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The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings).
Monozygotic twins result from the fertilization of one egg and the division of that single embryo forming two embryos. [4] However, just because a set of twins share the same genetic information, it does not mean they will exhibit the same traits and behaviors. There are different versions of a gene, which are called alleles. How a gene is ...
In 1979, Thomas J. Bouchard began to study twins who were separated at birth and reared in different families. He found that an identical twin reared away from his or her co-twin seems to have about an equal 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 ...
One way to determine the contribution of genes and environment to a trait is to study twins. [34] In one kind of study, identical twins reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. Twins reared apart are not assigned at random to foster or adoptive parents.
In the Minnesota Study of Twins Reared Apart, monozygotic twins separated shortly after birth were reunited in adulthood. [60] These adopted, reared-apart twins were as similar to one another as were twins reared together on a wide range of measures including general cognitive ability , personality , religious attitudes , and vocational ...
A limit of this design is the common prenatal environment and the relatively low numbers of twins reared apart. A second and more common design is the twin study in which the similarity of identical and fraternal twins is used to estimate heritability.
In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share on average half alleles and the correlation of their scores would be 0.50 if IQ were affected by genes ...
Thomas J. Bouchard Jr. (born October 3, 1937) is an American psychologist known for his behavioral genetics studies of twins raised apart. He is professor emeritus of psychology and director of the Minnesota Center for Twin and Adoption Research at the University of Minnesota.