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Between-Subject versus Within-Pair EEG Similarity of Twins

Genetic Similarity

The genetic similarity between monozygotic (MZ) co-twins is "1" since both co-twins share the same genome. While the parent-offspring genetic similarity is always "0.5" ireespective of the affection status of parents and offspring, the sib-sib genetic similarity is only on average "0.5", with similarity coefficients forming a normal distribution. In other words, the sib-sib genetic similarities cover a wide range from complete dissimilarity (comparable to that of unrelated individuals) to complete concordance (comparable to that of MZ twins). From the genetic point of view the genetic similarity between dizygotic (DZ) co-twins is identical with the sib-sib similarities among "ordinary" siblings. What distinguishes DZ twin pairs from "ordinary" sib pairs is the fact that the DZ co-twins have the same age while sharing the same environment (if raised together). Studies of MZ and DZ twin pairs where at least one co-twin suffers from schizophrenia provide very strong evidence for a genetic component in the etiopathology and pathogenesis of this illness as only genetics can explain why MZ co-twins brought up together have a 3.7-fold risk that both co-twins suffer from schizophrenia (55% concordance for schizophrenia) compared to DZ co-twins (15% concordance).

Twin Approach: Comparing MZ versus DZ twins Concordance

Traditionally, the phenotypic similarity between MZ and DZ co-twins is termed "within-pair concordance" and investigations into the genetic predisposition to a trait using within-pair concordances of MZ and DZ twins are called to "follow the twin approach". In fact, analyses of the within-pair concordance of quantitative traits such as "finger ridge count", "body height", "brain-wave patterns", "shoe size", "body weight", "schizophrenia" or "depression" yield quite amazing results. For additive traits distribution curves are found to be normal with the MZ:DZ ratio being exactly 2:1 irrespective of the actual size of the genetic component. For example, for finger ridge count we get 99:50, for body height 85:42, for brain wave patterns 82:41, for shoe size 75:37, and for body weight 65:34. By contrast, this ratio exhibits with 55:15 a highly significant deviation form this 2:1 ratio, thus indicating non-additiveness and complex inheritance encompassing multiple genetic and multiple non-genetic factors along with strong non-linear interactions.

Within-Pair Similarity of Brain Wave Patterns in MZ Twins

Our investigations into the within-pair concordance of brain wave patterns in MZ and DZ twins yielded with 82:41 clear support for the 2:1 ratio as expected for additive traits and no evidence for significant deviations. When corrected for the presence of artefacts in the EEG time series, heritability estimates lay in the range of 0.719-0.761. There was no difference between twins brought up together or reared apart. The distribution of within-pair similarity of brain wave patterns as derived from 24 MZ pairs brought up together are given in Figure 13 (upper half) along with the distribution of inter-individual similarities as derived from all possible pairings between unrelated individuals among these twins (lower half). Clearly, this latter distribution underestimates that of the general population.


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MZ frequency distribution
Fig. 13: Discrimination between the distributions of inter-individual (lower half) and within-pair (upper half) similarity coefficients based on 24 monozygotic male twin pairs brought up together. The experimental condition is quiet wakefulness (eyes closed; 4 channels: T3-T5, T5-O1, T4-T6, T6-O2).

Results derived from 28 monozygotic twin pairs reared apart (9 male, 19 female pairs) were comparable, thus suggesting that the within-pair EEG similarity of monozygotic twins does not differ from repeated assessments on the same individual.
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