Twin study

The twin research is a method of research in human genetics and psychology.

It is substantially dependent on a large amount of reliable data obtained by twin registers (eg Danish twin register or Swedish twin register) are provided.

Description

In the classical twin research are identical twins ( monozygotic twins, MZ) and dizygotic twins ( dizygotic twins, DZ ) were examined. If identical twins each other more closely resemble in terms of a particular trait than fraternal twins, this can be interpreted as an indication that the feature being examined is genetically influenced in particular.

By mathematical analyzes of the genetic component can ( heritability ) and the influence of environmental factors experienced jointly be determined (eg in the family) approximation. In conjunction with the methods of molecular genetics now individual genes can be studied in twin studies. Through linkage analysis and association analyzes, the heritability of single or multiple genes can be traced back here. This can, among others, the study of disease benefit (see Boomsma et al. ).

Heritability is often misunderstood as the proportion of the genetic material on the expression of a characteristic in a given individual. However, they determined the genetic proportion of the differences. People differ because they carry different gene variants and because they live in different environments. A heritability of, for example, 88 % of the body mass index would mean that 88% of the differences in body mass index in the population by genetic differences are due, but not that one man only to 12% is responsible for his weight.

Based on the assumption (see also below under " Comments ... " ), twins were subject largely the same environmental influences, were in criminological research (especially in the 1930s and 1940s ) twins ( both monozygotic (DC), as well as fraternal twins ( ZZ) ) to the effect examines whether criminal behavior due to the plant (ie, genetically determined ) is. For the subjects pairs to an existing concordance (or Diskonkordanz ) have been checked. The existence of a " criminal capture " both the one and the other part was considered concordant expression. In fact, this research indeed showed a higher concordance in the EZ - a sign of a confirmation of the hypothesis. However, the results can be found approximately by limiting the criminal acquisition on the bright field, the small number of each studied twins, as well as the retrospective nature of the studies question ( pre-selection ) to right. Empirical evidence for the genetic determination of criminal behavior not delivered the twin research so far, anyway.

In the era of National Socialism geneticist trying to figure out how to create identical twins and other multiple births. The reason for this was that they wanted to ensure the preservation of the " master race " by a very high birth rate. Targeted production of identical twins - the thinking goes - would have the number of "Aryan " children can be increased.

In the area of ​​psychological research on twins, which temporarily came into disrepute by the falsification of Cyril Burt Lodowic, the works of Kurt Gottschaldt be mentioned.

Separated twins raised

Occasionally, identical twins are examined, who grew up separately. Since identical twins are genetically almost identical, one can draw separately grown up by comparing identical twins conclusions as to which innate properties and which are learned. Show despite different environments common properties, so this is an indication ( not proof ) for a genetic determination. Compared with the classical twin research they focus not on the differences but the similarities. An example: Examined twins were similar in their weight characteristics ( overweight, normal weight ) rather their biological parents than their adoptive parents.

Separated growing up identical twins are extremely rare, this occurs for example when twins are up for adoption after birth.

Criticism

The premise of the research on twins is that identical and fraternal twins experience common environmental factors to the same extent and therefore differences in the similarity between one-and dizygotic twins alone by the different proportion of common genes (100 % vs. 50%) were related. This assumption is not without controversy, as the similarity of identical twins can lead to matching environmental factors ( gene-environment correlation). As a result, can result in an individual case the overestimate of heritability. However, the behavior of the parents towards monozygotic twins is not identical but are differentiated by behavior and needs of the individual twins. In addition, the relative influence of shared environmental factors, especially in the field of medical research on twins low or absent, the variability is mainly due to the genetic differences and individual environmental factors.

Another basic assumption is the wholly-owned genetic concordance between identical twins. Also, this premise is only restricted applicable because lead in the course of life mutations in differences. However, in the same way each individual is not identical with itself, since mutations occur locally and lead to genetic differences between different cells.

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