Research Group 'Psychiatric Genetics', Head: Prof. Dr. Hans H. Stassen

Department of Psychiatry, Psychotherapy and Psychosomatics

Psychiatric Hospital, University of Zurich


EEG Spectral Patterns of Dizygotic 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 Concordance of Brain Wave Patterns

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. Examples of spectral patterns derived from the DZ twins reared apart (2 highly concordant and 2 extremely discordant pairs) in Figure 19 give a visual impression of the "nature" of within-pair concordance and the distinct individuality of brain wave patterns.


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Stassen HH, Lykken DT, Propping P: Zwillingsuntersuchungen zur Genetik des normalen Elektroenzephalogramms. In: P. Baumann (ed): Biologische Psychiatrie der Gegenwart, Wien: Springer 1993, 139-144
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Stassen HH, Bomben G, Hell D: Familial brain wave patterns: study of a 12 sib family. Psychiat Genetics 1998; 8: 141-153
Stassen HH, Coppola R. Torrey EF, Gottesman II, Kuny S, Rickler KC, Hell D: EEG differences in monozygotic twins discordant and concordant for schizophrenia. Psychophysiology 1999; 36,1: 109-117
Dünki RM, Schmid GB, Stassen HH: Intraindividual specificity and stability of the human EEG: Linear vs. nonlinear approaches. Meth Inform Med 2000; 39: 78-82
Umbricht D, Koller R, Schmid L, Skrabo A, Grübel C, Huber T, Stassen HH: How specific are deficits in mismatch negativity generation to schizophrenia? Biol Psychiatry 2003; 53: 1120-1131
Stassen HH: EEG and evoked potentials. In: D. Cooper (ed) Nature Encyclopedia of the Human Genome. Nature Publishing Group, London 2003; 3: 266-269
Weisbrod M, Hill H, Sauer H, Niethammer R, Guggenbühl S, Stassen HH: Nongenetic pathologic developments of brain-wave patterns in monozygotic twins discordant and concordant for schizophrenia. Am J Med Genetics B 2004; 125: 1-9
Buckelmüller J, Landolt HP, Stassen HH, Achermann P: Trait-like individual differences in the human sleep EEG. Neuroscience 2006; 138: 351-356
EEG spectral pattern

Pairs of DZ twins reared apart: the upper half shows the EEG spectral pattern of the first co-twin and the lower half the pattern of the second co-twin. Spectral intensities are plotted on log-proportional scales along the vertical axes.
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