Behavioral Genetics Week 3: Heritability

This was week 3 of my Behavioral Genetics class, and this was the first week where (at times) I felt lost.  For Week 3, the professor taught some of the basic vocabulary words of genetics, gave a basic explanation for how heritability is calculated (this is the part where I got lost and I'm not even going to make an attempt to explain or remember it), and introduced the concept of gene environment interaction.  Part One of this blog post is vocabulary that I want to remember, so feel free to scroll on past.  Part Two of this blog post talks about gene-environment interaction, which is more generally interesting.

Part One:  Genetics Vocabulary

• Gene - a functional unit of inheritance
• Allele - alternative forms a gene governing a specific character can take (ie Y and G for color)
• Genotype - the two alleles one inherits
• Homozygotes - YY or GG
• Heterozygotes - YG or GY
• Genes are located on chromosomes, which are located in the nucleus of all our cells.
• Chromosomes 1-22 are autosomes. (Numbered roughly from largest to smallest.)  The third chromosome is the sex chromosome.
• Recombination - exchange of genetic material between homogous chromosomes
• Variance:  an index of the degree to which individuals differ for a quantitative trait 
• Biometrics:  Take a phenotypic variance and find what portion is associated with genetic variance and environmental variance.
• Additive genetic effects - the effect this gene adds does not depend on what genes it pairs with
• Non-additive genetic effects - the effect this gene adds does depends on what genes it pairs with
• Shared Environment - things individuals growing up in the same home share (income level, parents approach to child rearing, neighborhood)
• Non-shared Environment - environmental effects that individuals growing up in the same home do not share (peer group, accidents, differential parental treatment to each sibling

2 different heritability measures:

• Total heritability - indexes the effect of all genetic contributions.
• Additive heritability - measures the effect of additive genetic contributions.

ACE
A = Additive
C = Shared environmental factors
E = Non-shared environmental factors

• A usually explains at least half of traits (moderate to large effect)
• C often doesn't seem to matter (birth weight is an exception, type of religion is also an exception)
• E are always important (moderate effect)

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Part Two:  Gene Environment Interaction

Gene environment interaction:  The magnitude of the genetic effect depends on the environment.  Certain inherited genes are a "vulnerability", but they are only trigger a particular behavior if the person with the vulnerable gene also interacts with a particular type of environment. Here are some examples:

Aggression:  do genetic factors influence level of aggression?
 - If reared in a nurturing, protective home, then NO.
 - If reared in a chaotic, dysfunctional home, then YES.

Depression:  Is there gene environment interaction for depression?
- Depression is heritable and some of the genes for depression are not dependent on experiencing a certain environment.
- But, there are environmental influences that can influence the genetically based level of depression.
- Life stressors trigger the gene environment interaction for depression.
   - If you have low levels of life stress, then NO gene environment interaction.
   -  If you have high levels of life stress, then YES gene environment interaction.

IQ is an example of when there is NOT a gene environment interaction.
- Genes are important to IQ and environment is also important to IQ.
- But there are no genes which influence IQ that are triggered (or not) by experiencing a particular environment.

Why is gene-environment interaction important?
- In gene environment interaction, there are (potentially) two different points where you can intervene to help with problematic phenotypes.
– If you know that someone has an "at risk" gene, you can do environmental interventions to stop the vulnerable gene from being triggered - ie, Increase family cohesion in those at risk for aggression
– If you know that something is caused by a gene (as well as triggered by environment), then there is the possibility of pharmaceutical treatments.