19 Behavioural Genetics

Question 1

What is the measurable expression of a trait?

Answer 1

The phenotype

Question 2

How is phenotypic variance calculated?

Answer 2

Phenotypic variance = genetic variance + environmental variance + gene-environment interaction
Vp = Vg + Ve + Vge

Genetic variance =

Additive components/genotypes/sum of alleles +
Dominance components/dominant gene action +
Interaction between genes/epistasis

Vg = Va + Vd + Vi

Environmental variance = common environment + unique environment

Ve = Vc + Ve

Gene-environment interaction is non-additive.

Vp = Va + Vd + Vi + Ve [Vc+Ve] + Vge

Question 3

What is the difference between fraternal twins and identical twins?

Answer 3

Fraternal twins - different sperm and different egg, separate placentas

Identical twins - same sperm and same egg, shared placenta.

Monozygotic twins are genetically the same.

Question 4

How can heritability estimates be calculated from twin studies?

Answer 4

By measuring the difference in correlations between siblings on some phenotype:
rMZt (monozygotic, raised together), rDZt,
rMZa, rAS (adoptive siblings)

P = A+C+E= 1 (Vp =Va+Vc+Ve= 1)
rMZt = A+C = 1 - E
rDZt = 0.5A + C
rMZa = A
rAS = C

Heritability (or genetic variance) can be calculated by finding the correlation between rMZa and rMZt:
rMZa/rMZt = A / (A+C)

Question 5

What is the heritability quotient of a trait and how is it calculated?

Answer 5

Heritability is the component of variance of a phenotype that can be explained by heritable factors:
h^2 = Vg / Vp

Question 6

What is a quantitative genetic analysis of behaviour?

Answer 6

Statistical approach by analysing the relationship between genes and behavioural phenotypes by measuring changes and correlations in genetic code. This is done through mapping trait loci.

1. Identify sections or ‘markers’ of the chromosome that are linked to the trait. ‘Markers’ within genome vary from individual to individual (i.e. variation in genetic code). Calculate difference in these markers within genome.
2. Measure certain phenotypes (height, speed, etc.) or psychological factors
3. Calculate correlations and changes in expression of trait. Do variations in markers in genome (polymorphism) match variations in phenotype?
   Can infer statistically the number of genes responsible and how they interact.
4. Check by examining phenotypes between generations, i.e. in sets of progeny (offspring).

Question 7

Describe Tryon’s 1940 study which investigated maze running skill, and the genetic factors that contribute to this trait.

Answer 7

Tryon selectively bred ‘dull’(dd) and ‘bright’ (BB) rats. By cross-breeding strains, produced progeny with equal variances in errors in F1 and F2. (i.e. Aa strain performed at the same level as AA, Aa, aA, aa strain)
This indicates that the maze running skill is not a trait derived from a single allele, but from many genes (polygenic trait)

Question 8

What are the problems with quantitative analysis of behaviour?

Answer 8

1. Results only show sections of DNA highly associated with trait. We do not know whether individual candidate genes OR multiple genes each with small contributions to phenotypes.
2. Quantitative analysis requires large pedigrees and cross-mating, which is unlikely in studies with humans.
3. If a gene is missing there are often other genes that can take over, with unpredictable effects.
4. It regards the phenotype as stable, ignoring role of adaptation and developmental effects.
5. There is no evidence for a complex psychological trait being formed by single gene (except Huntington’s).

Question 9

What are the problems with twin studies?

Answer 9

1. External validity - assumes twins are representative, especially when results depend so much on context.
2. Mis-estimation of differences in Vg and Ve (much more complex, involves more variables)
3. There is no estimation of Vge (interaction effect)
4. Assumes MZ and DZ twins are similar - DZ usually treated differently by parents.
5. Excludes Vc and Vge
6. Assumes that twins raised apart live in different environments, but usually middle-class, white, liberal-democratic - underestimates effect of common environment.

Question 10

How are single gene studies of behaviour performed?

Answer 10

1. Studying the contribution of likely candidates for a phenotype based on what is known about neurobiology (eg. it makes sense that alcohol dehydrogenase should be implicated in alcoholism). This means testing different people with polymorphisms of the same gene.
2. Genetically modifying animals e.g. rat with ob gene mutation - leptin hormone regulation - body weight. You can then test how the presence of gene affects performance on some psych measure.

Question 11

What are single gene studies of behaviour?

Answer 11

Studying the contribution of a single candidate gene to a phenotype, based on a priori knowledge of biological and neurological function.
For example, alcohol dehydrogenase for alcoholism, COMT for cognition.

Question 12

What are the problems in explaining behaviour with genetics?

Answer 12

1. Cannot explain complex psychological trait by single gene.
2. Lack of consistency in polygenetic studies
3. A genetic marker can be predictive/useful but may also form the basis for discrimination. You know someone has the gene for SZ, but what then?

Question 13

What are the problems in explaining behaviour with genetics?

Answer 13

1. Cannot explain complex psychological trait by single gene.
2. Lack of consistency in polygenetic studies
3. A genetic marker can be predictive/useful but may also form the basis for discrimination. You know someone has the gene for SZ, but what then?
4. We don’t know how the gene actually brings about the behaviour/disorder. Massive black box.
5. The behaviour may also be hard to quantify, such as IQ. We’re assuming it’s an actual thing, but it may be reification – making up a construct to account for correlations. This is much easier with something like breast cancer, which is binary – you have it or you don’t.

Question 14

What is a quantitative genetic analysis of behaviour?

Answer 14

This is when you measure a behavioural phenotype in a population, then track how it changes across generations to infer statistically the number of genes responsible for the behaviour and their interactions. An example of this is Tryon’s classic 1940 study of artificially bred bright and dull rats. When cross-bred it was found their progeny had equal variance in next two generations (i.e. they were of average brightness/dumbness). This suggests maze-running skill is not derived from a single allele – it’s a polygenic trait.

Question 15

What are single gene studies of behaviour?

Answer 15

This is studying the contribution of likely candidates for a phenotype based on what is known about neurobiology. Eg. it makes sense that alcohol dehydrogenase should be implicated in alcoholism. Or COMT genotype in cognition. You can then test how the presence of gene affects performance on some psych measure. Or you can knock-out the gene in an animal and see what happens.

Question 16

What is the problem with quantitative analysis?

Answer 16

The results do not identify individual genes, only sections of DNA highly associated with a particular trait. And you can’t do pedigree and cross-mating studies with humans.

Question 17

What is the problem with single gene analysis?

Answer 17

It ignores the contribution of background genotype to phenotype. Eg. Snipping different bits of DNA has different effects on different strains of mice.

Isolating genes also misses out the contribution of genetic architecture.

Pleiotropy: one gene does more than one thing.

Also, if a gene is missing there are often other genes that can take over, with unpredictable effects.

It also regards the phenotype as stable, ignoring role of adaptation and developmental effects.

Ultimately, there is no evidence for a complex psychological trait being formed by single gene (except Huntington’s).