Humans are not the only animals capable of producing monozygotic twins. Armadillos are known for them and they have been reported in dogs, horses and pigs as well, but it is difficult to get a handle on how common they are in these species.
Without DNA testing, how do we know if two pups came from the same zygote? Humans have become more adept at it, with three to five in 1, live births being made up of monozygotic twins. The odds of having twins go up with advanced age, with the number of previous pregnancies, and with the use of assisted reproductive techniques, among other factors.
Twins have always fascinated humanity and they have been the source of many myths, including that they are gifted with a psychic connection. The answer is quite simple, as long-time friends and partners can attest: shared experiences. As for the astonishing stories of one twin knowing the other is in danger, these anecdotes tend to have more mundane explanations when we dig into them. The case of Gemma and Leanne Houghton sure seems paranormal. The two twins were teenagers and Gemma suddenly felt compelled to check on her sister.
But as the deputy editor for Skeptical Inquirer magazine, Benjamin Radford, pointed out in an article on twins, Leanne had a history of seizures and her sister had been told to keep an eye on her. No telepathy needed. But the most persistent myth about so-called identical twins is baked into the name. MZ female twins discordant for X-linked diseases: a review.
Acta Genet Med Gemellol Roma , 43 , pp. Wong, I. Phenotypic differences in genetically identical organisms: the epigenetic perspective. Hum Mol Genet, 14 , pp.
Petronis, I. Gottesman, P. Kan, J. Kennedy, V. Basile, A. Paterson, et al. Monozygotic twins exhibit numerous epigenetic differences: clues to twin discordance?. Schizophr Bull, 29 , pp. Bianchi, L. Integration of noninvasive DNA testing for aneuploidy into prenatal care: what has happened since the rubber met the road?.
Clin Chem, 60 , pp. Kruyer, M. Mila, G. Glover, P. Carbonell, F. Ballesta, X. Am J Hum Genet, 54 , pp. Ishii, Y. Makita, A. Ogawa, S. Amamiya, M. Yamamoto, A. Miyamoto, et al. The role of different X-inactivation pattern on the variable clinical phenotype with Rett syndrome.
Brain Dev, 23 , pp. SS Medline. Abbadi, C. Philippe, M. Chery, H. Gilgenkrantz, F. Tome, H. Collin, et al. Additional case of female monozygotic twins discordant for the clinical manifestations of Duchenne muscular dystrophy due to opposite X-chromosome inactivation.
Am J Med Genet, 52 , pp. Jorgensen, J. Philip, W. Raskind, M. Matsushita, B. Christensen, V. Dreyer, et al. Different patterns of X inactivation in MZ twins discordant for red-green color-vision deficiency. Am J Hum Genet, 51 , pp. Trejo, C. Derom, R. Vlietinck, W. Ollier, A. Silman, G. Ebers, et al. X chromosome inactivation patterns correlate with fetal—placental anatomy in monozygotic twin pairs: implications for immune relatedness and concordance for autoimmunity.
Mol Med, 1 , pp. Weksberg, C. Shuman, O. Caluseriu, A. Smith, Y. Fei, J. Nishikawa, et al. Hum Mol Genet, 11 , pp. Leonard, F. Bernier, N. Rudd, G. Machin, F. Bamforth, S. Bamforth, et al.
Two pairs of male monozygotic twins discordant for Wiedemann—Beckwith syndrome. 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. In the third step, analyses are applied only to disease-discordant MZ pairs. This design is ideal in controlling for potential confounding from genetic factors, as the cases and controls are genetically identical.
Thus, one is confident that an observed effect is not confounded by genetic predisposition. If the twin with the exposure in MZ pairs more often has a specific chronic disease, this will provide strong support for the likelihood that the exposure contributes to the causation of the disease. On the other hand, if an association exists in analyses of external controls among disease-discordant DZ pairs but not among MZ pairs, genetic effects have probably confounded the results.
As mentioned above, one can also focus on exposure-discordant pairs that are followed longitudinally for a disease outcome. In this case, t -tests or proportional hazard regressions can be utilized for estimating the relative risk between exposed and unexposed individuals, whereas matched analyses should be used in within-pair analyses, similar to the disease-discordant pairs.
Finally, as the twin registries contain longitudinal data on large samples, they can therefore be used for conventional epidemiological analyses disregarding twinship status. Several studies have been performed on the association between exposure and outcomes using the registries as a population-based cohort or as the basis for nested case-control studies. When using twin data for these types of studies, the dependency between the twins in a pair should be taken into account by using generalized linear models or other techniques.
Mx is a matrix algebra interpreter and numerical optimizer for structural equation modeling and other types of statistical modeling of data. Twin studies allow disentanglement of the shared genetic and environmental factors for the trait of interest. In the other study, twins were asked how many hours per night they typically sleep.
Both studies aimed to find out how much genes contribute to the traits of interest IQ and sleep time. To get an idea of how genetic a trait is, they compared how similar the pairs of identical twins were and how similar the pairs of non-identical twins were.
Because identical twins share all of their genes, their measurements of IQ and sleep time will be more similar the bigger role genes play in it i. On the other hand, we expect non-identical twins to be less similar than identical twins, but we still expect them to be somewhat similar because some of their genes are shared.
Keep in mind that what is being compared is not the trait measurements, but rather how similar the twins are both with high IQ, or both sleeping the same amount of time. In Figure 1 , similarity can be observed by how close the dots twin pairs are to the green line in the middle the perfect similarity line.
These studies showed that IQ is very heritable although the environment still plays a role in determining your IQ. We can tell this because the IQ of identical twins are almost always the same and non-identical twins are only sometimes the same. On the other hand, the graph for sleep of identical twins shows a big spread from the middle green line. This means that identical twins show big differences between them, and because they have the same genes this big differences are caused by the environment.
This means that sleep length has a low heritability. Studies like this have identified that personality, intelligence, poor eyesight, and even mental diseases, such as bipolar disorder and schizophrenia have a medium to high heritability i.
There is almost no limit to what kind of trait or disease can be studied. If it can be measured or classified, we can estimate its heritability!
We study twins to understand how much of the difference in a trait between people is caused by genes and how much is caused by the environment. These studies are important because they help scientists quantify genetic and modifiable environmental factors that increase the risk of certain diseases. Scientists have done many studies like these. Around 18, human traits, including height, body weight, and several diseases, have been studied so far [ 5 ].
So, when you see a pair of twins, remember how genetically special and valuable they are for science and health research. Mutations can arise suddenly at any time and are usually neither good nor bad for you.
Genes contain information about your traits. In this case, we measure how alike each twin is with his co-twin. For example, the amount of exercise you do.
0コメント