Quantitative genetic methods Flashcards
Mental health traits are classed as what?
complex traits
IQ, neuroticism, anxiety, food preference, and depressive symptoms are considered a quantitive or qualitative type of trait?
Quantitative traits
Quantitative traits often show continuous variation.
what does this mean?
The trait can take many different values, rather than only a few. For example height or weight.
The differences between values are usually small so that the distribution follows a smooth curve.
Many complex traits have discontinuous distribution meaning what?
and why do they have discontinuous distribution?
they show up in just a few categories rather than a small spread of values.
This could be simply because we don’t have the instruments to measure them on a continuous scale or because it makes clinical sense to represent them by means of categories.
Name so traits which express this?
schizophrenia, clinical depression, bipolar disorder, autism, hypertension, diabetes
What is a liability distribution?
We deal with these discontinuously varying traits by assuming that the observed categories are part of a continuous distribution that we’re not actually able to measure or observe
People’s liability may vary continuously, but they may only develop the disorder if they reach a what?
threshold liability
The best way to understand how we inherit complex traits is to do what?
contrast it to the inheritance of traits that are due to the effect of a single gene
Characteristics that are due to the effect of a single gene are called?
Mendelian traits
Much of our understanding of genetics comes from the work of Gregor Mendel. Who was he?
and what are discrete traits?
Gregor Mendel was an Austrian biologist and monk.
He studied how garden peas inherit different traits in the garden of his monastery
His explanation of heredity has formed the basis of modern genetics.
The traits Mendel studied are called discrete traits
- characteristics that only occur in one of two forms
- for example, short or long stems, white or purple flowers, yellow or green seeds, and round or wrinkled seeds
Mendel realized there were different or modes of inheritance. What are the two modes?
Dominant = fully expressed in the organism’s appearance.
Recessive = no noticeable effect on organism’s appearance.
Genes are located on chromosomes arranged in homologous pairs. Where are the pairs from?
one from the mother, one from the father.
Homologous chromosomes carry genes in the same order along their length, with possibly different alleles. True or False?
TRUE
The location of a gene along the chromosome is its what?
Locus
When two alleles are brought together in an individual, they stay together for life.
However, when that individual produces gametes (reproduction cells) what happens?
the alleles separate so each gamete only contains one
Which of these will be passed on is random or non- random?
Random
-you can’t be sure which allele a gamete is carrying.
What does this randomness ensure?
This ensures variation in the following generation
Genes are located on chromosomes that are arranged in pairs called?
Homologous pairs
Huntington’s disorder is an example of a Mendelian disorder, as it’s caused by the effect of?
One gene
Huntington’s is neurodegenerative meaning what?
abnormal protein builds up in nerve cells, causing the cells to die.
What are some of the symptoms of Huntington’s?
Symptoms depend on the brain areas affected:
muscle discoordination, cognitive decline or personality and psychiatric problems.
The Huntington’s allele is represented by capital H. Big H small h and small h small h are the genotypes
Having which allele at the gene will cause the individual to develop Huntington’s?
big H,
Francis Galton was interested in how inheritance worked for traits that were quantitative rather than qualitative.
He noticed what regarding children height?
• children’s height correlates with the average height of their parents in a complex way.
• tall parents have taller than average children.
• children of one tall and one short parent are likely to be
of average height.
Mendelians were looking for single-gene effects, biometricians argued Mendel’s laws could not apply to complex traits.
true or false?
TRUE
Mendel’s laws of inheritance of single genes also apply to complex traits but only when?
if the trait was due to the sum effect of many genes of small effect size.
If there is one gene with two alleles, big A and small a, only three genotypes exist with, at the most, how many phenotypes?
three phenotypes
What are phenotypes?
Phenotypes are the observable characteristics or classes in a trait.
If we add a 2nd gene to the system, there would be 9 genotypes, with how many phenotypes?
5 phenotypes
3 genes with 27 genotypes means how many phenotypes?
7 phenotypes
4 genes with 81 genotypes means how many phenotypes?
9 phenotypes
Who published the first general mathematical account of how the correlation between relatives could be explained by Mendelian inheritance?
Sir Ronald Fisher
the father of statistical genetics
If nature and nurture are both important, then how do we disentangle the contributions of each?
and what does A, C and E account for?
Designs such as adoption studies and twin studies are used to try and explain why there are differences within a population.
A - Additive genetic influences
C - Shared environmental influences
E - Non-shared environmental influences
Explain the reasoning behind family studies?
Family share on average what percent of their genes?
Relatives growing up in the same family share both genes and environment.
If you have a first degree relative with anxiety disorder, your chance of the same disorder is higher.
Families share on average 50% of their genes.
Genetic relatedness may be one reason why anxiety disorders cluster.
What is a limitation of family studies?
Family studies cannot tease apart genetic influences from shared environmental influences because members of the same family share the same family environment.
From family studies we can only derive ‘familiarity’ which is what?
the total effect of genetic influences and family environment
What are genetic relatives?
They share genes but not environment ie siblings adopted apart
Family and adoption studies for the aetiology of weight show what?
Weight is more genetic than shared environmental in origin
There are three main problems associated with adoption design. What are they?
- Families involved in adoption may not be representative of the population.
- Adoption studies don’t take account of the prenatal environment.
- The non-random nature of placements of adopted children.
What are monozygotic twins?
MZ (identical) twins develop from a single fertilised egg that divides completely to produce to two people.
What are Dizygotic twins?
They developed from two eggs
MZ twins do not share all their genes. True or false?
FALSE
They do share all their genes and, if they grow up in the same family, all their shared environment.
DZ twins share on average what % of their DNA?
50 %
If a divorce affected one twin more and had the effect of making that twin more anxious, this effect would be estimated as A, C, or E?
E: all the non-shared environmental influences and measurement error
How does the twin design enable estimation of genetic and environmental influences?
MZ and DZ twins have different genetic relatedness but the same correlation for C and E.
Any excess similarity between MZ over DZ can be interpreted as due to the greater proportion of genes shared by MZ twins, and gives an estimate of A.
‘A’ stands for what?
All the additive genetic influences
‘C’ stands for what?
All the shared environmental influences
Differences between MZ can only be due to non-shared environmental influences
True or false?
TRUE
In the twin method, if we see a greater correlation on a trait between MZ twins than DZ twins we conclude what?
that genetic influences are important for that trait.
Faulkner’s formula is used in twin studies to determine what?
the genetic heritability of a trait based on the differences between twin correlations.
What is the MZ correlation?
A + C
What is the DZ correlation
( .5 ) A + C
As non-shared environment is the only thing makes MZ twins different from one another it can be calculated as the total phenotypic variance
true or false?
TRUE
What is The Equal Environments Assumption (EEA)?
Some MZ twins are treated more similarly by their parents in comparison to DZ twin pairs
overestimation of genetic and underestimation of shared environmental effect.
How do we test the equal environment assumption?
If parental treatment is more similar for MZ than DZ twins, then DZ twins mislabelled at birth as MZ twins should be more alike than correctly labelled DZ twins
There are several important differences between twin and singleton births that may affect the ability to generalise results from twin studies. What are they?
- Twin births are more likely to be premature than non-twin births.
- Twins are 30% lighter than non-twins at birth.
- Language development is often slower in twins
twins do not appear to be different in terms of personality or psychopathology than singletons.
What does unipolar depression include?
Includes manic and depressive episodes
What age does depression peak?
age 19-21 years (girls more likely affected than boys)
Anxiety often begins in childhood - mean age of onset is?
11 years
What is a univariate study?
A twin study looking at a single trait
What can univariate studies tell you?
about genetic and environmental influences on a specific trait
Studies have found anxiety in childhood and adolescence is moderately heritable. At what percentage?
30- 40 %
Precise heritability estimates vary according to factors, including age and sex. Heritability is a population statistic therefore the results depend on population under study. True or false?
TRUE
Studies on two variables are called what?
Bivariate studies
These can look at genetic and environmental influences on what 2 things?
- comorbidity of two variables
* same variable at different developmental time points
E.g. of within-twin, cross trait correlation: Twin 1 anxiety correlated with Twin 1 depression
If cross-twin, cross-trait correlations are significant, it implies what?
common influences are due to genes (A) or shared environment (C).
Cross-twin, cross-trait correlation: twin 1 anxiety correlated with twin 2 depression
or
Cross-twin, cross-trait correlation: twin 1 depression correlated with twin 2 anxiety
If the cross-twin cross trait correlations in MZ and DZ pairs are similar, the cause for the overlap is probably due to what?
Shared environments
If the cross-twin cross trait correlation in MZ pairs is higher than in DZ pairs, the cause for the overlap in depression and anxiety is probably due to what?
Genetic factors (A)
If cross-twin cross trait correlation in MZ and DZ pairs is zero, the cause for the overlap is probably due to what?
non-shared environmental effects (E)
What can bivariate twin studies tell us about the comorbidity between different mental health disorders?
Highly co- morbid
The same genes important for anxiety are also important in depression.
Genetic factors act as general influences with environmental factors resulting in specific symptoms.
Why might heritability increase over development?
- ‘New’ genes associated with depression might get ‘switched on’ during puberty.
- Effect of shared environment might become less important.
- This may be related to gene-environmental correlation
Both anxiety and depression are relatively unstable over time. This means that if I have a lot of depressive symptoms at one time point I am NOT likely to report a lot of symptoms of depression at a later time point.
true or false?
FALSE
Both anxiety and depression ARE relatively stable over time
We can use bivariate twin studies to investigate what regarding stability?
genes or environment are stable over time.
• genes or environment account for stability.
Twin studies investigating anxiety and depression have shown that: (3 things)
• they are significantly heritable
• genes associated with depression and anxiety
seem to be overlapping
• genetic and environmental influences on traits change over development, but genes seem to be much more stable over time
