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By studying identical and fraternal twins and their families, we can estimate how genes and environment interact to influence character, strengths, vulnerabilities, and values.
Identical twins differ from fraternal twins in the number of genes they have in common.
Genes are the chemical units in the cells of the body.
They act as blueprints for guiding and directing growth.
Identical, or monozygotic (MZ), twins have the same genetic makeup.
Fraternal, or dizygotic (DZ), twins share 50% of their genes.
Most think that fraternal twins have two placentas and identical twins share one placenta, but this is not the case.
One way to answer this intriguing question is to compare MZ twins reared together with those separated at birth and raised in different families.
Similarity in twins reared together is due to genes and environment.
Similarity in twins reared apart is due only to genes because they don't share experiences together.
Most people would guess that twins reared together would be more alike than twins reared apart. But are they?
Here's what we found:
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The black bars represent pairs of identical (MZ) twins reared apart.
The grey bars represent pairs of identical (MZ) twins reared together.
The height of the bars show how similar, on average, the twins were in personality, interests, and attitudes. For example, look at the black bar above Personality. It shows that MZ twins reared apart were about 50% similar in personality. The grey bar above Personality shows that MZ twins reared together are also about 50% similar in personality.
What's the answer?:
An MZ twin reared away from his or her co-twin seems to have about as good a chance of being similar to the co-twin in terms of personality, interests, and attitudes as one who has been reared with his or her co-twin.
This leads us to believe that the similarities between twins are due to genes, not the environment.
Because the differences between twins reared apart must be due totally to the environment, and because these twins are just as similar as twins reared together, we can conclude that the environment, rather than making twins reared together alike, makes them different.
How can Minnesota Twin Studies help us to determine what's behind "that thing called LOVE"? Why do we feel that we have found our "One and Only"? Cupid's bow and arrow image may hold more truth than you think. Look below to see what we have found with the help of Minnesota twins and their families:
Step #1:
Step #2:
Step #3:
Can you figure out the connection between genes and divorce?
We have suggested that there is not a "divorce gene" but rather, the risk for divorce depends on many psychological charcteristics, all of which are genetically influenced.
Here's what we found:
By asking twins about divorces in their families, a divorce risk was calculated according to which family member was divorced.
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Minnesota Population
Rate: About 19% of Minnesota couples have been divorced.
Twin Family Rate: About 20% of the twins had divorce in
their families.
If Parent Divorced: The risk of divorce climbs to about 29%
if your parents are divorced.
If Spouse's Parent Divorced: The risk of divorce is about
31% if your spouse's parent is divorced.
If Fraternal (DZ) Twin Divorced: If you are a DZ twin and
your co-twin is divorced, your risk of divorce is 30%.
If Identical (MZ) Twin Divorced: ***If you are an MZ twin
and your co-twin is divorced, your risk of divorce to 45%, which is
25% above the rates for the Minnesota population. Since this was not
true for DZ twins, we can conclude that genes do influence the
likelihood of divorce.
How do we explain these results?
We know from other studies that genes strongly influence personality characteristics.
Identical (MZ) twins thus have very similar personalities.
We believe their similar divorce rates are due to their having genetically influenced personality characteristics that contribute to marital adjustment.
As twins grow older, do they become less similar to one another?
MZ (identical) twins become less similar in terms of physical traits like looks and weight over time. However, we recently found that MZ twins become more similar with respect to abilities such as vocabularies and arithmetic scores. Ultimately, twins like you will provide us with the answer to questions about whether personalities, interests, and opinions of MZ and DZ twins become more or less similar with age.
As DZ (fraternal) twins get older, as they
leave their rearing home, marry, start careers and families, and
develop their own circle of friends, they become less similar with
respect to vocabularies and arithmetic scores.
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The bar graph shown above
demonstrates that MZ twins tend to have more similar ages at the time
of death than DZ twins do. That is, MZ twins are more likely to die
at about the same age, and DZ twins are more likely to die at
different ages.
What is EEG and why is it important to measure in twins?
An EEG is a measure of brain activity or brain waves. By measuring brain waves, we can monitor a person's state of arousal, that is, whether he or she is asleep, awake and relaxed, or awake and excited.
This figure shows the different types of waves seen during different states of arousal.
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Alpha waves are seen
when one is awake and relaxed.
Beta waves are seen when one is awake and alert.
Delta waves are seen during sleep.
Theta waves are seen when one is drowsy.
Since identical (MZ) twins share all of their genes, and fraternal (DZ) twins share about 50% of their genes, we can estimate how much genes influence states of arousal by determining how similar EEG is for members of the two types of twin pairs.
It is difficult to determine twin similarity using squiggly EEG waves like those shown above. To simplify our task, we can use a computer to draw a picture, called a spectrum, that tells us how much of each type of EEG wave is present. Refer to the figures below to compare the EEG spectra of identical (MZ) twins and fraternal (DZ) twins.
These spectra were obtained from twins who were relaxing with their eyes closed. As you can see, what really makes the twins' spectra look the same or different is the amount of EEG alpha that is present. Some twins produce a great deal of alpha, causing their spectra to have a peak. Others generate little alpha and therefore have spectra that look relatively flat.
In this relaxed state, MZ twins tend to generate the same amount of alpha, producing strikingly similar EEG spectra. DZ twins show far less similarity. If EEG is determined mostly by genes, this is just what we would expect to see.
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What is a P300 and why is it important to study in twins?
A P300 is a tiny electrical response (a few millionths of a volt) that occurs in the brain when a person detects something that is unusual or interesting. For example, if a person were shown nine circles and one square, a P300 brain response would appear after seeing the square because it's different.
Like EEG, P300's are important to twin studies because we can assess what role genes or neurophysiological factors (which twins share) play in determining how the brain processes information.
In the graph below, identical (MZ) twin children from the Minnesota Twin Family Study have very similar looking P300's. By comparison, the children on the right side, who are fraternal (DZ) twins, do not show as much similarity in their P300's.
Note that for the members of MZ twin pairs, the shape and size of the P300 waves is very similar. Also, the peaks of the bumps tend to line up. For the members of DZ twin pairs, the P300 waves tend to have different shapes and sizes and sometimes the peaks do not line up.
These results indicate that the way the brain processes information may be greatly influenced by genes.
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