Partners:
Jena, Germany
London, UK
Utrecht, Netherlands
Amsterdam, Netherlands
Barcelona, Spain
Bonn, Germany
Heidelberg, Germany
Szeged, Hungary
Zurich, Switzerland
Marie Curie Action:
035987
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The Concept of Within-Pair Concordance
The concept of within-pair concordance is used to describe the similarity between
monozygotic (mz) and dizygotic (dz) co-twins with respect to a set of variables ("traits"),
preferably in a quantitative way. This concept is a special form of a more general
approach to quantifying between-subject and within-subject similarity for a given
set of features, for example, when addressing the question of "familiality" of
psychopathology syndrome scores within families, or when comparing the intra-individual
stability of brainwave patterns over time with the inter-individual differences in the
general population. Unlike correlation analysis, where the comparison of a feature
derived from all first-born co-twins with the same feature derived from all second-born
co-twins of the sample under investigation yields a single correlation coefficient, the
concordance approach evaluates the similarity for each twin pair separately, thus yielding
a distribution of similarity measures.
Distributions of Quantiative Traits
Quantitative similarity measures are under most circumstances normally distributed
and sample-independent. By contrast, qualitative measures often lack reproducibility
because they require the definition of "tresholds" for the underlying categories which
may be, to some extent, arbitrary. This is particularly true for the yes-no dichotomie
of the concordant/discordant models. Figure 2 shows the within-pair concordances in
mz and dz twins for the quantitative traits "finger ridge count", "body height",
"shoe size", "body weight", and "brain-wave patterns" [Lykken and Stassen: data of 1,300
dizygotic and 1,434 monozygotic twin pairs]. Mean values and variances are strongly
correlated (the higher the mean value the smaller the variance. All means values
display a mz:dz ratio of 2:1 independent of the observed magnitude.
Norm of Reaction — From Genotype to Phenotype
Genes code for proteins or RNA ("gene products") —which may interact among each other or
with their immediate environment within the cells in a variety of ways— and influence the
observable phenotypes ("traits of interest") only after a cascade of intermediate steps.
In consequence, empirically derived genotype-phenotype correlations are typically weak,
while the amount of phenotypic variance explainable through a single genomic locus is
generally modest, often tiny, in the range of a few percent. Close genotype-phenotype
correspondences with an approximate one-to-one mapping are rare. The cascade of intermediate
steps leading from a particular gene to its corresponding phenotype gives rise to
modulations and modifications through endogenous and exogenous factors, particularly in
cases where the gene under investigation is part of a self regulating system encompassing
a number of genes rather than a single gene. The modulations and modifications induced
by endogenous and exogenous factors lead to variations in the observed phenotypes which
are almost always normally distributed ("norm of reaction") with well-defined means and
standard deviations for ethnically homogeneous populations.
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Within-pair concordances in mz and dz twins for the quantitative traits
"finger ridge count", "body height", "shoe size", "body weight", and "brain-wave patterns".
The distributions are approximately normal with means ranging between 0.99 (finger ridge
count) and 0.65 (body weight). All mean values
display a mz:dz ratio of 2:1
[Lykken and Stassen: data of 1,300 dz and 1,434 mz twin pairs].
Please note: No more than 55% of mz and 15% of dz co-twins are concordant for schizophrenia, thus
displaying highly significant deviations from the expected mz:dz ratio of 2:1 for
additive traits.
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