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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).
A twin study compares the concordance rate of identical twins to that of fraternal twins. This can help suggest whether a disease or a certain trait has a genetic cause. [3] Controversial uses of twin data have looked at concordance rates for homosexuality and intelligence. Other studies have involved looking at the genetic and environmental ...
The word zygosity may also be used to describe the genetic similarity or dissimilarity of twins. [6] Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are dizygotic because they develop from two separate oocytes (egg cells) that are fertilized by two separate sperm.
No two humans are genetically identical. Even monozygotic twins (who develop from one zygote) have infrequent genetic differences due to mutations occurring during development and gene copy-number variation. [1] Differences between individuals, even closely related individuals, are the key to techniques such as genetic fingerprinting.
A second and more common design is the twin study in which the similarity of identical and fraternal twins is used to estimate heritability. These studies can be limited by the fact that identical twins are not completely genetically identical, potentially resulting in an underestimation of heritability.
There is evidence of heritability in personality traits. For example, one study found that approximately half of personality differences in high-school aged fraternal and identical twins were due to genetic variation - and another study suggests that no one personality trait is more heritable than another. [6] [8]
For example, it may be used for something transplanted from an identical twin. When the cells are collected from the same patient on whom they will be used, a graft is called autologous. Syngeneic refers to a graft transferred between genetically identical animals or people. [1] A syngeneic graft is known as an isograft. [2]
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.