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Federal government websites often end in. The site is secure. Twin studies are a special type of epidemiological studies designed to measure the contribution of genetics as opposed to the environment, to a given trait. Despite the facts that the classical twin studies are still being guided by assumptions made back in the s and that the inherent limitation lies in the study design itself, the results suggested by earlier twin studies have often been confirmed by molecular genetic studies later.
Use of twin registries and various innovative yet complex software packages such as the SAS and their extensions e. The debate of nature versus nurture is known since antiquity. The close resemblance of twins has been the subject of many works of fiction as well. Means of distinguishing between the effects of tendencies received due to genes at birth and those imposed by the different environments they were exposed to during their lives after birth have always been the subject of interest to researchers.
The objection to statistical evidence in proof of the inheritance of peculiar traits has always been blamed upon similar environmental conditions playing as a confounder. It has been one of the favorite research tools of behavioral geneticists and psychologists since long, mainly utilized to estimate the heritability of traits and to quantify the effect of a person's shared environment family and unique environment the individual events that shape a life on a trait.
The similarity between twins has been a source of curiosity since time immemorial. The idea of using twins to study the heritability of traits can be traced back to the British researcher Sir Francis Galton. Based on the similarities he found between twins from 80 questionnaires, Galton proudly announced his conclusion to the world that nature soundly beats nurture, though his sample was too small and consisted of all upper-class individals, without any control group.
The first reported classical twin study was a study performed by Walter Jablonski in , investigating the contribution of heredity to refraction in human eyes. Jablonski examined the eyes of 52 twin pairs and by comparing the size of within-pair differences between identical and nonidentical twins was able to infer the heritability of a trait.
Even later, in , Thomas J. Bouchard, Jr. Segal at the University of Minnesota conducted one of the most famous research studies on genetic influence in humans. They studied identical twins separated since birth and raised by different families adoption studies , and so assumed that similarities, if found any, must be those that are heavily influenced by a person's genetic heritage. The study was invoked by the sensational news reports of two identical twins reunited after a lifetime apart.
James Lewis and James Springer were separated 4 weeks after birth and each infant was taken in by a different adoptive family. When they were reunited at the age of 39, an extraordinary collection of coincidences emerged. Both had second marriages with women named Betty. Both had police training and worked part-time with law enforcement agencies. Both had childhood pets named Toy. They had identical drinking and smoking patterns, and both chewed their fingernails to the nub.
Furthermore, identical twins reared apart were eerily similar to identical twins reared together in various measures of personality, personal mannerisms, expressive social behavior, and occupational and leisure-time interests. However, they did not find outstanding similarities between identical twins on measures such as standardized personality tests. Still, Bouchard's findings can be interpreted as strong support for genetic influences on personality.
Bouchard's data set was unique and probably a one-time event in history because modern adoption agencies no longer break up sets of identical twins. The modern-day classical twin study design relies on studying twins raised in the same family environments, which provides control not only for genetic background but also for shared environment in early life. Some assumptions are also made in twin studies; one of them is the assumption of random mating, which assumes that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait.
If, instead, people tend to choose mates like themselves, then fraternal twins could share a greater percentage of their genes than expected. In the case of nonrandom mating, fraternal twins would have more genetically influenced traits in common than expected because the genes they receive from their mothers and fathers would be similar to each other. Similarly, the assumption of equal environments is also made, which assumes that fraternal and identical twins raised in the same homes experience similar environments.
It is assumed that genes and the environment typically make only separate and distinct contributions to a trait. In general, it is also assumed that only one type of genetic mechanism—usually additive—operates for a particular trait.
However, traits can be inherited through different genetic mechanisms. Additive genetic mechanisms mix together the effects of each allele. For example, if genes for curly hair were additive, a curly-haired father and a straight-haired mother might have a child who has wavy hair.
There can be variations in the classical model, which may sometimes provide an added advantage, for example if twins are followed up over longer duration of time in longitudinal manner to assess the development of adult-onset traits and conditions.
This slight deviation will allow for a more complete and accurate assessment of environmental factors over time. Similarly, on combining with molecular genetics, information about the presence or absence of specific genetic variants to determine the impact on the trait of interest can be explored.
The advances in molecular genetics have substantiated hypotheses generated by the traditional twin research design by pinpointing the effects of a particular gene. Depending on the objectives of the study, one may need only monozygotic or dizygotic twins, or a combination of the two.
A twin registry is a database of information about both identical twins and fraternal twins, which is often maintained on a country-wide level or by an academic institution, such as a university or other research institution. Registration of some twin registries are mandatory by law, for example Norway, where all births of twins since have been registered by the Norwegian government. The twin registry in some countries have also made extensive outreach efforts, for example examining hospital birth records and then making multiple follow-up efforts such as in-person visits to find the twins and have them agree to be registered.
The Danish Twin Registry is the oldest national twin register in the world, initiated in , and contains information about more than 88, twin pairs born in Denmark since , in addition to triplets and quadruplets. The Danish Twin Registry is used as a source for studies on genetic influence on normal variation in clinical parameters associated with clinical studies of specific diseases, the metabolic syndrome and cardiovascular diseases, and aging and age-related health problems.
In all cohorts the ascertainment has been population-based and independent of the traits studied, although different procedures of ascertainment have been employed. The Swedish Twin Registry STR , managed by the Karolinska Institute, is the largest population-based twin registry in the world containing approximately 1,70, twins in 85, pairs born There are 1, 37 twins still alive and living in Sweden.
It is a unique resource for clinical, epidemiological, and genetic studies. Information has been mainly collected for demographic, medical, and lifestyle characteristics, with special attention to general health, cardiovascular and respiratory disease, legal drug use, and dietary and psychosocial conditions.
It is currently in the final phase of a complete telephone interview screening of all twins born in or earlier regardless of gender composition or vital status of the pair.
The famous Minnesota Twin Registry is a registry of all twins born in Minnesota from to and from to ; it was started in Both studies include over individuals comprising twins, siblings, and parents. The MTFS began in , when it enrolled 1, pairs of identical and same-sex fraternal twins and their families from the upper Midwest.
Twins were identified through public birth records and invited to participate with their parents in a full-day intake assessment. SIBS is a study of adoptive as well as biological sibling and their parents.
It is one of the largest studies of adolescents and their families ever conducted. It is comprised of a volunteer cohort of 14, twins 7, pairs and sets of triplets, and a population-based cohort of 19, 9, pairs twins and 89 sets of triplets. Though there are many small-scale twin studies published in various journals related to metabolic syndromes, 19 , 20 , 21 cardiovascular diseases, 22 respiratory diseases, 23 cerebrovascular diseases, 24 epilepsy, 25 , 26 dermatology, 27 , 28 ophthalmology, 29 , 30 psychology 31 chromosomal disorders, 32 and dentistry, 33 , 34 , 35 , 36 among others, there exists no twin registry in India to documenting the details of twins borne.
In addition, there is no provision of any law for mandatory twin registration. There are many practical problems associated with registering twins borne, one of the important concerns being the large number of home deliveries.
In a country where recording the birth weight of every newborn is not yet possible, mandatory twin registration may prove a distant dream for the already overburdened health-work force. Still, outreach activities can be planned to register the twins. Apex medical institutes and tertiary care centers can take the initiative to maintain and analyze data regarding twins in their areas to find out various genetic as well as environmental confounders in various diseases.
The large pool of data related to twins gathered can be analyzed in various ways with the help of new, innovative as well as complex statistical softwares. Twin studies intend to measure the heritability of a trait, which can be determined by concordance rates. Concordance rate CR for a disease or trait among identical and fraternal twin pairs is actually a statistical measure of probability: If one twin has a specific trait or condition, what is the probability that the other twin has or will develop that same trait or disease?
When MZ concordances are greater than DZ concordances, genetic influences are indicated. Quantitative genetic analyses and heritability estimation, including comparisons of concordances or intraclass correlations and structural equation modelling, can also be used to investigate the relative importance of genetic and environmental influences on a particular trait or condition.
Linear structural equations and fit models over all types of twins can be used to describe the causes of variation in a phenotype. Structural equation modelling of data can provide further refinement in the results. The total variance in the trait can be partitioned into genetic variance, common environmental variance including shared familial environmental variance, and unique environmental variance. In order to estimate the parameters of interest, the equation for the twins is written and the parameters studied.
Heritability, the relative importance of genetic influences for variation in a trait, is defined as genetic variance divided by the total phenotypic variance.
It is calculated for two normally distributed phenotypic variables that are both expressed as a dichotomy disease or no disease and reflect the similarity of twin pairs. Thus, differences in correlations between various groups provide information about the presence of genetic effects. Multivariate analyses of twin data can additionally offer estimates of the extent to which allelic variants and environment may influence different traits and conditions.
The co-twin control analyses method is applied in situ ations where one wants to investigate the importance of an expected risk factor after controlling for genetic and shared environmental effects.
It should be noted that the co-twin control method may entail control of factors in the biological pathway between exposure and disease, which may cause an underestimation of the exposure studied. In studies of disease-discordant twins, two control groups usually are used: External controls and internal or co-twin controls. The analysis classically is conducted in three steps. Step 1 : Association between exposure and outcome comparison with external controls.
The first step, which is essentially a classic case-control study, is to compare twins diagnosed as cases with external controls other twins not related to the index probands , and to evaluate the risk for disease given an exposure.
This approach facilitates comparisons with results from ordinary case-control studies on singletons. Step 2 : Controlling for confounding from unmeasured early environment healthy co-twin as control. In the second step, the healthy co-twin in both MZ and DZ twin pairs can be used as a control for the diseased twin. Because twins share the same intrauterine environment and typically are reared together, the co-twin control method provides a very effective tool to minimize confounding by differences in an unmeasured childhood or adolescent environment.
If analyses with external controls show associations between exposure and disease and the relative risk remains similarly high in the within-pair co-twin analyses, it speaks in favor of a causal effect of the exposure on the disease. On the other hand, if the relative risk is not increased in the within-pair comparisons but only in the first-step analyses with external comparisons , this indicates that environmental factors early in life for example, fetal environment, maternal smoking, or childhood socioeconomic status SES are responsible for the initially observed findings.
If the relative risks from steps 1 and 2 differ, a direct test of significance of difference in risks can be performed by applying regression: The exposure on control status external versus internal control.
Step 3 : Controlling for unmeasured genetic background healthy monozygotic co-twin as control.
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