Establishing twin registers have an enormous potential for research on the genetics of complex traits. Some of them have existed for decades and have carefully collected longitudinal data on behavioural traits, diseases and environmental risk factors in large samples of twins and their families. By making comparisons between monozygotic (MZ) and dizygotic (DZ) twins, twin registers represent some of the best resources for evaluating the importance of genetic variation in susceptibility to disease. They are an excellent source for studying the significance of the genotype-environment interaction and of the contribution of specific npolymorphisms to the total genetic variance. Recent advances in statistical modeling allow simultaneous analysis of many variables in relatives such as MZ and DZ twins.
Classical twin studies.: The classical twin study compares phenotypic resemblances of MZ and DZ twins. MZ twins derive from a single fertilized egg and therefore inherit identical genetic material. Comparing the resemblance of phenotypic characters of MZ twins for a trait or disease with DZ twins offers the first estimate of the extent to which genetic variation determines phenotypic variation of that trait. If MZ twins resemble each other more than DZ twins, then the heritability (h2) of the phenotype can be estimated from twice the difference between MZ and DZ correlations.
Types of twin study and their applications
- Classical MZ-DZ comparison: These studies estimate the contributions of genetic and environmental effects to phenotypic variance, and test, for example, for age, cohort and sex differences in gene expression.
- Multivariate analyses: simultaneous analysis of correlated trait This type of analysis involves:
- direction of phenotypic casuality;
- causes of co-morbidity of two or more traits: multivariate modelling of
environmental and genetic correlations between traits;
- multivariate modeling to obtain genotypic (or environmental) values
- analysis of longitudinal data to study causes of phenotypic stability and
tracking over time; and
- testing of Genotype x Environment using measured environmental
- Co-twin control study: Case-control studies of MZ twins who are perfectly
matched for genes and family background; such studies can also be used to
study gene expression in discordant twins
Extended Twin Study: Studies of Twins and Their Families
- Parents of twins can be included to study cultural transmission and covariance,
- Parents of twins can be studied in a quasi-longitudinal design to determine genetic and environmental stability,
- Assortative mating can be studied if spouses of twins are included; social interactions and special twin effects, such as prenatal hormone transition, the ‘private language’ of twins and shared prenatal environment, can be studied if siblings of twins are included, and
- Maternal effects, Genotype Environment correlation and imprinting can be studied if offspring of MZ twins are included.
Twin studies are very helpful to understand the role of genes and environment for multifactorial traits (such as body height and weight, neuroticism and blood lipid levels) and complex diseases (such as obesity, depression and cardiovascular disease). Lifestyle risk factors such as smoking, exercise, diet etc. are important for the development of complex diseases are often considered to be ‘environmental’, they might themselves be influenced by genes .