be accounted for by the genetic difference among individuals

• Effect of heritable genetic disability can be huge for an individual, but have little effect on population– E.g., if trait only exists in 1 in 10,000 individuals, little

overall variation in trait for the entire population

– Many statistically significant environmental effects in behavioural sciences have small effects in population

Heritability

• The genetic effect size on individuals

• Often accounts for up to 50% of variance for behaviours

Relation Correlation Trait Heritability1st degree 0.5 100%1st degree 0.25 50%2nd degree 0.25 50%2nd degree 0.125 25%Unrelated 0.0 0%

Heritability Follow-Up• The genetic effect size on individuals• Often accounts for up to 50% of variance for

behaviours• Heritability estimated from correlations for

relatives• Consider 1st degree relative

– Correlations reflect half of the effect of genes because only share 50% genome

Relation Trait Heritability Correlation1st degree 100% 0.51st degree 50% 0.252nd degree 50% 0.252nd degree 25% 0.125Unrelated 0% 0.0

Origins• Heritability estimates go back to

Francis Galton (1889)– Data showing relationship between

offspring height as function of parental height

– Slope of line is 0.57, which approximates heritability of traits when offspring values regressed against average traits in parents

– If only one parent’s value is used, heritability is twice the slope

– (Note: this is the source of the term “regression” -- offspring values will always regress to the mean value of the population; the slope is always less than one) <upload.wikimedia.org/wikipedia/en/3/38/

Galton-height-regress.jpg>

Broad-Sense Heritability

• Reflects all possible genetic contributions to a population’s phenotypic variance (allelic variation, dominance variation, maternal and paternal effects)

• P = phenotype, G = genotype, E = environment so that P = G + E

• Considering variance, Var(P) = Var(G)+Var(E)• Broad-sense heritability, then, is H2 = Var(G)/Var(P)

Narrow-Sense Heritability

• Only the portion of the phenotypic variation that is allelic (“additive”) by nature

• So A = allelic contribution, so Var(A) is the allelic genetic variance such that h2 = Var(A)/Var(P)

Estimates• Only P can be observed directly• This makes estimating heritability difficult,

requiring sophisticated statistical methods to measure genetic and environmental variance

• Not so hard to do in non-human species– E.g., have same male mate with large number of

females, thereby producing large number of different, but related, offspring

– Ethically, can’t be done with humans

• Therefore, human heritability studies typically rely on adoption and twin studies

Correlation of Traits in Twins

• If MZ and DZ twin correlations are the same, then heritability is estimated as zero.

• If MZ twin correlations are 1.0 and DZ twin correlations are 0.5, then heritability is 100%.

• Basically, heritability should be about twice the difference between MZ and DZ twins.

Classic Twin Studies

• Start with assessing variance of a behaviour in a large group, and attempting to estimate how much is due to:

• Genetic effects (heritability)

• Shared environment

• Unique environment

ACE Model

• A (additive, or allelic, genetics)

• C (common environment)

• E (unique environment)

• (More complex models also examine non-additive genetics effects, D, for dominance)

MZ and ACE

• MZ twins raised in same family share 100% of their genes and all of the shared environment

• Differences due to their unique environment

• Correlation observed between MZ twins provides estimate of A+C

DZ and ACE

• DZ twins have the common shared environment, share 50% of their genes, and also have unique environment effects

• The correlation here is an estimate of 0.5A+C

Correlational Statistics

• rMZ = A+C and rDZ = 0.5A+C, then

• A = 2(rMZ – rDZ)

• C = rMZ – A

• E = 1 - rMZ

• where rMZ and rDZ are the correlations of the trait in MZ and DZ twins, respectively

Heritability Estimates

• Heritability for nine psychological traits as estimated from twin studies.

< upload.wikimedia.org/wikipedia/

en/5/54/Heritability-from-twin-correlations1.jpg>

All sources are twins raised together (sample sizes shown inside bars).

Concordances

• Disorders are diagnosed as either/or• Therefore, family resemblance is assessed by

concordances, not correlations• Concordance: an index of risk• Typically means presence of the same trait in both

members of a pair of twins• More correctly, the probability that a pair of

individuals will both have a certain characteristic, given that one of the pair has the characteristic

Concordances

• Twin concordances for seven psychological traits (sample sizes inside bars)

<upload.wikimedia.org/wikipedia/en/c/cf/Twin-concordances.jpg>

Concordances

• Because MZ twins are genetically identical, a genetic mutation carried by one will be carried by the other

• Thus, knowing the concordance rate for a characteristic should help establish the extent to which a trait is caused by a genetic mutation

Three Problems, Though

• A given mutation may not have 100% penetrance (i.e., may have different phenotypic consequences in genetically identical individuals)

• Unique environmental effects may be present in MZ individuals

• If the trait is polygenic it becomes even more difficult to evaluate

Penetrance

• Proportion of individuals carrying a particular variation in a gene (genotype) that also express a particular trait (phenotype)

Huntington’s Example

• Known mutations for disease of 95% penetrance– 5% with dominant allele for Huntington’s don’t

develop the disease and 95% do

• Trinucleotide repeat of CAG (coding for amino acid glutamine) forms a polyglutamine (polyQ) chain

• 36-39 repeats show reduced penetrance and/or later onset

Age Related Penetrance

• Example: multiple endocrine neoplasia 1 (MEN 1)– Parathyroid hyperplasia and pancreatic islet-

cell and pituitary adenomas

• Due to mutation in the menin gene on human chromosome 11q13

• Bessett et al. (1998) found age-related penetrance of MEN 1 to be 7% at age 10, but nearly 100% by age 60

Caveats

• Heritability describes what is in a particular population at a particular time

• Not what could be (or should be)

• Genetic and/or environmental changes will also alter heritability estimates

Within and Between Groups

• Causes of individual differences in groups have no implications for causes of average differences between groups.

• Heritability refers to the genetic contribution to differences among individuals within a group– Specific to time and place

• High heritability within a group doesn’t necessarily mean that average differences between groups are due to genetic differences– Environmental factors could be very significant