Modern twin studies have shown that almost all traits are in part influenced by genetic differences, with some characteristics showing a strong influence (e.g. height), others an intermediate level (i.e. IQ) and some more complex heritabilities, with evidence for different genes affecting different elements of the trait - for instance Autism.
While twins have been of interest to scholars since early civilization, such as the early physician Hippocrates (5th c. BCE), who attributed similar diseases in twins to shared material circumstances, and the stoic philosopher Posidonius (1rst c. BCE), who attributed such similarities to shared astrological sex circumstances, the modern history of the twin study derives from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior.
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), how much appears to be due to shared environmental effects, and how much is due to unique environmental effects - events occurring 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 (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:
Monozygous (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 50% of their genes: so the correlation between DZ twins is a direct estimate of ½A + C . If r is the rate observed for a particular trait, then:
These two equations allow us to derive A, C, and E :
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. 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).
An initial limitation of the twin design is that is does not afford an opportunity to consider both Shared Environment and Non-additive genetic effects simultaneously. This limit can be addressed by including additional siblings to the design.
A second limitation is that gene-environment correlation is not detectable as a distinct effect. Addressing this limit requires incorporating adoption models, or children-of-twins designs, to assess family influences uncorrelated with shared genetic effects.
The Twin Method has been subject to criticism from Statistical Genetics, Statistics and Psychology, with some arguing that conclusions reached via this method are ambiguous or meaningless. Core elements of these criticisms and their rejoinders are listed below:
For example, Peter Schonemann has criticized methods for estimating heritability developed in the 1970s. He has also argued that the heritability estimate from a twin study may reflect factors other than shared genes. Using the statistical models published in Loehlin and Nichols (1976), the narrow heritability’s of HR of responses to the question “did you have your back rubbed” has been shown to work out to .92 heritable for males and .21 heritable for females, and the question “Did you wear sunglasses after dark?” is 130% heritable for males and 103% for females
Twins are not a random sample of the population, and they differ in their developmental environment. In this sense they are not representative
For example: Dizygotic (DZ) twin births are affected by many factors. Some women frequently produce more than one egg at each menstrual period and, therefore, are more likely to have twins. This tendency may run in the family either in the mother's or father's side of the family, and often runs through both. Women over the age of 35 are more likely to produce two eggs. Women who have three or more children are also likely to have dizygotic twins. Artificial induction of ovulation and in vitro fertilization-embryo replacement can also give rise to DZ and MZ twins .
The effects of genes depend on the environment they are in. Possible complex genetic effects include G*E interactions, in which the effects of a gene allele differ across different environments. Simple examples would include situations where a gene multiplies the effect of an environment (in this case the slope of response to an environment would differ between genotypes). A second effect is "GE correlation", in which certain allelles occur more frequently than others in certain environments. If a gene causes a person to enjoy reading, then children with this allele are likely to be raised in households with books in them (due to GE correlation: one or both of their parents has the allele and therefore both accumulates a book collection and passes on the book-reading allele). Such effects can be assessed by measuring the purported environmental correlate (in this case books in the home) directly.
Often the role of environment seems maximal very early in life, and decreases rapidly after compulsory education begins. This is observed for instance in reading (Byrne etal 2006) as well as intelligence (Deary et al, 2006). This is an example of a G*Age effect and allows an examination of both GE correlations due to parental environments (these are broken up with time), and of G*E correlations caused by individuals actively seeking certain environments (Plomin et al., 1987).
While concordance studies compare traits which are either present or absent in each twin, correlational studies compare the agreement in continuously varying traits across twins.
For a group of twins, pairwise concordance is defined as C/(C+D), where C is the number of concordant pairs and D is the number of discordant pairs.
For example, a group of 10 twins have been pre-selected to have one affected member (of the pair). During the course of the study four other previously non-affected members become affected, giving a pairwise concordance of 4/(4+6) or 4/10 or 40%.
For a group of twins in which at least one member of each pair is affected, probandwise concordance is a measure of the proportion of twins who have the illness who have an affected twin and can be calculated with the formula of 2C/(2C+D), in which C is the number of concordant pairs and D is the number of discordant pairs.
For example, consider a group of 10 twins that have been pre-selected to have one affected member. During the course of the study, four other previously non-affected members become affected, giving a probandwise concordance of 8/(8+6) or 8/14 or 57%.
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Several academic bodies exist to support behavior genetic research, including the Behavior Genetics Association , the International Society for Twin Studies, and the International Behavioural and Neural Genetics Society Behavior genetic work features prominently in several more general societies, for instance the International Society of Psychiatric Genetics.
The following Twin Studies are ongoing studies that are recruiting subjects:
Largest Twin Study of Age-Related Macular Degeneration Finds Genetics and Environment Play Large Role in Development of the Blinding Disease; Results Published in March Issue of Archives of Ophthalmology.
Mar 14, 2005; Byline: Massachusetts Eye & Ear Infirmary BOSTON, March 14 (AScribe Newswire) -- Age-related macular degeneration (AMD) is the...