twin studies - senecio7の日記

Evans DM, Gillespie NA & Martin NG 2002 Biometrical genetics. Biol Psychol 61:33-51.

twin studies
dominant genetic and common environmental components of variance cannot be estimated simultaneously
these components are negatively confounded in a study of twins reared together
genetic dominance acts to inflate the correlation between MZ twins relative to the correlation between DZ twins
common environmental effects inflate the correlation between DZ twins relative to the correlation between MZ twins

when the correlation between MZ twins is less than twice the DZ correlation, we estimate σ_C² and assume that genetic dominance is absent
conversely when the MZ correlation is more than twice the DZ correlation we estimate σ_D² and assume that σ_C² is zero

twins differ from singletons in several important aspects
results derived from twin studies do not generalize to the rest of the population
most of these studies have examined young twins, and have not matched twins with singletons in terms of genetic or environmental backgrounds
a number of studies comparing older twins with singletons have failed to find differences in physical characteristics, cognitive abilities or in the prevalence of many adult diseases, suggesting that any differences between twins and singletons are 'washed out' early in development
the other major assumption of the classical twin study is the 'Equal Environments Assumption' that MZ twin pairs experience the same degree of environmental similarity as DZ twin pairs
there is strong evidence that MZ twins are treated more similarly than their DZ counterparts
it is questionable whether this environmental similarity causes increased phenotypic concordance