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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).
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 ...
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 ...
Falconer's formula is a mathematical formula that is used in twin studies to estimate the relative contribution of genetic vs. environmental factors to variation in a particular trait (that is, the heritability of the trait) based on the difference between twin correlations. [2]
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.
From twin studies is typically estimated at 0 because the correlation between monozygotic twins is at least twice the correlation for dizygotic twins. When using the Falconer variance decomposition ( 1.0 = a 2 + c 2 + e 2 {\displaystyle 1.0=a^{2}+c^{2}+e^{2}} ) this difference between monozygotic and dizygotic twin similarity results in an ...
Nearly three years ago, Mount Holly resident Amy Mann connected with her 22-year-old biological twins for the first time thanks to a 23andMe DNA test.
The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. [5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, [6] [7] [8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may ...