genetic fingerprinting genome, diploid organisms have two homologous sets, polyploid organisms have many sets sometimes from the same ancestor (autopolyploids) and sometimes from different ancestors (allopolyploids). Interspecific hybrids also have two or more difference genomes.
netic engineering raises many legal and ethical issues, and the introduction of genetically modified organisms into the environment requires strict controls and monitoring.
genetic fingerprinting A technique for identifying individuals by means of their DNA. The DNA being tested is extracted from cells (from blood, semen, tissue fragments, etc.) and broken into fragments of 600–700 bases each, using restriction enzymes. The human genome contains many loci where short base sequences are repeated in tandem, with great variation between individuals in the number of such repeats. These so-called variable number tandem repeats (VNTR) can be identified using special DNA probes, thus providing a virtually unique set of markers for any given individual. This technique is used in veterinary and human medicine to establish the parentage of individuals, and in forensic science to identify individuals from traces of body tissue or fluids. Even minute amounts of DNA can now be amplified, using the POLYMERASE CHAIN REACTION, to provide sufficient material for genetic fingerprinting.
genotype The genetic make-up of an organism. The actual appearance of an individual (the phenotype) depends on the dominance relationships between alleles in the genotype and the interaction between genotype and environment. Compare phenotype.
genus (pl. genera) A taxonomic category involving a collection of similar species. Genera may be subdivided into subgenera, and also, especially in plant taxonomy, into sections, subsections, series, and subseries. The scientific name of a species always includes the genus name (or its abbreviation) as the first word of the binomial. Similar genera are grouped into families. See binomial nomenclature.
geological time scale A system of measuring the history of the earth by studying the rocks of the earth’s crust. Since new rocks are generally deposited on top of existing material, those lower down are oldest, although this is often disrupted by e.g. volcanic activity, tectonic movements and erosion. The strata of rock are classified according to their age, and a time scale corresponding to this can be constructed. The main divisions (eras) are the PALEOZOIC, MESOZOIC, and CENOZOIC. These are further subdivided into periods and epochs.
genetic map A map showing the sequence of particular genes or segments of DNA on a chromosome. Restriction enzymes and DNA probes are used to determine the exact sequence, but it is possible to get a rough estimate by analyzing the frequency of recombination between the alleles of linked genes. See chromosome map; DNA probe; restriction endonuclease. genetics The term coined by the British biologist Bateson (1861–1926) to describe the study of inheritance and variation and the factors controlling them. Today the subject has four main subdivisions – Mendelian genetics (classical genetics), population genetics, cytogenetics, and molecular or biochemical genetics.
geophyte A plant whose perennating buds are situated below ground as, for example, in plants with bulbs, corms, tubers, or rhizomes. See Raunkiaer’s plant classification.
genome A complete haploid chromo-
germ cell Any cell in a cell lineage that
some set. Haploid organisms have one
eventually produces gametes.
geotropism See gravitropism. cell
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