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Chromosome Breakdown
Chromosomes
Genes
DNA
Nucleotides
With reference to relevant research
studies, to what extent does genetic
inheritance influence behavior?
Gene Mapping
• Some scientists are
trying to determine the
effect of a particular
gene on behavior such
as temperaments or
psychological disorders.
Caspi et al. (2003)
The role of the 5-TT gene in depression after stressful events.
• Longitudinal Study (vs
Cross Sectional)
• The 5-TT gene
influences the level of
serotonin, which is
known to play a role in
mood.
Caspi et al. (2003)
The role of the 5-TT gene in depression after stressful events.
• There are two types of 5HTT genes.
• Long- a little over 50% of
population
• Short- a little less than
50%
• The researchers
compared participants
with normal 5-HTT (long)
genes with shorter alleles.
• Specifically after stressful
events.
Caspi et al. (2003)
The role of the 5-TT gene in depression after stressful events.
Results
• After stressful events,
the participants with
the shorter 5-TT gene
were more likely to
become depressed.
• The 5-TT gene could
indicate vulnerability to
depression.
Caspi et al. (2003)
The role of the 5-TT gene in depression after stressful events.
Evaluation
• Since so many people
carry it (almost half the
population), it is hard to
say that in is the gene
that made the difference.
• Maybe it was the
environmental factors
(stressful events) that
caused the depression.
• Did not show a causeeffect relationship.
Chromosomal Abnormalities
• Gender comes from
23rd pair of
chromosomes…men
have XY…woman have
XX.
• Turner’s syndrome is
single X.
• Klinefelter’s syndrome
is extra X…XXY
• XYY- Super male
Let’s take a better look at Twins….
• Monozygotic Twins (MZ
or identical) come from
the same sperm and egg
and share 100% of there
genes.
• Dizygotic Twins (DZ or
fraternal) come from two
different eggs and share
about 50% of their genes.
• Regular siblings also share
around 50% of their
genes.
Twins
In twin studies, researches
focus on something
called a concordance
rate…
• The probability that if
one individual has the
trait the other will also
have it.
• It tells us if a trait is
inherited.
Schizophrenia
Fraternal Twins: 15%
Identical Twins: 50%
• Usually in twin studies, one
twin acts as a control for the
other twin.
• In good twin research sets
of MZ are compared to sets
of DZ for a trait of disorder.
• High concordance rates
with MZ and lower with DZ
indicates the trait is due to
genes (inherited).
• Differences within pairs of
MZ are thought to be
environmental factors.
Bouchard et al. (1990)
Twin study investigating genetic inheritance in intelligence
• This study used a selfselected sample of MZ
twins who had been
reared together (MZT)
and MZ twins who had
been reared apart
(MZA).
• Looked at a lot of
factors but we will focus
on intelligence.
Bouchard et al. (1990)
Twin study investigating genetic inheritance in intelligence
Results:
• Used the WAIS to
measure intelligence.
• The concordance rates
for MZA was 69%, for
MZT it was 88%.
Bouchard et al. (1990)
Twin study investigating genetic inheritance in intelligence
• The researchers
concluded that
environmental factors do
play a role in
development of
intelligence but IQ is to a
large extent inherited.
• 70% of the observed
variation in the sample
could be attributed to
genetic variation.
Bouchard et al. (1990)
Twin study investigating genetic inheritance in intelligence
• They claim that in their
sample (white, middle
class, industrialized
nation) genetic
inheritance in IQ accounts
for around 2/3 of
observed variance of IQ.
• They also said that their
findings do not indicate
that IQ cannot be
increased by
environmental factors.
Bouchard et al. (1990)
Twin study investigating genetic inheritance in intelligence
Evaluation
• Correlational data cannot
establish cause/effect
relationships.
• Concordance rates were
high, but far from 100%.
• No control for the effect
of environmental
variables.
• Self-selected sample.