genomic imprinting Occurs when DNA receives
biochemical marks instructing a cell how and when to
express certain genes. Resulting gene expression is usu-
ally from one copy of a gene, either from the maternal
orpaternal gene.
genomic library A collection of clones made from a
set of randomly generated overlapping DNA fragments
that represents the entire genome of an organism.
genotype The complete genetic makeup of an organ-
ism, which may not show in physical appearance; the
pair of alleles at a particular locus. The percentage of a
particular genotype in a population is called the geno-
type frequency. Also, in taxonomy, the type species of a
genus.
See alsoPHENOTYPE.
genus Taxonomic classification of a group of related
or similar objects or organisms. A genus has one or
more species. Groups of similar genera, plural of genus,
make up a family. In the scientific name of an organ-
ism, it is the first word followed by a second word to
complete the binomial. For example, Homo is the
genus name for humans, while the entire binomial,
Homo sapiens,is the species name. Genus and species
names are italicized.
See alsoTAXON.geographic range The total area or range occupied
by a species or population.geological time The span of time that has passed
since the creation of the Earth and its components; a
scale used to measure geological events millions of
years ago. Measured in chronostratic or relative
terms, where subdivisions of the Earth’s geology are
set in an order based on (a) relative age relationships
based on fossil composition and stratigraphic position
or (b) chronometric or absolute time where the use of
radiometric dating techniques give numerical ages.geological time 143Types of virusesAdenovirusHuman
immunodeficiency
virusBacteriophageAn adenovirus, a grouping of DNA-containing viruses that cause respiratory disease, including one form of the common cold.
Adenoviruses can be genetically modified and used in gene therapy to treat cystic fibrosis, cancer, and potentially other diseases.
(Courtesy of Darryl Leja, NHGRI, National Institutes of Health)