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5. GENETIC BASIS OF SUBSTANCE DEPENDENCE

common types of genetic studies in humans and the types of information
these studies provide are summarized below. Animal studies are also briefly
presented.

Family, twin and adoption studies: estimations of heritability

Family, twin and adoption studies can be used to determine whether or not
there is a genetic contribution to psychoactive substance use and
dependence, but they do not provide evidence to determine which particular
gene is involved. Twin and adoption studies also help to dissociate
environmental factors from genetic factors.
Family studies examine the inheritance of traits through a family, in order
to find out about patterns of inheritance and the relative risk of inheriting a
disorder.
Twin studies are based on the fact that monozygotic (identical) twins share
identical genetic material, while dizygotic (fraternal) twins share the same
degree of genetic similarity as non-twin siblings. Presumably, twins raised
together share very similar environments. If genetic effects are present, then
monozygotic twins should be more alike, with respect to those effects, than
dizygotic twins. This allows an estimation of the genetic contribution to
psychoactive substance dependence. These types of studies provide evidence
that variation in the vulnerability to substance dependence in populations is
influenced by both individual genotypes and environmental differences
(Heath et al., 1999a; Vanyukov & Tarter, 2000).
Adoption studies are capable of almost completely separating genetic and
environmental influences on the variation in the vulnerability to a disorder
(except contributions of antenatal and early postnatal environmental factors)
(Heath et al., 1999a; Vanyukov & Tarter, 2000); in this way they complement
the more traditional twin studies. Using adoption studies, environmental
factors can be separated from genetic factors, since children adopted at birth
are raised in an environment that is different from that of their genetic family.
In this way, environmental factors such as socioeconomic status, learning
about substance use, exposure to psychoactive drugs, etc., are randomized.
For example, if a particular family shows a high level of substance dependence
from generation to generation, it is difficult to know how much of this is
attributable to shared genes, and how much is attributable to the shared
environment. With adoption studies, the effect of the environment is factored
out, and thus, it is easier to determine more clearly the contribution of
genetics.

Identifying chromosomal locations of interest: linkage studies
Twin and adoption studies give an estimation of the proportion of variation
in a trait that is due to genetics; however, they do not provide any information
about which genes or chromosomes are involved. Linkage and association

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