Identical Twins - mrzimmerman.org

Identical Twins:
The Original Clones
Overview
• Biology of twins
– Dizygotic twins
– Monozygotic twins
• History of twin studies
– Types of twin studies
– Major players in the history of twin studies
• Case studies and scientific studies
• Problems with twin studies
• Controversy of twin studies
The Biology of Twins
The Biology of Twins
• Dizygotic (fraternal) twins: The result of
two separate fertilization events involving
two egg cells and two sperm cells
The Biology of Twins
• Monozygotic (identical) twins: The result of
one fertilization event followed by the
separation of blastomeres into two groups
during cleavage
– Separation before trophoblast tissue formation at
day 5 ‡ embryos have two separate chorions
and amnions
– Separation between day 5 and day 9, when the
amnion lining forms ‡ embryos have one shared
chorion and two amnions
– Separation after day 9 ‡ embryos share one
chorion and one amnion
Monozygotic Twins
• Genetically identical
• Share DNA, but not fingerprints
• Have identical brain wave patterns, which
are almost as unique as fingerprints
among singletons
• Occur once every 254 births
• 1/3 of all twin births are monozygotic
• Incidence of identical twin births not
under the influence of genetic factors
History of Twin Studies
Types of Twin Studies
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Pre-natal and birth experiences of twins
Trait-similarity studies
Longitudinal studies of twins
Comparison studies of twins and singletons
Psychological relationships between twins
Co-twin control studies
Studies of cultural attitudes toward twins
Monozygotic twins reared apart (MZA)
Francis Galton
“I have…[sought] some
new method by which it
would be possible to weigh
in just scales the effects of
Nature and Nurture, and to
ascertain their respective
shares in framing the
disposition and intellectual
ability of men. The lifehistory of twins supplies
what I wanted.”
-- Francis Galton, in his article “History of
Twins” (Inquiries into Human Faculty and
its Development 1875)
Josef Mengele
• Auschwitz's senior physician
and “Angel of Death”
• Fascinated with identical
twins, on whom he
conducted varied “genetic
experiments”
• Believed that the 1500 sets
of twins he studied would
help give scientific
fortification to Hitler’s dogma
of genetic superiority
Sir Cyril Burt
• Three studies, in 1943, 1955,
and 1966, revealed results of IQ
tests of twins reared apart
• Results strongly suggested that
intelligence is highly heritable
• After Burt’s death in 1971,
critics began to question the
soundness of the data
• Correlation coefficients of the 3
studies were suspiciously similar,
despite the addition of many
more subjects
• The number of twin pairs
reared apart (53) was highly
questionable
The Jim Twins
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Jim Springer and Jim Lewis
Separated four weeks after birth
Reunited at age 39
Similarities were striking. Both men…
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had first wives named “Linda” and second wives named “Betty.”
named their sons “James Allan.”
owned dogs names “Toy.”
drove the same color and same model Chevy.
chain smoked Salem cigarettes.
chewed their fingernails.
vacationed in the same spot each year.
got headaches at the same time of the day.
enjoyed mechanical drawing and carpentry.
had excelled at math in school and struggled with spelling.
Thomas Bouchard, Jr.
• Springer and Lewis became the first pair of subjects to
enroll in a retrospective twin study by Thomas Bouchard
of the University of Minnesota
• This research set the foundation of the Center for Twin
and Adoption Research
• Minnesota Study of Twins Reared
Apart (MISTRA) became the primary
research project
• One personality test administered to
the Jim twins found the differences in
their scores to be the same as
differences that would be expected
between the scores of one person
taking two consecutive tests
Twin Studies: The Present
• Swedish Twin Registry:
largest in the world with
70,000 pairs registered
• Twin Research: Bimonthly
journal devoted to studies
concerning twins
• Since the study’s outset
MISTRA has collected data
on over 100 sets of twins
separated before 5 months
of age until thirty years
later
Percent Correlation of IQ Tests
The same person tested twice
Identical twins reared together
Identical twins reared apart
Fraternal twins reared together
Biological siblings
Parents and children living together
Parents and children living apart
Adopted children living together
Unrelated people living apart
100% = perfect identity
87%
86%
76%
55%
47%
40%
31%
0%
0%
0% = random difference
Gay Twins: A Case Study
In 1980, a man entered a gay bar while
visiting another city and was surprised that
many of the bar’s patrons recognized him, yet
called him by a name that was not his. The
man wondered if he had a long-lost brother
and began searching for such an individual.
Upon meeting each other, the identical twins
found that they’d both had childhood speech
impediments, were both emotionally unstable,
and had both realized they were gay around
age fourteen. (The two later became lovers!)
Gay Twins: An observational study
• Eckert ED, Bouchard TJ, Bohlen J, Heston LL.
• Studied six monozygotic twins, reared apart (two
male, four female)
• At least one twin of each pair was self-reportedly
gay or bisexual
• All female pairs were discordant for sexual
orientation
• One male pair was concordant, and the other was
not clearly concordant or discordant
• Suggests that female homosexuality may be an
acquired trait, whereas male homosexuality may
involve an interaction of factors, in which genes
play a large role
Migraine Headaches in Twins
• Svensson DA, Larsson B, Waldenlind E, Pedersen NL
(2003)
• Compared migraine reports of 314 twin pairs reared
apart to 364 matched control pairs reared together
from Swedish Twin Registry
• Results: Non-significant shared rearing environmental
effects on men or women
• Heritability for men: 38%
• Heritability for women: 48%
• Conclusion: Genetic factors, not environmental ones,
account for migraine prevalence within certain families
Tobacco consumption in twins
• Kendler, KS; Thornton, LM; and
Pedersen, NL (2000)
• Compared regular tobacco use (RTU) of
778 twin pairs, male and female, reared
together and reared apart, identical and
fraternal, who were registered in
Swedish Twin Registry
• Percent variance attributed to:
Men
Women
Genetics
61
63
Environment
20
15
Individual
19
22
Problems With Current Twin Studies
• Confounding factors:
– affect of being adopted
– twins raised separately are more similar than those
raised together because they don't need to
individualize
• Small sample sizes
• Bouchard has not allowed other researchers to
check his data
• Anecdotal, headline worthy evidence tends to
overshadow statistical data
• Twin studies don’t allow the study of genes as
they interact with different places in time
Controversy:
Why is this the recurring theme?
• Twins are, and always have been, a preview
to one aspect of the cloning debate
• To determine if such a thing as intelligence is
largely inherited through genetics could
answer a question that some would rather be
left unanswered
• The Nature v. Nurture debate is a passionate
one, and twins (especially identical twins
reared apart) have come the closest to
resolving the debate