Twin and Adoption Studies: Practices & Findings - …
Thomas of the University of Minnesota did the most famous research on genetic influences in humans. He studied . Identical twins come from a single egg, fertilized by a single sperm, which splits after the egg starts to develop. Therefore identical twins are closer to being genetically identical than any other humans. By studying identical twins who were separated at birth and raised by different families, Bouchard could see which similarities might emerge despite a different family environment. These similarities might be those that are heavily influenced by a person's genetic heritage.
Twin and adoption studies - PHG Foundation
The most common research methodologies are family studies, twin studies, and adoption studies. Environmental influences can be divided into two classes, shared and nonshared (or unique) environment.
Adoption studies are one form of clinical genetic study designed to evaluate genetic and environmental influences on phenotype. In adoption studies the index cases and controls are adoptees. The adoption study design provides a unique opportunity to examine gene and gene by environment interactions for a variety of behavioural, psychiatric and medical disorders. Adoption genetic studies provide some challenges related to the issues related to adoption (atypical biological parent profiles, differences in adoptive parents compared to nonadoptive parents and the stress of learning about being an adoptee). Recently, molecular genetic strategies have been applied to samples of adoptees. Advancing privacy issues have made it more difficult to design and carry out adoption research.
Twin Studies- Genetics | Twin | Adoption - Scribd
The results of this study should be interpreted in the light of the concept of heritability. Heritability does not imply an invariant, immutable genetic influence such as occurs in the case of hair or eye color. It describes instead the genetic influences found among persons living in a particular range of environmental conditions. Under different environmental conditions, different estimates of heritability might be obtained. Nevertheless, the conditions in which the subjects of our study live are those of much of Western society, and our results should apply to persons in that society. Similar results have been obtained by other studies in Western society, whether of twins in America , or of adoptees in Denmark , or America. Genetic factors appear to be major determinants of the body-mass index in Western society, and they may account for as much as 70 percent of the variance.
• Why might adoption studies at times be preferrable to twin studies
The values for height and weight that we used were based on self-reports rather than direct measurements; the latter are clearly preferable. Self-reported heights and weights, however, are often quite accurate and are increasingly used in epidemiologic studies if direct measurements are not available. The self-reported values used in our study corresponded closely to measured values. In a sample of more than one third of the twins, the correlations between self-reported and measured height and weight were very high. It should be noted that estimates of genetic influence in twin studies are not affected by over- or under-estimates of height and weight unless there is a differential bias in reporting between monozygotic and dizygotic twins. There was no evidence of such bias.
Twin and Adoption Studies – psychologyrocks
Like the studies described above, our study assessed genetic influences on the body-mass index. Unlike those studies, however, ours involved few frankly obese persons. The relevance of these results to obesity thus depends on how the middle range of values for body-mass index relates to the extreme that characterizes obesity. In this regard it is reassuring that three adoption studies have found that genetic influences extend across the range of weight from thin to very obese.
Potential limitations / problems with adoption & twin studies
The third finding of this study is that neither the shared rearing environment nor correlated environments contributed to variation in the body-mass index. This finding is supported by two lines of evidence. First, among the SATSA twins the intrapair similarity in height and weight was not influenced by either age at separation or degree of separation. Second, adoption studies have not found any effect of the childhood rearing environment. , Estimates of the heritability of the body-mass index that are based on data from family members who have shared the same environments (twins reared together or parents and offspring) do not appear to have been biased by the effects of shared or correlated environments. The significant environmental influences on body-mass index are not attributable to shared family influences but are unique to the individual person.