the cell or organism. Such genes, which are contin- ually transcribed, show constitutive expression. Other genes are transcribed only under certain environmen- tal conditions or at specific times during develop- ment. These genes show conditional expression. Most genes are expressed in direct proportion to their ploidy levels in the nucleus. However, there is a small number of genes whose transcription rates are increased disproportionally, or even decreased, as polyploidy levels rise. See Appendix C, 1999, Galitski et al.; constitutive mutation, derepression, DNA meth- ylation, inducible system, operon, parental imprinting, repressible system, selector gene, selfish operon. gene family See multigene family. gene flow the exchange of genes between differ- ent populations of the same species produced by mi- grants, and commonly resulting in simultaneous changes in gene frequencies at many loci in the re- cipient gene pool.
gene-for-gene hypothesis the proposal that dur- ing their evolution a host and its parasite develop complementary genetic systems, with each gene that provides the host with resistance matched by a gene in the parasite that confers susceptibility. The inter- acting genes from the two species are called corre- sponding genes, since for each gene that conditions resistance in the host there is a corresponding gene that conditions avirulence in the parasite, and the products of the two genes interact. The product of the resistance gene serves as a receptor for a ligand produced by the parasite, directly or indirectly through expression of an avirulence gene. The bind- ing of receptor and ligand is the recognition event that elicits through cellular signal transduction (q.v.), a cascade of defense responses that constitute the resistant phenotype. See Appendix C, 1955, Flor; coevolution, Linum usitatissimum, Melampsora lini. gene frequency the percentage of all alleles at a given locus in a population represented by a specific allele. Also referred to as allelic frequency (q.v.). gene fusion the union by recombinant DNA tech- niques of two or more genes that code for different products, so that they are subject to control by the same regulatory systems. gene gun See particle-mediated gene transfer.
gene insertion any technique that inserts into a cell a specific gene or genes from an outside source, including cell fusion, gene splicing, transduction, and transformation. gene interaction interaction between different genes residing within the same genome in the pro- duction of a particular phenotype. Such interactions often occur when the products of the nonallelic genes under study function at steps in a sequence of reactions that result in compounds which generate the phenotype in question. These interactions can produce variations from the classical genetic ratios. An example would be the inheritance of aleurone color in Zea mays.
In order for the corn kernel to possess colored aleurone, at least one A and one C gene must be present. Given A and C in the hetero- zygous or homozygous condition and, in addition, R in the heterozygous or homozygous condition, then a red pigment is produced. Purple pigment is syn- thesized if P is present in addition to A, C, and R. All four genes reside on different chromosomes. Thus, if a plant of genotype AaCCRRPp is self-polli- nated, the offspring will contain the following aleu- rone classes: purple, red, and white in a 9 : 3 : 4 ratio. Here the 9 : 3 : 3 : 1 ratio has been converted into a 9:3:4 ratio, because the P gene cannot be ex- pressed in the absence of A. gene knockout See knockout. gene library See genomic library. gene machine an automated DNA synthesizer for producing short DNA probes (generally 15-30 base pairs long) or primer DNA (q.v.) for use in a poly- merase chain reaction (q.v.).
gene manipulation the formation of new combi- nations of genes in vitro by joining DNA fragments of interest to vectors so as to allow their incorpora- tion into a host organism where they can be propa- gated. See DNA vector, genetic engineering. gene mapping assignment of a locus to a specific chromosome and/or determining the sequence of genes and their relative distances from one another on a specific chromosome. gene networking the concept that there exist func- tional networks of genes which program early devel- opment, and that genes which encode proteins with multiple conserved domains serve to cross-link such networks. Thus, a set of genes containing domain A and a set containing domain B are linked by genes containing both domains. The segmentation gene paired (prd) of Drososphila illustrates the theory. It contains a homeobox (q.v.) and a histidine-proline repeat domain. This prd-specific repeat occurs in at least 12 other genes, while the homeobox defines a second gene set. Presumably, the prd product can in- teract with products of genes containing only the homeobox sequence or the histidine-proline repeat, or both. The conserved domains are thought to serve as the sites to which the proteins bind to specific
chromosomal regions to regulate neighboring genes. See Appendix C, 1986, Noll et al.; eyeless. Genentech, Inc. the first company to specialize in recombinant DNA technology. It is located in South San Francisco, California, and its name is a contrac- tion of Genetic Engineering Technology. See Appen- dix C, 1976, Boyer and Swanson; 1982, Eli Lilly. gene pair in a diploid cell, the two representative genes (either identical or nonidentical alleles) at a given locus on homologous chromosomes. gene pool the total genetic information possessed by the reproductive members of a population of sex- ually reproducing organisms. gene probe See probe. gene product for most genes, the polypeptide chain translated from an mRNA molecule, which in turn is transcribed from a gene; if the RNA tran- script is not translated (e.g., rRNA, tRNA), the RNA molecule represents the gene product. gene 32 protein the first DNA unwinding protein (q.v.), to be isolated. It is the product of gene 32 of phage T4 and is essential for its replication. The pro- tein has a molecular weight of 35,000 daltons and binds to a stretch of DNA about 10 nucleotides long. See Appendix C, 1970, Alberts and Frey. generalized in evolution theory, an unspecialized condition or trait, usually considered to have a greater potential for evolving into a variety of alter- native conditions than that possessed by a highly specialized one.
Primitive traits tend to be general- ized; derived or advanced traits tend to be more spe- cialized. generalized transduction See transduction. generation time (Tg) the time required for a cell to complete one growth cycle. See doubling time. gene redundancy the presence in a chromosome of many copies of a gene. For example, the nucleolus organizer of Drosophila melanogaster contains hun- dreds of duplicate copies of the cistrons that code for the 18S and 28S rRNA molecules. gene silencing a phenomenon in which genes near certain chromosomal regions, such as centro- meres or telomeres, are rendered transcriptionally inactive. See antisense RNA, DNA methylation, het- erochromatin, position effect, RNA interference, so- matoclonal variation, telomeric silencing. gene splicing See recombinant DNA technology.
gene substitution the replacement of one gene by its allele, all the other genes (or all other relevant genes) remaining unchanged. gene superfamily a collection of genes that are all products of gene duplication and have diverged from one another to a considerable degree. The repeated copies of an ancestral gene can follow three evolu- tionary pathways: (1) they can be inactivated by mu- tation, (2) they can acquire new functions, or (3) they can retain their original function. The globin superfamily of genes provides examples of genes that (1) mutated to pseudogenes (q.v.), (2) acquired new functions (the gene for myoglobin [q.v.] versus the alpha chain gene of hemoglobin), and (3) re- tained their original functions (the Gγ and Aγ genes). See hemoglobin genes. gene targeting a technique for inserting into labo- ratory mice genetic loci modified in desired ways. Standard recombinant DNA techniques are used to introduce desired chemical changes into cloned DNA sequences of a chosen locus. The mutated se- quence is then transferred into an embryo-derived, stem-cell genome, where it is allowed to undergo homologous recombination (q.v.).
Microinjection of mutant stem cells into mouse blastocysts is then per- formed to generate chimeras. The stem cells come from a black mouse line, and the recipient embryos are from a white strain. Therefore, chimeras can be identified by their variegated coat colors. Large numbers of these chimeras are mated together, and in the F1 rare black progeny are observed. Some of these will be homozygous for the targeted gene. If the mutation represents a null allele (q.v.), the func- tions of the normal allele can be inferred from the abnormal phenotypes shown by the homozygotes.
See Appendix C, 1988, Mansour et al.; knockout. gene therapy addition of a functional gene or group of genes to a cell by gene insertion (q.v.) to correct a hereditary disease. See Appendix C, 1983, Mann, Mulligan, and Baltimore; 1990, Anderson; 1996, Penny et al.; ex vivo. genetic anticipation the occurrence of a heredi- tary disease with a progressively earlier age of onset in successive generations. In those diseases caused by expansion of trinucleotide repeats (q.v.) anticipation results from an intergenerational increase in repeat lengths. However, reversion of the triplet repeat lengths to the normal size range can also occur, al- though this happens less often. genetic assimilation the process by which a phe- notypic character initially produced only in response to some environmental influence becomes, through
a process of selection, taken over by the genotype, so that it is formed even in the absence of the envi- ronmental influence that at first had been necessary. genetic background all genes of the organism other than the one(s) under consideration; also known as the residual genotype. genetic block a block in a biochemical reaction generally due to a mutation that prevents the syn- thesis of an essential enzyme or results in the forma- tion of a defective enzyme.
If the defective enzyme has limited activity, the block may be a partial one, and the mutant is referred to as “leaky.” genetic bottleneck See bottleneck effect. genetic burden See genetic load. genetic coadaptation See coadaptation. genetic code the consecutive nucleotide triplets (codons) of DNA and RNA that specify the sequence of amino acids for protein synthesis. The code shown below is used by most organisms, but there are exceptions (see universal code theory). The mRNA nucleotide sequences are written 5′ to 3′ left to right, respectively, because that is the direction in which translation occurs. Thus, an mRNA segment specifying proline-tryptophan-methionine would be (5′) CCU-UGG-AUG (3′), whereas its complemen- tary, antiparallel DNA template strand would be (3′) GGA-ACC-TAC (5′).
The code is degenerate in that all amino acids, except methionine and tryp- tophan, are specified by more than one codon. Most of the degeneracy involves the third nucleotide at the 3′ end of the codon (see wobble hypothesis). The code is read from a fixed starting point, in one direc- tion, in groups of three consecutive nucleotides. The start codon is AUG, and in bacteria it specifies the insertion of N-formylmethionine (q.v.). When AUG occupies an internal position in the mRNA, it speci- fies methionine. See Appendix C, 1961, von Eh- renstein and Lipmann, Crick et al., Nirenberg and Matthaei; 1966, Terzaghi et al.; 1967, Khorana; 1968, Holley et al.; 1979, Barrell et al.; 1985, Horo- witz and Gorowski, Yamao; codon bias, strand termi- nologies, transcription unit. genetic code dictionary See amino acid, universal code theory. genetic colonization introduction of genetic ma- terial from a parasite into a host, thereby inducing the host to synthesize products that only the parasite can use. See Agrobacterium tumefaciens, opine. genetic counseling the analysis of risk of produc- ing genetically defective offspring within a family, and the presentation to family members of available options to avoid or ameliorate possible risks. See in- formed consent.
See amino acids entry for three-letter symbols. S = Start codon for met for eukaryotes (N- formylmethionine for prokaryotes). I = Internal codon for met. Ter = Termination codon.