What is genetic inheritance
[engl. heritability], [BIO, PER], is the ratio of two variances, namely that of the genetic variance divided by the observed (phenotypic) variance. Heredity, i.e. the proportion of the observed differences between individuals, which can be traced back to genetic differences between them. A distinction is made between heredity and e. S. of heredity i. w.S. Heredity i. e. Regarding the share of the observed differences which is passed on from parents to their children via genetic means. This is the proportion of the additive genetic variance taking into account the extent of selective partner choice (behavioral genetics). It can be estimated from the similarity of adopted children to their birth parents (adoption studies). Heredity i. w.S. regarding the contribution of all genetic influences to the observed differences and includes both additive and non-additive gene effects (behavioral genetics). Heredity i. w.S. can especially be deduced from twin studies. In general, it is the case that the heredity of a trait is higher, the more closely the similarity of people in this trait goes hand in hand with their genetic similarity. The genetic similarities are e.g. B. 1 for identical twins (EZ), ½ for dizygotic twins (ZZ), for siblings as well as for parents and their biological children and 0 for unrelated persons (e.g. adoptive siblings).
Heredity of IQ: Table 1 reports correlations determined by Bouchard & McGue (1981) from 111 studies (r) between the IQ values (intelligence) of relatives and adoptees. According to Loehlin (1989), these findings suggest a heredity of the IQ of around 50%. Another approx. 25% are due to the influence of the shared environment, 15% to the influence of the specific Environment and 10% due to measurement errors (behavioral genetics). Furthermore, the environment contributes more to the similarity of twins than to the similarity of siblings and least of all to the similarity of parents and children. The findings in the table, however, relate to people of very different ages. A differentiation according to age groups shows that in adulthood, compared to childhood, the importance of genetic influences is greater and the importance of the shared environment is less. Longitudinal twin and adoption studies are particularly informative here. Both in the Louisville twin study (LTS; Wilson, 1983) and in the Twins Early Developmental Study (Trouton et al., 2002) found high similarities between both types of twins with regard to their cogn. Development (in the LTS these correlations were at the age of 3 months r =, 66 for EZ and r =, 67 for ZZ). On the other hand, both studies showed higher similarities of EZ than ZZ at the age of 2 years, which indicate a moderate influence of genetic factors and a continued strong influence of the shared environment. By the age of 15, the difference in the similarities of EZ (r =, 88) and ZZ (r =, 54), which indicates a growing proportion of genetic influences on intelligence. This conclusion is supported by data from a longitudinal adoption study: Plomin et al. (1997) report from the Colorado adoption project that in natural families the parent-child correlation from birth to the age of 12 of r = .12 to approx. r = .30 increased and remained at this level. In contrast, in adoptive families the parent-child correlation reached at the age of 3 years r = .20 its maximum and then sank to r = .00 from.
Heredity of personality traits: As a result of a literature search, Loehlin (1992) arrived at the relative correlations reported in Table 2 for the most frequently examined personality traits extraversion and neuroticism. Heredity estimates for these personality traits on the basis of twin studies accordingly amount to approx. 40%, and the remaining 60% are related to effects of the specific Environment and measurement errors (Plomin et al., 1999). The common environment, on the other hand, does not significantly contribute to the similarity of people who have grown up together (Loehlin, 1992). However, adoption studies result in lower heredity estimates, because all relatives except EZ are hardly alike. This discrepancy in the heredity estimates from twin and adoption studies can be resolved to the effect that non-additive genetic effects, especially epistasis, are significant (Plomin et al., 1998): Epistasis increases the similarity EZ, but not the ZZ as well as of parents and Children (behavioral genetics).
Heredity of mental disorders: Numerous twin and adoption studies have gone into psychological heredity. Disorders, especially schizophrenia, the prevalence rate of which in the general population is 1%. This basic risk increases systematically if family members suffer from schizophrenia, namely in the case of one of the following: (a) second-degree relatives to 4%, (b) first-degree relatives to 9%, (c) ZZ to 17%, and (d) EZ to 48% (Gottesman, 1991). Apparently, the risk of developing schizophrenia is significantly influenced by a genetic disposition. However, since the concordance for development cooperation is far below 100%, both genetic and environmental factors are involved in the occurrence of the disorder. Schizophrenic disorders differ in terms of their severity and are evidently subject to polygenetic inheritance (Faraone & Tsuang, 1985): It can be assumed that a quant. there is a graded risk of developing schizophrenia. In this respect, the demarcation between affected and normal people is subject to a certain arbitrariness. Polygenetic inheritance also suggests that the relatives of affected persons who do not suffer from schizophrenia will systematically deviate from the population mean in the direction of the disorder, proportional to the degree of their genetic similarity to the sick person (DeFries & Fulker, 1985). This assumption forms the basis for genetic extreme group analyzes, which investigate the extent to which the significance and type of genetic and environmental influences generalize across the range of characteristics. Also for other disorders, e.g. For example, bipolar depression (bipolar disorders), genetic influences have been demonstrated (Plomin et al., 1999).
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