592
Behavior Genes
Childhood fatigue
Attention deficit
hyperactivity disorder
Bulimic symptoms
Depression (unipolar)
Manic depression(bipolar)
Autism
Schizophrenia
0
0.2
0.4
0.6
0.8
1
Correlation coefficient
Dizygotic twins
Monozygotic twins
Fig. 4
Monozygotic and dizygotic twin correlations for various disorders.
(Taken from McGufFn, P., Martin, N. (1999) Science, medicine, and the future.
Behaviour and genes,
BMJ
319
, 37–40.)
MZ twins
b
=
1.0
Full sibs
b
=
0.50
b
=
0.25
Fig. 5
The genetic
relationships (
β
) between the
offspring of monozygotic twins.
be differentiated using adoption studies,
although for practical reasons, these have
been used less extensively than twin stud-
ies. There are three main variations on the
adoption design. Firstly, the adoptee study
design compares the frequency of a disor-
der or trait in the adopted-away offspring
of parents who have the trait and adopted-
away offspring of parents who do not. A
second type is the adoptee’s family study.
This involves taking affected adoptees as
the index cases. The prevalence of the trait
in biological relatives can be compared
with that in adoptive relatives or to that in
the relatives of control adoptees. A higher
rate of disorder in those biologically related
to the index cases indicates a genetic ef-
fect. One can also compare the prevalence
of the disorder in the biological parents
of adopted-away probands with that in the
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