Twins

Question 1

What is an allele?

Answer 1

An alternative form of a gene at a particular locus

Question 2

What makes identical twins similar?

Answer 2

Shared genes @ 100% and shared environments

Question 3

What makes non-identical twins similar?

Answer 3

Shares genes (50%) and shared environments

Question 4

What does A + C + E =

Answer 4

The full variance, this is usually standardised to 1

Question 5

Mz correlation = .8
Dz = .6

What is the correct model to fit to these correlations and what can the total variance for the phenotype in question (height) be attributed to?

Answer 5

ACE model (as diff. Between MZ and dz correlation is equal to less than half the mz correlation… Otherwise ADE)

A = .4 
C = .4
E = .2

Question 6

In a path diagram what are observed variables, latent variables, causal pathways and covariance (correlational) pathways represented by?

Answer 6

Observed variables = squares
Latent variables = circles
Causal pathway = single headed arrow
Covariance pathway = double headed arrow

Question 7

What do path diagrams enable us to do?

Answer 7

They enable us to visually represent a model

And make it possible for us to calculate the aetiology of a phenotype by following path tracing rules and carrying out some simple sums.

Question 8

What are some advantages of statistical model fitting?

Answer 8

We can fit complex models

We obtain information about confidence intervals

Question 9

What are a couple of problems with statistical

Model fitting?

Answer 9

It is based on the principle of parsimony

• this often leads to C being dropped
• A and E then absorb the variance from C and can become over estimate

Models are based on implausible assumptions

e. g. No rGE or GxE
- this leads to inaccurate estimations

Question 10

The twin method relies on several assumptions, list 5 of these

Answer 10

• genetic correlation between MZ twins = 1
• no gxe interactions and correlations
• equal environment assumption
• chorionicity
• generalisability

Question 11

One of the assumptions of the twin method is that MZ twins have a genetic correlation of 1, is this true?

Answer 11

MZs show small variation in their genetic composition

E.g. Differences in CNVs (Brunder et al., 2008)

• overall, DNA differences between MZs are small (validity of assumption probably acceptable)

Question 12

How can MZ twins show different gene expression?

Answer 12

Gene expression is the process from the DNA sequence through transcription and translation to the protein produced and is influenced by a process of methylation which instructs this process as to how much expression of a gene should happen at a particular time

MZ twins can differ in gene expression but environmental factors may account for these differences in gene expression (Evans and Martin,2000)

Question 13

What are three types of gxe correlations?

Answer 13

Passive: from parents child gets genes and environments, parents good readers more books in the home (environment) and good reading genes are passed on

Reactive: the environment is shaped around genetic propensities, if a child is a good reader family members may buy more books as presents and teachers may ask the child to read with younger children thus increasing a child’s exposure to environments which strengthen the ability

Active: seeks out and modifies environment based on genetic propensities

Question 14

What are gene x environment interactions

Answer 14

This is where the influence of the environment actually interacts with a genotype to create the phenotype

Question 15

Outline Caspi (2003) study

Answer 15

Stressful Life events before the age of 21 and long and short genotype for the 5-httplr gene and depression at age 26
1000 p’s split into groups based on genotype
SS
LL
SL

Stressful life events recorded included items on employment, health, finances.

*there were no sig. Differences
In the number of stressful life events experienced between the three genotyped groups… Suggesting that the genotype didn’t influence life events.

Individuals who were either ss or so were found to show a greater amount of depressive symptoms than the ll genotype

Question 16

Outline the three basic principles that underly quantitative genetics

Answer 16

Behavioural genetics methods compare he similarity of family members who share different levels of genes and environments

If pairs of individual who share more of their genes are more similar to each other than pairs of individuals who share fewer genes, this suggests a genetic influence on behaviour

If lots of individuals who share their environments resemble one another more than those who do not, this suggests an influence of shared environment.

Question 17

What is the equal environments assumption?

Answer 17

This is the assumption that MZ and DZ twins share the same amount of their shared environment.

Question 18

In what way has the equal environments assumption been questioned?

Answer 18

Increased physical similarities seen in MZs = means that they are treated more similarly than DZs

MZs = more likely to be dressed alike and shared bedrooms (loehlin and nicholls, 1976)

Question 19

Goodman & Stevenson (1991) suggested what about the equal environments assumption?

Answer 19

They showed that MZ twins mislabelled as DZ twins are treated as similarly as correctly labelled MZ twins

*research suggests that zygosity attribution alone does not lead to more similar treatment of MZs vs. DZs.

Question 20

What is the chorion?

Answer 20

Is the outermost sac enclosing the embryo

Question 21

Discuss the assumption of chorionicity

Answer 21

It is assumed that the only reason MZ twins are more similar to DZs is because of increased genetic similarity… But what about likelihood of sharing a chorion?

All DZs = dichorionic
1/3 of MZs = dichorionic

Monochorionicy could make twins more similar

This could result in artificially inflated genetic estimates

BUT

Monochorionic twins = more likely to experience birth defects = makes then less similar

This could result in artificially decreased genetic estimates

Question 22

What do we know about twins in the context of whether twin research s generalisable?

Answer 22

Twins are more likely to…

Be born prematurely
Experience adverse intrauterine environments
Have lower birth weight
Have slower language
Have older mothers
Be born following in vitro fertilisation

However twins are similar to singletons on most traits and within a few years of development have caught up with singletons.

Question 23

Historically twin samples often focused on volunteers what issues does this present?

Answer 23

Sampling bias

How representative are the samples? For example MZ and concordant pairs are possibly more likely to take part. This could affect differences between MZ and DZ twin pairs

Question 24

How can we combat sampling bias in twin samples?

Answer 24

Use general population epidemiological samples

Question 25

Selecting twin samples based on birth registers is advantageous, however what issues do they still have?

Answer 25

Non-participation
Longitudinal studies have high attrition rates

Attrition may be selective

However, benefits of large scale population samples such as teds means that it is possible to check how representative the sample is in regards to the wider population and then gives more support for how generalisable findings are. From teds many measures have been checked such as ethnic percentages and cognitive abilities with the biggest discrepancy being found in the number of mothers staying at home compared to that of singletons. This is likely to be due to the extra work involved in raising twins and can potentially influence behaviours.

Question 26

Why is there an issue around sampling bias when researchers are looking at a specific psychopathology such as depression?

Answer 26

May be a problem as referral may be influenced by the characteristics of the co-twin (concordant pairs more likely to seek referral?)
This would inflate the correlations between twins reducing non- shared environmental influences.

Not such a problem with serious mental disorders such as schizophrenia

Question 27

Limitations of the standard twin study have led to what extensions?

Answer 27

Twins reared apart

RATIONALE
If MZ twins are separated at birth and brought up in different families, resemblance (beyond chance) must be due to genes

LIMITATIONS

1. Very unusual to separate twins
2. Separation typically doesn’t occur at birth
3. Adoptive families may be similar for both twins
4. Offspring a twins

RATIONALE
the children of MZ twins are like half-siblings (e.g. Aw would be if they have the same
Mother but different father). The children of dz twins are like standard cousins

Limitations
- less statistical power (as looking at half siblings vs cousins)

Question 28

Adoption design. Discuss the comparison of adoptive and biological sibling pairs

Answer 28

What makes adoptive sibling pairs similar? Shared environments only

What makes biological siblings pairs similar?
Shared genes (50%) and shared environm eye 

We can compare adoptive vs biological siblings to conclude about genetic and environmental influences on traits

Question 29

Other than siblings what other comparisons can be made using an adoption design?

Answer 29

Resemblance between adopted child and birth parents is due to genetics; resemblance between adoptive child and adoptive parents is due to shared environment)

Question 30

What are 4 assumptions of the adoption design?

Answer 30

Representativeness

Prenatal environment

Selective placement

Assortative mating

Question 31

How representative are adoption designs?

Answer 31

Research has suggested that biological parents, adoptive parents and adopted children are representative of the general population but there are certain differences (e.g. Adoptive parents likely to be older)

Question 32

Prenatal environments may make adoptive children more similar to their mothers in addition to shared genes, has research shown this?

Answer 32

Research has shown that adopted children are as similar to their birth fathers as mothers which would suggest that the prenatal environment is not having a huge effect

Question 33

What is selective placement?

Answer 33

This is where adoption agencies try to match biological and adoptive parents on various attributes

Question 34

Outline family designs

Answer 34

This is where you use the genetic relatedness of siblings not twins

Full siblings living together (50% of genes and 100% C)

Half siblings living together (25% of genes and 100% of C)

Full siblings living apart (50% genes 0% C)

Half siblings living apart (25% genes and 0% C)

Unrelated siblings living together (0% of genes and 100% of C)

Question 35

What are the advantages of a family design?

Answer 35

1. Sample ascertainment:
   - rates of marriage breakdown and cohabitation = high
   - samples of blended families = easier to obtain that twin and adoption samples
2. Testing hypotheses:
   - if behaviour is genetically influenced we would expect greatest correlations for MZs, then DZs and full siblings, then half siblings, then unrelated siblings

Question 36

What are the disadvantages of the family design?

Answer 36

• differences in genetic relatedness are accompanied by differences in environmental influences (e.g. Half-siblings are likely to live together for less time than full-siblings)
• therefore, hard to distinguish genetic and environmental influences