PSYC 3102: Introduction to Behavioral Genetics
Lecture 12
XYY syndrome continued:
Witkin in Denmark examined Danish army enlistment

Small country, no federal vs. state governments

Compulsory draft into state service for all men

Series of tests then placed in Army or other state service for a few years

Provided a large cohort of males

Incoming draftees were all karyotyped
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Only 12 XYY cases found

5 out of those 12 (~42%) ended up having criminal records compared to base rate of ~12%
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Significant difference
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However, crimes were largely petty offenses (shoplifting, etc.), not violent or sexual crimes
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The picture that emerges:
o These individuals tend to suffer from some learning disabilities, higher rate than
average, but not severe
o THIS is a predicting factor of delinquency, generally petty crimes

Conclusion: No predisposition for aggression or violence, but predisposed to learning
problems, which in turn, increases risk of getting into trouble with the law
2 Major Myths:
1. Hyper-masculined sexual psychopaths due to increased male hormones (hypertestosteronized)
2. Perfectly normal “there is NO association with criminal records”
Microdeletion

Small section of chromosomal material is missing (thousands to hundreds of thousands of
nucleotides, 5-6 genes)

Prader-Willi Syndrome & Angelman’s Syndrome are examples of microdeletions
Microduplication

Areas get duplicated
Genetic Technology
Can’t go into great detail—it is its own course
Taking a very simplistic view

Most important in modern genetics-- Ability to identify polymorphism

Polymorphic gene = more than one common allele
Huntington’s Disease

Discovered in 1870s-1880s

Later realized it is a dominant gene
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Identify the gene to find out the protein it codes for, find treatment or interventions

Start with linkage analyses to find the general area

Problem—not enough marker genes (genes that are known and mapped)

1980s—better techniques to detect polymorphism and place their location

For 75 years nothing panned out in HD research

But in 1984 there was the first report of general linkage
Electrophoresis

Purpose: Separate molecules by molecular size

Start with gel (viscous material)

Insert a series of molecules

Run current

Molecule with certain electric charges will move to one end of gel

Size makes them faster or slower in moving through the gel

Need to dye molecules in order to see them

Probes are single stranded sections of DNA or RNA that binds to the DNA (complimentary(
that carries a “light bulb”

Fluorescent dyes are mainly used now, but radioactive labels used to be used more
commonly

Used for proteins, DNA
RFLP

Restriction Fragment Length Polymorphism

A method of finding and detecting polymorphisms

Identifies alleles at restriction sites where restriction enzymes cut
Restriction Enzyme:

An enzyme that recognizes a specific nucleotide sequence, then cuts at the DNA at that
sequence

Ex: EcoRI recognizes a certain sequence and cuts the DNA in to 2 fragments
For example… can be used in forensic and paternity cases
(really today they use more precise tests, this is one of the first methods)

EcoRI enzyme cuts at a specific sequence

2 individuals: enzyme may cut different fragments (or may not cut one at all)

After cut up, do electrophoresis and then bathe in the probe

Results:
Smith / Jones / Perp
or Pappa

Results: Jones didn’t do it/isn’t the father; NOT that Smith did do it/is the father!!

Need to keep testing more and more genes until you get acceptable probabilities that it is a
match
Other tools:
Polymerase Chain Reaction (PCR)

Purpose is to “amplify” DNA = make a large # of copies of a desired section of DNA

Ex. make lots of copies of a single gene
Advantages:

can do a lot of genotyping from a small amount of sample (used to take white blood cells and
make a cell culture, now can get a buccal sample from the cells in the mouth)
Pre-Processing Steps:
1. Purify DNA from the cell
2. Cut up long strands of DNA into more manageable pieces (with restriction enzymes)
Major Steps for PCR:
1. Heat it – denature double-stranded DNA into single-stranded DNA
+
2. Add primers – small section of single stranded DNA that will bind to the strands
primers
3. Add replication material – enzymes that will replicate DNA (polymerase) and free
nucleotides
now have 2 copies of DNA
doubled the amount of DNA

Repeat steps 1-3 to make more & more copies

After ~20 cycles, you magnify the gene of interest ~100,000 fold

Signal for the one gene will overpower the other genes in the sample, then you can
SEQUENCE
Types of Polymorphisms
1. Protein / enzyme variations
2. RFLP
3. Tandem Repeat polymorphisms – simple, different number of repeats (think of DNA
stuttering), sequence can be as short as 2 nucleotides or as long as ~44 nucleotides
4. SNP – single nucleotide polymorphisms consist of variations in only one nucleotide
Increased number of known polymorphisms help in gene detection
Can test for polymorphic phenotypes
Example:

In the serotonin receptor 1A gene there is a SNP (A to G) which gives a different amino acid
in that position, which will cause the protein product to fold slightly differently

Serotonin is believed to be involved in depression

So… Compare depressives to controls
Example 2:

Polymorphism in the serotonin transporter gene doesn’t make a different transporter

It is a tandem repeat in the promoter region (4-11 repeats of a 44 nucleotide sequence)

It is thought to influence the rate of transcription

So… Genotype and compare people
Take home points:
Not important to know nitty-gritty details
Don’t need to explain PCR or show RFLP
But KNOW DEFINITIONS
Read text regarding sequencing, but don’t need to know details