Twin studies reveal the absolute and relative importance of environmental and genetic influences on individual's in a sample. Twin research is considered a key tool in behavioral genetics and in content fields, from biology to psychology. Twin studies are one part of a broader field – behavior genetics which includes all data that are genetically informative – siblings, adoptees. pedigree data etc. Twins are a valuable source for observation because they allow study of varying family environments (across pairs) and widely differing genotypes: monozygotic (MZ) or "identical" twins, share nearly 100% of their genetic polymorphisms, which means that most variation in pairs' traits (measured height, susceptibility to boredom, intelligence, depression, etc.) is due to their unique experiences. Dizygotic (DZ) or "fraternal" twins share only about 50% of their polymorphisms. Thus powerful tests of the effects of genes can be made. Twins share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) by virtue of being born in the same time and place. Discordance in MZ twins provides a powerful window into environmental effects (see below). The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (they are for most traits) this implicates their higher genetic similarity as a cause of variance in the behavior. By comparing many hundreds of families of twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior. 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 (e.g. intelligence quotient) and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait — as in the case of autism. METHOD
The power of twin designs arises from the fact that twins may be either monozygotic (identical (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 identical and fraternal 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);
shared environment - events that happen to both twins, affecting them in the same way; unshared, or unique, environment - events that occur to one twin but not another, or events that affect each twin in a different way. Typically these three components are called A (additive genetics) C (common environment) and E (unique environment); hence the acronym "ACE". It is also possible to examine non-additive genetics effects (often denoted D for dominance (ADE model); see below for more complex twin designs). The ACE model indicates what proportion of variance in a trait is heritable, versus the proportions which are due to shared environment or unshared environment. Research is carried out using SEM programs such as OpenMx, however the core logic of the twin design is the same, as described below: Monozygotic (identical - MZ) twins raised in a family share both 100% of their...
Please join StudyMode to read the full document