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In this chapter we take the univariate model as described in
Chapter 5, and apply it to twin data. The main goals of this chapter are i) to enable the readers
to apply the models to their own data, and ii) deepen their
understanding of both the scope and the limitations of the method.
In Section 6.2.1 a
model of additive genetic (A), dominance genetic (D), common
environment (C), and random environment (E) effects is presented
although D and C are confounded when our data have been obtained from
pairs of twins reared together. The first example concerns a
continuous variable: body mass index (BMI), a widely used
measure of obesity, and Section 6.2.2 describes how these
data were obtained and summarized. In Section 6.2.3 we fit
this model to authentic data, using Mx in a path coefficients
approach. Section 6.2.5 illustrates the univariate model
fitted with variance components. An alternative treatment which may
be skipped without loss of continuity. The results of initial
model-fitting to BMI data appear in Section 6.2.6 and two
extensions to the model, the use of means (Section 6.2.7)
and of unmatched twins (Section 6.2.8), are
described before drawing general conclusions about the BMI analyses in
Section 6.2.9. In Section 6.3 the basic model
is applied to ordinal data. The second example (Section 6.3.1)
describes the collection and analysis of major depressive disorder in a
sample of adult female twins. This application
serves to contrast the data summary and analysis required for an
ordinal variable against those appropriate for a continuous variable.
In most twin studies there is considerable heterogeneity of age
between pairs. As shown in Section 6.4, such
heterogeneity can give rise to inflated estimates of the effects of
the shared environment. We, therefore, provide a method of
incorporating age into the structural equation model to separate its
effects from other shared environmental influences.

** Next:** 2 Fitting Genetic Models
** Up:** 6 Univariate Analysis
** Previous:** 6 Univariate Analysis
** Index**
Jeff Lessem
2002-03-21