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).
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.
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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Braun S, Bridler R, Müller N, Schwarz MJ, Seifritz E, Weisbrod M, Zgraggen A, Stassen HH:
Inflammatory Processes and Schizophrenia: Two Independent Lines of Evidence from a Study
of Twins Discordant and Concordant for Schizophrenic Disorders. Eur Arch Psychiatry Clin
Neurosci 2017; 267: 377-389
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Braun S, Bridler R, Müller N, Schwarz MJ, Seifritz E, Weisbrod M, Zgraggen A, Stassen HH:
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