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PSYC 3102: Introduction to Behavioral Genetics
Lecture 13
Association Studies
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How a gene relates to a trait
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2 kinds: Pop’n Based and Family Based
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Population-based designs – controls are from the general population
o Get people with disorder (ex: major affect disorder), then get controls
o Then genotype for particular gene or genes (like serotonin transporter)
o Conclude if that locus is associated with MAD
(ex. Short and long forms because of a repeat polymorphism)
phenotype
ADHD
SS
SL
10
LL
Total
35
55
100
40
40
100
(# of people)
Control
20
o Easy to conduct
o But population stratification is a problem – allele frequencies are associated with
sub-populations; allel frequencies can be associated with ethnicity
Example:
phenotype
Sickle cell
Normal
Skin Pigmentation Gene
LL
DL
DD
Low-Zero
High Number
High Number
Total
100
100
But skin pigmentation does NOT have any kind of causal relationship with the disease!!
- vast majority of sickle cell individuals have African ancestry
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Family-based designs – use genetic relatives as controls
o This avoids problems of ethnicity and population stratification
o Often use siblings as controls
Example:
Family MAD sibling Normal sibling #L in ADHD - #L in normal
Smith
LL
SL
2–1=1
Jones
SL
SS
1–0=1
Etc.
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If there is no association, the mean of the last column should be about 0
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If mean of last column is >0; L allele confers risk
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If mean is <0; S allele confers risk
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Much more difficult to execute these studies
Discussion of Article
Comments of Class:
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Very vague
Unnecessary N vs. N debate
Not enough mention of environment
Learn to control behavior
Biased to Nature, but courts are currently very Nurture-biased
Test for gene only after crime is committed??? Or before?
Not full penetrance
Prediction and prevention… false positives
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What about male/female differences in aggression?
Family
Criminal
Normal
Smith
XY
XX
# of Y
chromosomes
1–0=1
Jones
XY
XX
1–0=1
Etc.
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9 to 1 ratio of men to women with violent crimes
In this sense the Y chromosome has a VERY high association with violent crimes, it is a
genetic marker in this sense
But, does the Y chromosome cause crime?????
This is just a statistical association
HOW do genes and environment interact?
Y is a predictor, but not the whole picture
Association studies don’t tell you HOW!!
Case by case
Issues of genetic discrimination; examples: cancer and insurances; lay persons interpretations
Fragile X child example – hit sibling with a bat, caused major brain damage – do you treat
that child the same? Or based on genes?
Can treat differently without being more lenient
What is sentencing designed to do? Rehabilitate or Stop from recurring or Retribution?