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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

BOX 5.1

Commonly used genetic terms
Allele: One member of a pair of homologous genes in a diploid cell. An individual
with identical alleles at a genetic locus is a homozygote; one with non-identical
alleles is a heterozygote. In a case in which one allele leads to an observable
gene product and the other has no phenotype, the functional allele is said to
be dominant and the non-functional allele recessive.
Candidate genes: Genes with perceived relevance to the trait in question, which
can be used to compare allele frequencies between affected and non-affected
groups.
Gene: In genetics, a unit inferred from the pattern of inheritance; in molecular
biology, defined narrowly as a section of DNA that is expressed as RNA or,
more widely, as a coding sequence of DNA and associated regulatory
sequences.
Gene locus: The specific place on a chromosome where a gene is located.
Heritability: The proportion of phenotypic variance that can be attributed to
additive genetic variance.
Genotype: The genetic make-up of any organism.
Linkage: The more-frequent-than-random occurrence of two traits together due
to the proximity of their corresponding genes on the same chromosome. The
likelihood of a recombination event separating the two genes decreases with
their increasing proximity on the chromosome.
Linkage studies: These studies use multiply-affected families to examine traits
that are inherited together. The concept is based on the fact that genes that
are located close to one another will be more likely to be inherited together
from one parent than two genes located further apart.
Phenotype: The outward physical manifestation of the cell or individual due to
actual expression of the alleles that are present.
Polygenic: A trait arising from more than one gene.
Polymorphism: The occurrence of something in several forms, e.g. the
occurrence in a population of two or more alleles of a gene at a single genetic
locus.

genetic research attention. However, one overwhelming finding from genetic
studies of psychoactive substances is that the heritability (i.e. genetic
contribution) of dependence for one substance correlates highly with
dependence for other substances. Thus, there may be some common genetic
components to substance dependence in general, as well as to dependence
for specific psychoactive substances. There is also a high degree of association
of substance dependence with mental illness (see Chapter 6). The most

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