Methods
The power of twin designs arises from the fact that twins may be either monozygotic (MZ: developing from a single fertilized egg and therefore sharing all of their alleles) – or dizygotic (DZ: developing from two fertilized eggs and therefore sharing on average 50% of their polymorphic alleles, the same level of genetic similarity as found in non-twin siblings). These known differences in genetic similarity, together with a testable assumption of equal environments for MZ and DZ twins creates the basis for the twin design for exploring the effects of genetic and environmental variance on a phenotype.
The basic logic of the twin study can be understood with very little mathematics beyond an understanding of correlation and the concept of variance.
Like all behavior genetic research, the classic twin study begins from assessing the variance of a behavior (called a phenotype by geneticists) in a large group, and attempts to estimate how much of this is due to genetic effects (heritability), and how much appears to be due to shared or unique environmental effects - events that affect each twin in a different way, or events that occur to one twin but not another.
Typically these three components are called A (additive genetics) C (common environment) and E (unique environment); the so-called ACE Model. It is also possible to examine non-additive genetics effects (often denoted D for dominance (ADE model); see below for more complex twin designs).
Given the ACE model, researchers can determine what proportion of variance in a trait is heritable, versus the proportions which are due to shared environment or unshared environment. While nearly all research is carried out using SEM programs such as the freeware Mx, the essential logic of the twin design is as follows:
Monozygotic (MZ) twins raised in a family share both 100% of their genes, and all of the shared environment. Any differences arising between them in these circumstances are random (unique). The correlation we observe between MZ twins provides an estimate of A + C . Dizygous (DZ) twins have a common shared environment, and share on average 50% of their genes: so the correlation between DZ twins is a direct estimate of ½A + C . If r is the correlation observed for a particular trait, then:
- rmz = A + C
- rdz = ½A + C
Where rmz and rdz are simply the correlations of the trait in MZ and DZ twins respectively.
Twice difference between MZ and DZ twins gives us A: the additive genetic effect (Falconer's formula). C is simply the MZ correlation minus our estimate of A. The random (unique) factor E is estimated directly by how much the MZ twin correlation deviates from 1. (Jinks & Fulker, 1970; Plomin, DeFries, McClearn, & McGuffin, 2001).
The difference between these two sums, then, allows us to solve for A, C, and E. As the difference between the MZ and DZ correlations is due entirely to a halving of the genetic similarity, the additive genetic effect 'A' is simply twice the difference between the MZ and DZ correlations:
- A = 2 (rmz – rdz)
As the MZ correlation reflects the full effect of A and C, E can be estimated by subtracting this correlation from 1
- E = 1 – rmz
Finally, C can be derived:
- C = rmz – A
Read more about this topic: Twin Study
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