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.
- studying the significance of the genotype-environment interaction
- the contribution of specific polymorphisms 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 for individuals;
- analysis of longitudinal data to study causes of phenotypic stability and tracking over time; and
- testing of Genotype × Environment using measured environmental indices.
- 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 G × E 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 (Blickstein et al, 2005).
Case studies for Nature versus Nurture Debate
Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart” (1990), by Thomas J. Bouchard Jr, David T. Lykken, Matthew McGue, Nancy L. Segal and Auke Tellegen
According to the authors, the monozygotic reared apart twins showed similarities in behavior, interests, religion, and intelligence regardless of their rearing environment. The IQ scores were also consistent between the monozygotic reared apart twins. Bouchard and his colleagues compare the correlation of IQ between monozygotic reared apart twins and the correlation of IQ between monozygotic twins that are reared together, and found that those values were similar. That similarity led them to conclude that rearing monozygotic twins together or apart has no significant effect on IQ. Based on that comparison, Bouchard and his colleagues conclude that monozygotic twins reared apart were just as similar as monozygotic twins reared together.
Finally, the authors state three different possibilities that could cause the similarity between monozygotic twins reared apart. First, the authors state that the genetic factors strongly influence the behavior and general intelligence of individuals and accounts for about 70 percent of the variation in IQ. Second, the authors claim that environmental factors did not have a significant effect on the development of monozygotic twins when they were reared apart which caused them to be similar in social attitude due to the genetic factor. Finally, the authors state that genetic factors control the effects of environmental factors. That means the even if environmental factors had a significant effect, the genetic similarity between the monozygotic reared apart twins might have influenced the twins to focus on, and retain similar environmental influences and, as a result, the twins become similar.
The Minnesota twin studies has been cited over 1500 times. It also raised many questions about the importance of heredity and the environment in shaping human behavior. The evidence that Bouchard and his colleagues provided through their study supported the argument that genetic factors and inheritance play a large role in the development of individuals and the interests and characteristics they show.
Chicago study : MZ twin reared apart shown different IQ level , reveal to us impact of environment on IQ level.
Thus both case studies proved that IQ has both genetic as well as environment effect, concludes it it as nature and nurture shape traits rather than nature vs nurture.