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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).
Fraternal twins are dizygotic because they develop from two separate oocytes (egg cells) that are fertilized by two separate sperm. Sesquizygotic twins are halfway between monozygotic and dizygotic and are believed to arise after two sperm fertilize a single oocyte which subsequently splits into two morula .
Twin studies are one method of testing genetic and environmental influences, although they cannot reveal what kind of environmental influence this may include (social or non-social). [29] Identical or monozygotic twins share their genes, while fraternal or dizygotic twins are only as genetically similar as any other sibling pair.
Adoption and twin studies are used to investigate the complex interplay between genes and the environment. These studies typically involve the comparison of identical (monozygotic) and fraternal (dizygotic) twins to determine the extent to which genetic factors and environmental influences contribute to variations in traits or behaviors.
One way to consider it is using trait X in twin 1 to predict trait Y in twin 2 for monozygotic and dizygotic twins (i.e. using twin 1's IQ to predict twin 2's brain volume); if this cross-correlation is larger for the more genetically-similar monozygotic twins than for the dizygotic twins, the similarity indicates that the traits are not ...
Studies of human heritability often utilize adoption study designs, often with identical twins who have been separated early in life and raised in different environments. Such individuals have identical genotypes and can be used to separate the effects of genotype and environment.
Unlike monozygotic twins, dizygotic twins result from the fertilization of two eggs by two separate sperms within the same pregnancy. This causes the set of twins to have genetic variations, so their genetic information is unique from one another. In studies conducted between 1924 and 1976, there were more left-handed monozygotic twins.
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 ...