Twins Flashcards
What is an allele?
An alternative form of a gene at a particular locus
What makes identical twins similar?
Shared genes @ 100% and shared environments
What makes non-identical twins similar?
Shares genes (50%) and shared environments
What does A + C + E =
The full variance, this is usually standardised to 1
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?
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
In a path diagram what are observed variables, latent variables, causal pathways and covariance (correlational) pathways represented by?
Observed variables = squares
Latent variables = circles
Causal pathway = single headed arrow
Covariance pathway = double headed arrow
What do path diagrams enable us to do?
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.
What are some advantages of statistical model fitting?
We can fit complex models
We obtain information about confidence intervals
What are a couple of problems with statistical
Model fitting?
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
The twin method relies on several assumptions, list 5 of these
- genetic correlation between MZ twins = 1
- no gxe interactions and correlations
- equal environment assumption
- chorionicity
- generalisability
One of the assumptions of the twin method is that MZ twins have a genetic correlation of 1, is this true?
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)
How can MZ twins show different gene expression?
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)
What are three types of gxe correlations?
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
What are gene x environment interactions
This is where the influence of the environment actually interacts with a genotype to create the phenotype
Outline Caspi (2003) study
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
Outline the three basic principles that underly quantitative genetics
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.
What is the equal environments assumption?
This is the assumption that MZ and DZ twins share the same amount of their shared environment.
In what way has the equal environments assumption been questioned?
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)
Goodman & Stevenson (1991) suggested what about the equal environments assumption?
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.
What is the chorion?
Is the outermost sac enclosing the embryo
Discuss the assumption of chorionicity
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
What do we know about twins in the context of whether twin research s generalisable?
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.
Historically twin samples often focused on volunteers what issues does this present?
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
How can we combat sampling bias in twin samples?
Use general population epidemiological samples
Selecting twin samples based on birth registers is advantageous, however what issues do they still have?
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.
Why is there an issue around sampling bias when researchers are looking at a specific psychopathology such as depression?
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
Limitations of the standard twin study have led to what extensions?
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
- Very unusual to separate twins
- Separation typically doesn’t occur at birth
- Adoptive families may be similar for both twins
- 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)
Adoption design. Discuss the comparison of adoptive and biological sibling pairs
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
Other than siblings what other comparisons can be made using an adoption design?
Resemblance between adopted child and birth parents is due to genetics; resemblance between adoptive child and adoptive parents is due to shared environment)
What are 4 assumptions of the adoption design?
Representativeness
Prenatal environment
Selective placement
Assortative mating
How representative are adoption designs?
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)
Prenatal environments may make adoptive children more similar to their mothers in addition to shared genes, has research shown this?
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
What is selective placement?
This is where adoption agencies try to match biological and adoptive parents on various attributes
Outline family designs
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)
What are the advantages of a family design?
- Sample ascertainment:
- rates of marriage breakdown and cohabitation = high
- samples of blended families = easier to obtain that twin and adoption samples - 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
What are the disadvantages of the family design?
- 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
