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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).
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.
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.
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.
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]
Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence.
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.
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 ...