So they looked at 968 dizygotic twins to make sure this wasn't the case.
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Fraternal -- or dizygotic -- twins form from two eggs that have been fertilized by two of the father's sperm, producing two genetically unique siblings.
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This is the most common type of twin. Dizygotic twins, like any other siblings, will practically always have different sequences on each chromosome, due to chromosomal crossover during meiosis. Dizygotic twins share only 50 percent of each other's genes, which resemble amongst siblings that are conceived and born at different times. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age.
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Unlike monozygotic twins, dizygotic twins result from the fertilization of two eggs by two separate sperms within the same pregnancy. This causes the set of twins to have genetic variations, so their genetic information is unique from one another. Considering the pathological left handedness syndrome, it was also assumed that monozygotic twins have a higher prevalence for left handedness than dizygotic twins because monozygotic twins experience more birth complications than dizygotic twins. In studies conducted between 1924 and 1976 there were more left-handed monozygotic twins.
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IVF techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.
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As of 2013, the cohort was predominantly female (83%) and middle-aged or older, with about an equal division of monozygotic and dizygotic twins.
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This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%; see the references section, below, for more details. Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in London in 1993 to a white mother and Caribbean father.
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According to the theory, genes play a strong role in the development of alcoholism. Twin studies, adoption studies, and artificial selection studies have shown that a person's genes can predispose them to developing alcoholism. Evidence from twin studies show that concordance rates for alcoholism are higher for monozygotic twins than dizygotic twins—76% for monozygotic twins and 61% for dizygotic twins. However, female twin studies demonstrate that females have much lower concordance rates than males.
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Monozygotic, or identical, twins share 100% of their genes, while dizygotic, or fraternal, twins share on average 50% of their genes. The classic twin study compares monozygotic and dizygotic twins. If the monozygotic twins resemble each other much more closely than the dizygotic twins, then it is likely that genetics play a strong role in the development of the trait of interest. These studies showed that the genetic influences on obsessive- compulsive symptoms were 45 to 65% in children.
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Women who have three or more children are also likely to have dizygotic twins. Artificial induction of ovulation and in vitro fertilization-embryo replacement can also give rise to fraternal and identical twins.
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Specifically, 15 percent of monozygotic twins were left-handed while 13 percent of dizygotic twins were left-handed. In another study the frequency of right-handed and left-handed pairs of dizygotic twins is about 23%, while twins with both individuals displaying left-handedness is less than 4% and the frequency of pairs of monozygotic twins in which only one twin is left-handed is about 21% and in which both twins are left-handed is less than 4%. However, there was no difference in the handedness frequency between monozygotic and dizygotic twins. Currently there is not much evidence to further prove the idea that monozygotic twins have a higher prevalence for left handedness using the pathological left-handedness syndrome because of the improvements within medicine causing a decrease in birth defects and complications.
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Monozygotic twins had the most overlap in their peer networks (82%), followed by same-sex dizygotic twins (67%), same-sex virtual twins (e.g., unrelated peers matched on certain characteristics; 62%), friend-friend pairs (48%), opposite-sex dizygotic twins (42%), same-sex full siblings (39%), opposite sex virtual twins (37%), and opposite-sex full siblings (27%). Genetics, sex (same- or opposite-sex), age, and relationship intimacy affected rates of peer overlap. Another example used pairs of corporations engaged in a business alliance as the focal unit, and found that the more common partners (i.e.
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Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.
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Occasionally two follicles reach the 10 mm stage at the same time by chance and as both are equally sensitive to FSH both survive and grow in the low FSH environment and thus two ovulations can occur in one cycle possibly leading to non-identical (dizygotic) twins.
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Piontelli, Alessandra (1992). From Fetus to Child: An Observational and Psychoanalytic Study, London 1992. See also: Alessandra Piontelli, Luisa Bocconi, Chiara Boschetto, Alessandra Kustermann, Umberto Nicolini: Differences and similarities in the intra-uterine behaviour of monozygotic and dizygotic twins. In: Twin Research. 1999, 2, S. 264–273 (S.
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Sibling influence, gender roles, and the sexual socialization of urban early adolescent girls. The Journal of Sex Research. 40(1). 101-110. Biological studies show that dizygotic twins with the opposite- sex co-twin gender show more sex-typed behaviors than same-sex twins.Mehta, C. M., Strough, J. (2010).
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Evidence for the genetic basis of facial recognition abilities in the general population, however, comes from studies on face perception in twin participants by Wilmer, J. B. et al. in 2009, in which the facial recognition scores on the Cambridge Face Memory test were twice as similar for monozygotic twins in comparison to dizygotic twins. This finding was supported by a twin study on the genetic bases of facial recognition by Zhu, Q. et al. in (2009) which found a similar difference in facial recognition scores when comparing monozygotic and dizygotic twins and Shakeshaft, N. G. & Plomin, R. (2015), which determined the heritability of facial recognition to be approximately 61%, using a similar set of twin studies.
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Individual twin studies and meta-analyses of twin studies have estimated the heritability of risk for schizophrenia to be approximately 80% (this refers to the proportion of variation between individuals in a population that is influenced by genetic factors, not the degree of genetic determination of individual risk), but the heritability estimate varies from 41 to 87%. Concordance rates between monozygotic twins vary in different studies, approximately 50%; whereas dizygotic twins was 17%. Some twin studies have found rates as low as 11.0%-13.8% among monozygotic twins, and 1.8%-4.1% among dizygotic twins, however. Family studies indicate that the closer a person's genetic relatedness to a person with schizophrenia, the greater the likelihood of developing the disorder.
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The monozygotic twins also chose spouses and best friends who were more similar to their co-twins' friends and spouses than did dizygotic twins. The authors said there was a substantial genetic contribution to these effects in the twins. Similarity to social partners was higher on more heritable characteristics than on less.
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Heteropaternal superfecundation is common in animals such as cats and dogs. Stray dogs can produce litters in which every puppy has a different sire. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits.
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They looked at a sample of 18 families. Based on the comparison of a series of monozygotic and dizygotic twins, Freire-Maia (1961) concluded that the preference in the type of hand clasping was affected by certain genetic factors, and (perhaps) to a significant extent.Freire-Maia A. (1961): Twin data on hand clasping: a reanalysis. Acta Genet. Statist. Med.
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Weinberg was born in Stuttgart and studied medicine at Tübingen, Berlin and Munich, receiving an M.D. in 1886. He returned to Stuttgart in 1889, where he remained running a large practice as a gynecologist and obstetrician until he retired to Tübingen a few years before his death in 1937. Much of his academic life he spent studying genetics especially focusing on applying the laws of inheritance to populations. Additional contributions by Weinberg to statistical genetics included the first estimate of the rate of twinning – Realizing that identical twins would have to be same-sex, while dizygotic twins could be either same or opposite sex, Weinberg derived the formula for estimating the frequency of monozygotic and dizygotic twins from the ratio of same sex and opposite twins to the total of maternities.
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Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac.
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Discordance, in genetics typically means that a similar trait is not shared between twin members. Studies of twins have shown that genetic traits of monozygotic twins are fully concordant whereas in dizygotic twins, half of genetic traits are concordant, while the other half are discordant. Discordant rates that are higher than concordant rates express the influence of the environment on twin traits.
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His work involved investigating the phenomenon of tolerance and transplantation immunity. He collaborated with Rupert E. Billingham and they did research on problems of pigmentation and skin grafting in cattle. They used skin grafting to differentiate between monozygotic and dizygotic twins in cattle. Taking the earlier research of R. D. Owen into consideration, they concluded that actively acquired tolerance of homografts could be artificially reproduced.
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Researchers have used this method to investigate genetic correlates of musical pitch recognition in both monozygotic and dizygotic twins. Drayna, Manichaikul, Lange, Snieder and Spector (2001) determined that the variation in musical pitch recognition is primarily due to highly heritable differences in auditory functions not tested by conventional audiologic methods. Therefore, the DTT method may provide a benefit to advancing research studies similar to this one.
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When carried out between members of a shared social group (e.g., a village, a tribe, a nation, humanity) people offer "fair" (i.e., 50:50) splits, and offers of less than 30% are often rejected.See One limited study of monozygotic and dizygotic twins claims that genetic variation can have an effect on reactions to unfair offers, though the study failed to employ actual controls for environmental differences.
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Genetic and hereditary causes are being considered and several epidemiologic findings indicate considerable genetic influence especially for early onset cases. First degree relatives have a 2.5-fold risk, and nearly 6-fold risk when considering early onset cases. Monozygotic twins have double concordance rate for hysterectomy compared to dizygotic twins. Expansion of uterine fibroids occurs by a slow rate of cell proliferation combined with the production of copious amounts of extracellular matrix.
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The strong genetic component is borne out in studies on monozygotic twins, with a concordance of 38–55%, with an even higher concordance of circulating thyroid antibodies not in relation to clinical presentation (up to 80% in monozygotic twins). Neither result was seen to a similar degree in dizygotic twins, offering strong favour for high genetic aetiology. Hashimoto's thyroiditis is associated with CTLA-4 (cytotoxic T-lymphocyte antigen-4) gene polymorphisms. CTLA-4 downregulates.
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Mixed twins are fraternal twins born to multiracial families which differ in skin color and other traits considered to be racial features. From a biological point of view, the differences in these fraternal or dizygotic twins from two biracial parents are not surprising. In humans, a relatively small number of genes are thought to be responsible for human skin color. Different alleles or gene variants code for differences in the melanin found within the skin.
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Twin studies manipulate the environmental factors of behavior by examining if identical twins raised apart are different from twins raised together. Before the advancement of molecular genetics, twin studies were almost the only mode of investigation of genetic influences on personality. Heritability was estimated as twice the difference between the correlation for identical, or monozygotic, twins and that for fraternal, or dizygotic, twins. Early studies indicated that personality was fifty percent genetic.
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PCOS is a heterogeneous disorder of uncertain cause.Page 836 (Section:Polycystic ovary syndrome) in: There is some evidence that it is a genetic disease. Such evidence includes the familial clustering of cases, greater concordance in monozygotic compared with dizygotic twins and heritability of endocrine and metabolic features of PCOS. There is some evidence that exposure to higher than typical levels of androgens and the anti-Müllerian hormone (AMH) in utero increases the risk of developing PCOS in later life.
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Referencing the mean proportions of left handedness singletons are 8.5 percent, dizygotic twins are 14 percent and monozygotic twins are 14.5 percent. Using this data, it is theorized that twins have higher prevalence for left handedness because of prenatal complications. For example, the pathological left-handedness syndrome has been speculated to contribute to why twins having a higher prevalence for left handedness. Pathological left-handedness syndrome states that when an injury occurs during early development it effects lateralization and ultimately handedness.
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Russell, Wells, and Rushton (1985) reanalyzed several previous studies on similarities between spouses and concluded there is higher similarity on the more heritable characteristics. Rushton examined blood group genes and found that sexually interacting couples had more similar blood group genes than randomly paired individuals. Rushton and Bons (2005) examined personality, attitude, and demographic characteristics for similarity among different groups of people. Monozygotic twins resembled one another (r = 0.53) more than dizygotic twins (r = 0.32), pairs of spouses (r = 0.32), and pairs of best friends (r = 0.20).
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Lee Anne Thompson is an American psychology professor known for her work in behavior genetics and the biological processes involved in intelligence. Thompson earned her B.A. from Case Western Reserve University in 1982, then attended University of Colorado at Boulder, earning an M.A. in 1985 and her Ph.D. in 1987. She currently teaches at Case Western and is on the editorial board of Intelligence. Thompson co-authored a widely cited twin study on communication disorders which found higher concordance in monozygotic twins than dizygotic twins.
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The reasons as to why twins may have a higher left-handedness prevalence than singletons are still being debated and nothing have yet to be conclusively determined. Many studies have used both monozygotic and dizygotic twins to analyze how the environment and genetics may influence writing behaviors to try to answer the question of why twins have a higher frequency of left-handedness, but no differences were observed. As previously stated, more research needs to be conducted to find the correlation between twins and handedness.
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Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes.
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However, dizygotic twins may also look very different from each other (for example, be of opposite sexes). Studies show that there is a genetic proclivity for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.
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Some of the susceptibility genes may be population specific. Genetic studies of the rates of disease in families supports the genetic basis of this disease with a heritability of >66%. Identical (monozygotic) twins were found to share susceptibility to the disease at >35% rate compared to fraternal (dizygotic) twins and other full siblings who only showed a 2–5% concordance in shared inheritance. Since SLE is associated with many genetic regions, it is likely an oligogenic trait, meaning that there are several genes that control susceptibility to the disease.
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The pattern of cortical gyri and sulci is not random; most of the major convolutions are conserved between individuals and are also found across species. This reproducibility may suggest that genetic mechanisms can specify the location of major gyri. Studies of monozygotic and dizygotic twins of the late 1990s support this idea, particularly with regards to primary gyri and sulci, whereas there is more variability among secondary and tertiary gyri. Therefore, one may hypothesize that secondary and tertiary folds could be more sensitive to genetic and environmental factors.
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This skewing can then be inherited by progeny cells, or increased by secondary selection. The X-chromosome controlling element (Xce) gene in mice has been found to influence genetically mediated skewing. It is unknown whether a similar gene plays a role in human X-inactivation, although a 2008 study found that skewing in humans is mostly caused by secondary events rather than a genetic tendency. There is a much higher concordance rate in genetically identical (monozygotic) twins compared to non-identical (dizygotic) twins, which suggests a strong genetic input.
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Some research has suggested that satisficing/maximizing and other decision-making strategies, like personality traits, have a strong genetic component and endure over time. This genetic influence on decision-making behaviors has been found through classical twin studies, in which decision-making tendencies are self-reported by pairs of twins and then compared between monozygotic and dizygotic twins. This implies that people can be categorized into "maximizers" and "satisficers", with some people landing in between. The distinction between satisficing and maximizing not only differs in the decision-making process, but also in the post-decision evaluation.
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Twin studiesClassical Twin Studies and Beyond, www.nature.com provide a way to understand how genotype affects an observable characteristic (called a phenotype). In short, identical (monozygotic) twins carry the same alleles for 100% of their genes whereas fraternal (dizygotic) twins will carry different alleles at 50% of the genes for which their parents had different genotypes. So if some characteristic (say, depression) that is observed in one identical twin is always observed in the other one, but this does not hold for fraternal twins, then one can conclude that heredity plays an important role in causing the condition.
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For example, philosopher Daniel Dennett has proposed that humans are genetically predisposed to have a theory of mind because there has been evolutionary selection for the human ability to adopt the intentional stance. The intentional stance is a useful behavioral strategy by which humans assume that others have minds like their own. This assumption allows you to predict the behavior of others based on personal knowledge of what you would do. In 1951, Hans Eysenck and Donald Prell published an experiment in which identical (monozygotic) and fraternal (dizygotic) twins, ages 11 and 12, were tested for neuroticism.
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Over time, this can result in measurable differences between biological and chronological age. Epigenetic changes have been found to be reflective of lifestyle and may act as functional biomarkers of disease before clinical threshold is reached. A more recent study, where 114 monozygotic twins and 80 dizygotic twins were analyzed for the DNA methylation status of around 6000 unique genomic regions, concluded that epigenetic similarity at the time of blastocyst splitting may also contribute to phenotypic similarities in monozygotic co-twins. This supports the notion that microenvironment at early stages of embryonic development can be quite important for the establishment of epigenetic marks.
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He published a book in 1947 called Dimensions of Personality, describing the personality dimensions of Extraversion and Neuroticism. Gray, a student of Eysenck, studied personality traits as individual differences in sensitivity to rewarding and punishing stimuli. The significance of Gray's work and theories was his use of biology to define behavior, which stimulated a lot of subsequent research.. In 1951, Hans Eysenck and Donald Prell published an experiment in which identical (monozygotic) and fraternal (dizygotic) twins, ages 11 and 12, were tested for neuroticism. It is described in detail in an article published in the Journal of Mental Science.
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It removes the possible influence of dominance and epistatic effects which, if present, will tend to make monozygotic twins more similar than dizygotic twins and mask the influence of shared environmental effects. This is a limitation of the twin design for estimating c^2. However, the general conclusion that shared environmental effects are negligible does not rest on twin studies alone. Adoption research also fails to find large (c^2) components; that is, adoptive parents and their adopted children tend to show much less resemblance to one another than the adopted child and his or her non-rearing biological parent.
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Pillard is also well known for a series of studies he coauthored with the psychologist J. Michael Bailey, which examined the rate of concordance of sexual identity among monozygotic twins, dizygotic twins of the same sex, non- twin siblings of the same sex, and adoptive siblings of the same sex. In all studies they found rates of concordance variantly consistent with the hypothesis that homosexuality has a significant genetic component. The Council for Responsible Genetics and other researchers have criticized this work for using a self-selected sample, a problem which later studies have attempted to remedy.
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GDF9 mutations are present in women with premature ovarian failure, in addition to mothers of dizygotic twins.Yan, C., Wang, P., DeMayo, J., DeMayo, F., Elvin, J., Carino, C., Prasad, S., Skinner, S., Dunbar, B., Dube, J., Celeste, A. and Matzuk, M. (2001). Synergistic Roles of Bone Morphogenetic Protein 15 and Growth Differentiation Factor 9 in Ovarian Function. Molecular Endocrinology, 15(6), pp.854-866 Three particular missense mutations GDF9 P103S, GDF9 P374L and GDF9 R454C have been found, although GDF9 P103S is present in women with dizygotic twins as well as women with premature ovarian failure.
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That is, estimates of shared environmental effects (c^2) in human studies are small, negligible, or zero for the vast majority of behavioural traits and psychiatric disorders, whereas estimates of non-shared environmental effects (e^2) are moderate to large. From twin studies c^2 is typically estimated at 0 because the correlation (r_{MZ}) between monozygotic twins is at least twice the correlation (r_{DZ}) for dizygotic twins. When using the Falconer variance decomposition (1.0 = a^2 + c^2 + e^2) this difference between monozygotic and dizygotic twin similarity results in an estimated c^2=0. It is important to note that the Falconer decomposition is simplistic.
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In genotyping studies where DNA is directly assayed for positions of variance (see SNP), concordance is a measure of the percentage of SNPs that are measured as identical. Samples from the same individual or identical twins theoretically have a concordance of 100%, but due to assaying errors and somatic mutations, they are usually found in the range of 99% to 99.95%. Concordance can therefore be used as a method of assessing the accuracy of a genotyping assay platform. Because a child inherits half of his or her DNA from each parent, parents and children, siblings, and fraternal (dizygotic) twins have a concordance that averages 50% using this measure.
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One proposed explanation is that people with different genes tend to seek out different environments that reinforce the effects of those genes. The brain undergoes morphological changes in development which suggests that age-related physical changes could also contribute to this effect. A 1994 article in Behavior Genetics based on a study of Swedish monozygotic and dizygotic twins found the heritability of the sample to be as high as 0.80 in general cognitive ability; however, it also varies by trait, with 0.60 for verbal tests, 0.50 for spatial and speed-of- processing tests, and 0.40 for memory tests. In contrast, studies of other populations estimate an average heritability of 0.50 for general cognitive ability.
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For example, in one approach to twin studies, researchers compare the personality scores of monozygotic (MZ) or identical twins reared together to dizygotic (DZ) or fraternal twins reared together. Because both types of twins in this design are reared together, all twin pairs are regarded as having shared a 100% common environment. In contrast, the monozygotic twins share 100% of their genes whereas the dizygotic twins only share about 50% of their genes. Therefore, for any given personality trait, it is possible to parcel out genetic influences by first obtaining the MZ correlation (reflecting 100% common environment and 100% shared genes) and subtracting the DZ correlation (reflecting 100% common environment and 50% shared genes).
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There is a great deal of evidence to show that schizophrenia is a heritable disease. One key piece of evidence is a twin study that showed that the likelihood of developing the disease is 53% for one member of monozygotic twins (twins with same genetic code), compared to the 15% for dizygotic twins, who don't share the exact DNA. Others question the evidence of heritability due to different definitions of schizophrenia and the similar environment for both twins."The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes" Author Jay Joseph The fact that even monozygotic twins don't share a 100% concordance rate suggests environmental factors play a role in the vulnerability and development of the disorder.
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Blood phobia is often caused by direct or vicarious trauma in childhood or adolescence. Though some have suggested a possible genetic link, a study of twins suggests that social learning and traumatic events, rather than genetics, is of greater significance.. The inclusion of “blood-injury phobia” within the category of specific or simple phobias in classificatory systems reflects a perception that fear has a primary role in the disorder. Consistent with this assumption, blood-injury phobia appears to share a common etiology with other phobias. Kendler, Neale, Kessler, Heath, and Eaves (1992) have argued from data comparing monozygotic with dizygotic twins that the genetic factor common to all phobias (agoraphobia, social phobia, and specific phobias), strongly predisposes a person to specific phobias.
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In a 1991 study, Bailey and Pillard conducted a study of male twins recruited from "homophile publications", and found that 52% of monozygotic (MZ) brothers (of whom 59 were questioned) and 22% of the dizygotic (DZ) twins were concordant for homosexuality. 'MZ' indicates identical twins with the same sets of genes and 'DZ' indicates fraternal twins where genes are mixed to an extent similar to that of non-twin siblings. In a study of 61 pairs of twins, researchers found among their mostly male subjects a concordance rate for homosexuality of 66% among monozygotic twins and a 30% one among dizygotic twins. In 2000, Bailey, Dunne and Martin studied a larger sample of 4,901 Australian twins but reported less than half the level of concordance.
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The difference between identical twins and fraternal twins is the basis of the classical Twin Method, and first appeared in Siemens' work "Zwillingspathologie", or "Twin Pathology", in 1924.Bataille, V. "The use of the twin model to investigate the genetics and epigenetics of skin diseases with genomic, transcriptomic and methylation data". Journal of the European Academy of Dermatology and Venereology, 2012. While the twin method is generally attributed to Francis Galton's 1875 article "The History of Twins, as a Criterion of The Relative Powers of Nature and Nurture", Galton did not suggest the comparison between identical (monozygotic) and fraternal (dizygotic) twins; instead, Galton suggested using twins to test the power of the environment to change the similarity level between identical twins.
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In a recent analysis it was even determined that there is no specific developmental complication that contributes to the higher prevalence of left-handedness between monozygotic and dizygotic twins. Evidently, there is no conclusive evidence to support the idea that a certain type of twin may have a higher prevalence of left handedness because the results from studies conducted contradict one another. Even studies analyzing how gender within monozygotic and dizygotic populations may have a prevalence for left handedness, some found that males have a higher prevalence while other studies show that gender does not have an impact on handedness. Further studies addressing the topic needs to be performed to come to a conclusive answer on whether a type of twin or gender effects handedness.
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In 2013, a twin study combined a survey of pairs of twins where one or both had undergone, or had plans and medical approval to undergo, gender transition, with a literature review of published reports of transgender twins. The study found that one third of identical twin pairs in the sample were both transgender: 13 of 39 (33%) monozygotic or identical pairs of assigned males and 8 of 35 (22.8%) pairs of assigned females. Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 (2.6%) pairs where both twins were trans. The significant percent of identical twin pairs in which both twins are trans and the virtual absence of dizygotic twins (raised in the same family at the same time) in which both were trans would provide evidence that transgender identity is significantly influenced by genetics if both sets were raised in different families.
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The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual's adulthood. During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio. Importantly, there has been no correlation between the sex hormone levels of an adult and the individual's 2D:4D, which implies that it is strictly the exposure in utero that causes this phenomenon. A major problem with the research on this topic comes from the contradiction in the literature as to whether the testosterone level in adults can be predicted by the 2D:4D ratio.
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Twin and family studies have long been used to estimate variance explained by particular categories of genetic and environmental causes. Across a wide variety of human traits studied, there is typically minimal shared-environment influence, considerable non-shared environment influence, and a large genetic component (mostly additive), which is on average ~50% and sometimes much higher for some traits such as height or intelligence."Meta-analysis of the heritability of human traits based on fifty years of twin studies", Polderman et al 2015 However, the twin and family studies have been criticized for their reliance on a number of assumptions that are difficult or impossible to verify, such as the equal environments assumption (that the environments of monozygotic and dizygotic twins are equally similar), that there is no misclassification of zygosity (mistaking identical for fraternal & vice versa), that twins are unrepresentative of the general population, and that there is no assortative mating. Violations of these assumptions can result in both upwards and downwards bias of the parameter estimates.
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