How can something so tiny make such a big difference?
Rebecca A. Lundwall
Psychology Department
Brigham Young University

Classical (Mendelian) genetics
Developmental genetics
Evolutionary genetics
Genomics (sequences)
Medical genetics
Population genetics
◦ Quantitative genetics
Genetic epidemiology
Psychiatric genetics
Behavioral genetics
◦ Molecular genetics

 Individuals are different in behavioral as well as physical traits.
 Some of the differences might be influenced by genes.

Behavior
Genetics
The attempt to establish heritability: that
any genes influence a particular behavior.
Initial approaches often use twin and/or adoption studies.

Fraternal “twins” from separate eggs are not any more genetically alike than other siblings.
Identical twin: Same sex only
Twin & Adoption
Studies
Assess the impact of nature & nurture:
 study traits of siblings vs. identical twins
 see if siblings vary more than twins
Fraternal twin: Same or opposite sex

Studies of twins in adulthood show that identical twins are more alike than fraternal twins (even if reared apart) in:
 personality traits such as extraversion (sociability) and neuroticism (emotional instability).
 behaviors/outcomes such as the rate of divorce.
 abilities such as overall Intelligence test scores.
Study the traits of identical twins as they grow up together or separately
(Minnesota Twin
Family Study).

The probability that the second twin will have a certain trait if the first does. If the
MZ concordance is higher than the DZ concordance that suggests genetic influence to the extent that the MZ twins were treated no more similarly than the DZ twins.

Even if raised apart, identical twins are still more similar to each other than to family they were raised by on:
 personality
Adopted children also seem to be more similar to their genetic relatives than their environmental/nurture relatives
 abilities/IQ
 attitudes
 interests
 specific fears
 brain waves, heart rate
All of this means that it is worth investigating genetic influence on these behaviors.

Behavior
Genetics
Molecular
Genetics
Molecular genetics is an extension of behavior genetics that asks the question, “Which genes influence a particular behavior?”

The Nucleus contains Chromosomes which in turn contain DNA (deoxyribonucleic acid)

Segments within DNA consist of genes that make proteins including enzymes and neurotransmitters.

 46 chromosomes in 23 sets matched sets
 X and Y chromosomes are not a matched set in males, who are missing some genes on the Y.
 Each person’s chromosomes have the same gene locations as everyone else
 Each biological parent donates half a set of chromosomes to his/her offspring.

 This genetic protein assembly can be turned on and off by the environment, regulatory regions of the gene itself, or by other genes.
 Almost any trait we see is a result of the complex interactions of many genes and countless other molecules.

Recall that a gene (and DNA generally) consist of sequences of nucleotide bases:
◦ A= adenine
◦ T = thymine
◦ G= guanine
◦ C =cytosine
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SNPs
Parent 1: A A C C T C TA C A C
Parent 2: A A C C T T TA C A C G
= Possible genotypes: CC, CT, TT
VNTRs
3 repeats: CCACTCAGG CCACTCAGG CCACTCAGG
2 repeats: CCACTCAGG CCACTCAGG
= Possible genotypes: 3R/3R, 3R/2R, 2R/2R
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 Vary dose of some treatment drug.
 Associate any behavioral outcome with drug dose.
 A common pattern is a drug dose-response.
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 A single SNP can influence the rate or quality of the protein produced.
 We can associate any behavioral outcome with genotype.
 A common pattern is a gene doseresponse.
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We have found significant differences in RT by genotype on COMT at SNP
#rs4680
Significant associations between a SNP or VNTR and a behavior commonly explain 3-12% of the variation in behavior.

We’ve also found significant differences in RT slope across trials by genotype on COMT at SNP #rs4680. Less than 25% of variance explained.

Polygenic Inheritance – Process by which several genes interact to produce a certain trait; responsible for our most important traits.
Pleiotropic inheritance – the effect of a single gene on more than one characteristic trait trait trait trait

• Genes can influence traits, which influence responses, which influence the environment, which can influence gene activity.
• For example, a genetic predisposition influences a child to be restless and hyperactive evokes an angry response from his parents.
• Or, a stressful environment can trigger genes to manufacture neurotransmitters that lead to depression.

 Genes can turn each other on and off in response to environmental conditions
Example: shortened daylight triggers animals to hibernate
Example: obesity in adults can turn off weight regulation genes
 Epigenetics: the environment acts on the surface of genes to alter their activity

Histones can be modified by enzymes. One of the most common modifications is methylation
(adding a methyl group). This slows transcription (the copying of DNA into RNA so a protein can be built).

Longitudinal Study (vs Cross Sectional)
The SLC6A4 gene influences the level of serotonin, which is known to play a role in mood.

There is a VNTR on the SLC6A4 gene.
Long version - a little over 50% of population
Short version - a little less than 50%
The researchers compared participants with a normal SLC6A4 (long) VNTR with participants with shorter alleles.
Specifically after stressful events.

Results
After stressful events, the participants with the shorter SLC6A4 VNTR were more likely to become depressed.
The SLC6A4 gene could be involved in vulnerability to depression.

We can look at infant siblings of kids who have Autism
The infants don’t have autism, but they share 50% of their genes with their biological siblings
If the infants do poorly on some skill related to autism and then later go on to develop autism, we can identify which genes influence the development of autism
Identifying children earlier allows us to intervene and enroll infants in parentinfant programs that prevent the development of some symptoms and ameliorate the development of others
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Knowing How to Change the Environment
Some genes do not confer risk for problem behavior, but sensitivity to the environment
BDNF and MAOA are two of these
Children are more likely to grow up to be delinquent only if they have both a risk allele and they grow up in a harsh home environment
If we are kind to them, they grow up amazingly well – otherwise . . .
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Finding Better Treatments for Intractable
Psychiatric Disorders
Treatment Refractory Depression and Treatment
Resistant Obsessive –compulsive disorder
Finding a biological pathway is associated with particular, more severe symptoms may make these types of disorders easier to treat with medications
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