A Dictionary of Genetics phần 4 pot

166 fragile X syndrome the males in this F 2 generation are retarded. See Ap- Preparations made in this way provide useful infor- mation concerning three-dimensional organizationpendix C, 1969, Lubs; 1991, Verkerk et al.; CpG is- land, DNA methylation, fragile chromosome site, pa- of protein particles embedded in the lipoidal mem- branes of cells.rental imprinting, trinucleotide repeats. fragile X syndrome See fragile X–associated mental freeze fracture a method for preparing samples for retardation. electron microscopy; frozen samples are fractured with a knife and the complementary surfaces are frameshift mutation See reading frame shift. cast in metal. See Appendix C, 1961, Moor et al. framework region the highly conserved, relatively frequency the most studied gene, which controls invariant portion of the variable (V) region of an im- the biological clock of Neurospora. Mutations of frq munoglobulin chain, as distinguished from the hy- either shorten or lengthen the period between co- pervariable segments of the V region. nidiations. The frq gene has been cloned, and it gives rise to at least two processed transcripts. The longer fraternal twins See twins. transcript arises from an ORF of 2,364 base pairs. free energy that component of the total energy of Although the predicted protein product of frq has a system that can do work. See thermodynamics, sec- an amino acid sequence that shows no extended ond law of. similarities to any previously described protein, there is a segment about 50 amino acids long with freemartin a mammalian intersex arising due to sequence similarities to the per gene of Drosophila. the masculinization of a female twin by hormones See period. from its male sibling when the fetal circulations are continuous. frequency-dependent fitness a phenomenon in which the adaptive value of a genotype varies with free radical an unstable and highly reactive mole- changes in allelic frequencies. For example, in Bate- cule, bearing an atom with an unpaired electron, sian mimicry, mimics have greater fitness when they that nonspecifically attacks a variety of organic struc- are rare relative to their models. See mimicry. tures, including DNA. The interaction of ionizing radiation with water can generate hydroxyl and hy- frequency-dependent selection selection involv- droperoxyl groups (free radicals that are potent oxi- ing frequency-dependent fitness (q.v.). See Appendix dizing agents). See superoxide anion. C, 1937; L’He ´ ritier and Tiessier; 1951, Petit; minor- ity advantage. free radical theory of aging aging due to the pro- duction of reactive free radicals of oxygen that inflict Freund adjuvant a widely used adjuvant contain- molecular damage to cell organelles, especially ing killed, dried mycobacteria suspended in the oil DNA. These lesions accumulate with time and cause phase of a water-in-oil emulsion. The bacteriologist a progressive reduction in viability. See antioxidant Jules T. Freund developed this infusion and found enzymes. that it prolonged antibody synthesis when injected together with the antigen. freeze-drying a method of dehydrating a cell or solution by rapidly freezing its moisture content to Friend leukemia virus (FLV) a virus inducing leu- ice. The solid material is then dried in the frozen kemia in mice and rats. It was discovered in 1956 state under vacuum, so that ice sublimes directly to by Charlotte Friend, who later showed that it was a water vapor with a minimization of shrinkage. See retrovirus (q.v.). It proved to be superior to work lyophilize. with than the two other mammalian cancer viruses previously discovered (mouse mammary tumor virus freeze-etching a technique for preparing biologi- and Gross mouse leukemia virus). cal material for electron microscopy. Live or fixed specimens are frozen in a liquid gas, such as freon or Fritillaria a genus of lilies. Species of this genus are nitrogen, and then placed in a Balzer freeze-fracture widely used in cytogenetic investigations because of apparatus. This is an instrument that allows frozen their large chromosomes. In fact the largest C value tissues to be sectioned in a vacuum. The exposed (q.v.) so far recorded for a plant is 12.5 × 10 10 bp of surface is allowed to sublime slightly (to etch), so DNA for Fritillaria uva-vulpis. that surface irregularities that reflect the type and distribution of cell constituents are accentuated. The frizzle a feather mutation in domestic fowl. FF in- dividuals are “extreme frizzle,” with bristle featherssurface is then replicated, and the replica is stripped away and viewed under the electron microscope. that wear off easily; whereas Ff individuals are “mild fusion gene 167 frizzle,” with more normal curly feathers. Frizzle fundamental theorem of natural selection a the- orem developed by R. A. Fisher according to whichfeather keratin shows a poorly ordered crystalline structure, and its amino acid composition is ab- the increase in fitness of a population at any given time is directly proportional to the genetic variancenormal. in fitness of its members. fructification 1. a reproductive organ or fruiting Fungi the kingdom that contains yeasts, molds, body. 2. the generation of fruit or spore-producing smuts, rusts, mushrooms, and other saprophytes. structures by plants. These organisms are placed at the bottom of most fructose a six-carbon hexose sometimes called lev- phylogenies that show the evolution of the eukaryo- ulose. It is a component of sucrose. tic kingdoms. Their primitive characters include mi- tochondria with plate-shaped cristae, a Golgi made up of individual dictyosomes, and mitosis with an endonuclear spindle. Fungi cannot perform endocy- tosis, and they lack undullipodia and centrioles. They reproduce by forming spores. See Appendix A, Kingdom 3; opisthokonta. funiculus the plant stalk bearing on ovule. fused gene See fusion gene. fructose intolerance a disorder of carbohydrate fused protein a hybrid protein molecule produced metabolism inherited as an autosomal recessive. Pa- when a gene of interest is inserted by recombinant tients lack fructose-1,6-diphosphatase. Symptoms DNA techniques into a recipient plasmid and dis- disappear if dietary fructose is restricted. places the stop codon for a plasmid gene. The fused protein begins at the amino end with a portion of fruit the ripened ovary of the flower that encloses the plasmid protein sequence and ends with the pro- the seeds. tein of interest. Compare with polyprotein. See Ap- pendix C, 1970, Yourno et al. fruit fly See Drosophila. fushi tarazu (ftz) one of the pair rule selector FSH follicle-stimulating hormone (q.v.). genes of Drosophila. The name means segment defi- cient in Japanese. The ftz gene is located at 3–47.5, F − strain Escherichia coli behaving as recipients within the Antennapedia complex, and it is first ex- during unidirectional genetic transfer. pressed within 7 vertical stripes of cells in the early F + strain Escherichia coli behaving as donors during embryo. It has later functions during metamorphosis unidirectional genetic transfer. See F factor. when it specifies the identities of individual neurons in the developing central nervous system. FTZ, the F test See analysis of variance. protein encoded by ftz, contains a PEST sequence Fugu rubripes Takifugu rubripes (q.v.). (q.v.) and a homeobox (q.v.). FTZ functions as an activator of the transcription of segment polarity functional cloning in human genetics, the identi- genes such as engrailed. But FTZ can also function as fication of the gene responsible for a disease from a a suppressor of transcription for genes such as wing- knowledge of the underlying molecular defect. If the less. See zygotic segmentation mutants. protein encoded by the gene is known, it is often possible to isolate the appropriate mRNAs and to fusidic acid an antibiotic that prevents translation use them, or cDNAs derived from them, as probes by interfering with elongation factor G. for the gene. This was the first method used success- fusion gene 1. a hybrid gene, composed of parts fully to clone genes responsible for certain hereditary of two other genes, arising from deletion of a chro- diseases, such as sickle cell anemia, Tay-Sachs dis- mosomal segment between two linked genes or by ease, and phenylketonuria. Contrast with positional unequal crossing over. Hemoglobin Lepore (q.v.)is cloning. See cDNA, hereditary disease, mRNA, probe. an example of such a fused gene. See cone pigment genes (CPGs), Philadelphia chromosome. 2. a labora-fundamentalism a conservative religious ideology that holds the origin and diversity of life is by divine tory construct consisting of regulatory elements from one gene ligated to the structural elements of another.creation, based upon a literal interpretation of the biblical account of Genesis. See creationism. Transgenic animals (q.v.) often carry fused genes. fusome a cytoplasmic organelle that is required for mutant shown to have fragmented fusomes was called otu because it formed ovarian tumors made up of hun-the proper formation of germ line syncytia during gametogenesis in both male and female insects. The dreds of cells, most of which were not connected by ring canals and which never differentiated into eitherfusome arises from endoplasmic reticulum that tra- verses the ring canals (q.v.) formed during successive oocytes or nurse cells. Mitotic effectors such as cyclin A(q.v.) have been shown to bind transiently with fu-cycles of incomplete mitotic germ cell divisions. After each division, a plug of fusomal material accu- somes during G2 and prophase, and this suggests that the fusomal system plays a role in the timing, syn-mulates in each newly-formed ring canal. This mate- rial then fuses with the fusome(s) formed from the chronization, and eventual cessation of cystocyte divi- sions (q.v.). Among the other components identifiedprevious division(s), and ultimately a mature, branched structure called a polyfusome is produced. in fusomes are alpha and beta spectrins, ankyrin, an adducin-like protein, dynein, and a protein encodedThe polyfusome entry illustrates how this structure directs the pattern of cystocyte interconnections by by the bag of marbles gene. Mutations in this gene also produce ovarian tumors. See adducin, bag of marblesanchoring one pole of each mitotic spindle, thus ori- enting the plane of cell division. The first Drosphila (bam), hu-li tai shao (hts). 168 G G galactosemia a hereditary disease in humans in- herited as an autosomal recessive due to a gene on the short arm of chromosome 9. Homozygotes suf- g gravity; employed in describing centrifugal fer from a congenital deficiency of the enzyme ga- forces. Thus, 2,000 × g refers to a sedimenting force lactosyl-1-phosphate uridyl-transferase, and galac- 2,000 times that of gravity. tose-1-phosphate accumulates in their tissues. They exhibit enlargement of the liver and spleen, cata- G guanine or guanosine. racts, and mental retardation. Symptoms regress if G 0 ,G 1 ,G 2 See cell cycle. galactose is removed from the diet. Prevalence 1/ 62,000. See Appendix C, 1971, Meril et al. ga/gigaannum one billion years. The age of the earth is 4.6 ga. See Appendix C, 1953, Patterson. galactosidase See alpha galactosidase, beta galac- tosidase. gain of function mutation a genetic lesion that causes a gene to be overexpressed or expressed at Galapagos finches See Darwin’s finches. the wrong time. Such mutations often affect up- Galapagos Islands a cluster of 14 islands that stream elements that control the time in the life cy- straddle the equator 650 miles west of Ecuador. cle when a gene is turned on or the specific tissue in Many of its species are found nowhere else in the which it is expressed. Gain of function mutations are world, such as marine iguanas, flightless cormorants, often dominant. Contrast with loss of function muta- giant tortoises, and a special group of finches. The tions. five weeks Darwin spent exploring these islands in GAL4 a transcriptional activator protein encoded 1835 were the crucial weeks of his scientific life. See by the gal4 gene of yeast and required for the ex- Appendix C, 1837, Darwin; Darwin’s finches; hot pression of genes encoding galactose-metabolizing spot archipelago. enzymes. The GAL4 protein consists of two separa- Galapagos rift See rift. ble but essential domains (q.v.): an N-terminal (q.v.) domain which binds to specific DNA sequences up- gall an abnormal growth of plant tissues. stream (q.v.) from the various target genes, and a C- gallinaceous resembling domestic fowl. terminal (q.v.) domain which is required to activate transcription. These properties of GAL4 have been Gallus gallus domesticus the domesticated exploited to develop the yeast two-hybrid system chicken, the bird for which the most genetic infor- (q.v.) for detecting protein-protein interactions and mation is available. Its ancestor is the Asian red jun- the Drosophila targeted gene expression technique gle fowl, from which it was domesticated around (q.v.) for studying the functioning of master control 8,000 BC . Its genome size is about 1 gbp and its genes in various targeted tissues. See Appendix C, number of protein coding genes is ϳ23,000. Birds, 1989, Field and Song; 1995, Halder et al. snakes, and lizards have two classes of chromo- somes: macrochromosomes and microchromosomes. galactose a six-carbon sugar that forms a compo- In chickens there are 10 macrochromosomes and 29 nent of the disaccharide lactose and of various cereb- microchromosomes. The fifth chromosome in length rosides and mucoproteins. See beta galactosidase. is the metacentric Z, which occurs in duplicate in males. The female has one Z and a smaller w chro- mosome. The macrochromosomes including the Z replicate synchronously. All the microchromosomes replicate late and so does the w of females. Roughly 60% of all the protein coding genes in the chicken have human orthologs. The haploid chromosome number is 39. The female is the heterogametic sex (ZW), whereas the male is the homogametic sex (ZZ). There are over 200 genes that have been 169 170 Galton apparatus mapped. Estimated genome size is 1.125 × 10 9 base employed in mathematical models of species com- petition.pairs. See Appendix A, Chordata, Aves, Galliformes; Appendix E; comb shape, plumage pigmentation gametic disequilibrium the nonrandom distribu- genes, poultry breeds. tion into the gametes in a randomly mating popula- tion of the alleles of genes occupying different loci. Galton apparatus an apparatus invented by Fran- The nonrandom distribution may result from linkage cis Galton (illustrated below) consisting of a glass- of the loci in question or because the loci interact faced case containing an upper reservoir where balls with respect to their effects on fitness. See linkage are stored. Below the reservoir are arranged row disequilibrium. after row of equally spaced pegs that stand out from the wall, and below these is a series of vertical slots. gametic meiosis See meiosis. The balls are allowed to fall one at a time through a central opening at the bottom of the reservoir. Since gametic mutation any mutation in a cell destined each ball after striking a peg has an equal probability to become a gamete, and therefore potentially he- of bouncing to the left or right, most will follow a reditary. Compare with somatic mutation. zigzag course through the pegs and will eventually gametic number the haploid number of chromo- land in a central slot. The final distribution of balls somes (symbolized by N) characterizing a species. in the slots will be a bell-shaped one. The apparatus demonstrates how the compounding of random gametoclonal variation the appearance of new events will generate a family of bell-shaped curves. traits in haploid plants that grow in tissue culture See Appendix C, 1889, Galton. from anthers or other reproductive material rather than from diploid body tissue as in somatoclonal variation (q.v.). gametocyte a cell that will form gametes through division; a spermatocyte or oocyte. gametogamy the fusion of gamete cells or nuclei. gametogenesis the formation of gametes. gametophore a branch bearing a gametangium or gametangia. gametophyte the haploid phase (of the life cycle of plants undergoing an alternation of generations) during which gametes are produced by mitosis. See sporophyte. gamma chain one of the two polypeptides found in fetal hemoglobin (q.v.). gamma field a field where growing plants may be exposed to chronic irradiation from a centrally placed multicurie 60 Co gamma-ray source. gamma globulin an antibody-containing protein Galton apparatus fraction of the blood. See Appendix C, 1939, Tiselius and Kabat. gametangium an organ in which gametes are gamma ray an electromagnetic radiation of short formed. See antheridium, oogonium. wavelength emitted from an atomic nucleus under- going radioactive decay. gamete a haploid germ cell. See Appendix C, 1883, van Beneden. gamogony a series of cell or nuclear divisions that eventually lead to the formation of gametes. game theory a mathematical theory dealing with the determination of optimum strategies where the gamone a compound produced by a gamete to fa- cilitate fertilization. Chemotactic sperm attractantspolicies adopted depend on the most likely behav- iors of two or more competitors. Game theory is produced by eggs are examples. gas chromatography 171 gamont the haploid adult form of those protoctists that have both haploid and diploid phases in their life cycles. Gamonts function in sexual reproduc- tion; they undergo gametogony to produce diploid agamonts. Meiosis takes place in agamonts, and the haploid agametes that result disperse, undergo mi- totic divisions, and differentiate into gamonts, com- pleting the cycle. gap genes a class of Drosophila genes that control embryogenesis. Loss of function mutations result in gamontogamy the aggregation of gamonts during the loss of contiguous body segments, and therefore sexual reproduction and the fusion of gamont nuclei gaps appear in the normal pattern of segmented to produce agamonts. structures in the embryo. See zygotic segmentation mutants.ganglion a small nervous-tissue mass containing numerous cell bodies. gargoylism a term covering two genetically dis- ganglioside a family of complex lipids containing tinct hereditary diseases of connective tissue in hu- sphingosine, fatty acids, carbohydrates, and neura- mans, Hunter syndrome (q.v.) and Hurler syndrome minic acid. The Gm2 ganglioside that accumulates (q.v.). in the brain of patients with Tay-Sachs disease (q.v.) Garrod disease See alkaptonuria. is shown below. See Appendix C, 1935, Klenk. gap the position where one or more nucleotides gas chromatography a chromatographic technique in which an inert gas is used to sweep through a col-are missing in a double-stranded polynucleotide con- taining one broken chain. umn the vapors of the materials to be separated. Gm2 ganglioside 172 gas-flow radiation counter gas-flow radiation counter a counter in which an the amino acid substitutions often determine whether the condition will be mild or severe. Five commonappropriate atmosphere is maintained in the sensi- tive volume by allowing a suitable gas to flow slowly mutations collectively account for over 95% of the cases of Gaucher disease in the Ashkenazi Jewishthrough it. population. These alleles occur in a frequency far gastrin a hormone secreted by the stomach that higher than could be sustained by mutation. Hetero- causes secretion of digestive enzymes by other stom- zygotes can be identified because their peripheral ach cells. leukocytes show lower enzyme levels. Homozygotes can be detected in utero by assays run on cells ob- Gastropoda the class of molluscs containing the tained by amniocentesis. Adjacent to the Gaucher snails. See Appendix A. disease gene is a pseudogene that also contains 11 gastrula the stage of embryonic development exons. In the regions present in both sequences, 96% when the gastrulation movements occur. of the nucleotides are identical. The pseudogene is smaller because of large deletions in four of the in- gastrulation the complex cell movements that trons and small deletions in two of the exons. Some carry those cells whose descendants will form the fu- alleles responsible for Gaucher disease symptoms ture internal organs from their largely superficial appear to have arisen by rearrangements between position in the blastula to approximately their de- the structural gene and the pseudogene. The first de- finitive positions inside the animal embryo. Prior to scription of the disease was published in 1882 by Dr. gastrulation, the amphibian embryo relies on RNA Philippe Gaucher, hence the eponym. See Appendix molecules pre-loaded into the ooplasm during oo- C, 1989, Horowitz et al.; Ashkenazi, cerebrosidase, genesis. During gastrulation, however, newly synthe- lysosomal storage diseases, Tay-Sachs disease. http:// sized nuclear gene products are required. www.gaucherdisease.org. Gaucher disease the most common hereditary Gaussian curve See normal distribution. disorder of glycolipid metabolism, due to mutations G banding See chromosome banding techniques. in a gene on chromosome 1 at position q21. The gene contains 11 exons, that encode a 7,500 base gb, gbp See gigabase, gigabase pairs. pair transcript that specifies the enzyme glucocere- GDB Genome Data Base (human) See Appendix E. brosidase. This cleaves glucose from cerebrosides, and glucocerebrosides accumulate in lysosomes when Geiger-Mueller (G-M) counter a sensitive gas- the enzyme is defective. Although the enzyme defi- filled radiation-measuring device. ciency exists in all the cells of persons with the dis- gel diffusion technique See immunoelectrophore- ease, the cell primarily responsible for the syndrome sis, Ouchterlony technique, Oudin technique. is the macrophage (q.v.). Macrophages swollen by the accumulation of glucocerebrosides are called Gaucher gemma (plural , gemmae) a multicellular, asexual cells. An effective treatment involves infusing the reproductive structure, such as a bud or a plant frag- patient with a genetically engineered glucocerebrosi- ment. dase, which is specifically targeted to Gaucher cells. gemmules pangenes. See pangenesis. Unfortunately, the treatment is tremendously ex- pensive, amounting to $200,000 per year or more. gene the definition of a gene changes as more of its properties are revealed. In the classical literatureThe most common mutations of the cerebrosidase gene are missense mutations, and the positions of it is defined as a hereditary unit that occupies a spe- Characteristics of Genes Responsible for Five Human Diseases Gene Size mRNA Size No. of Genetic Disease Protein Product (Kbp) (Kb) Introns sickle-cell anemia beta chain of hemoglobin 1.6 0.6 2 hemophilia B factor 9 34 1.4 7 phenylketonuria phenylalanine hydroxylase 90 2.4 12 cystic fibrosis CF transmembrane- 250 6.5 26 conductance regulator Duchenne muscular dystrophy dystrophin, plus shorter 2300 14 78 isoforms gene expression 173 cific position (locus) within the genome or chromo- ang et al.; amplicon, genomic equivalence, Podospora anserina, Rhynchosciara, TAOS 1.some; a unit that has one or more specific effects upon the phenotype of the organism; a unit that can gene bank See genomic library. mutate to various allelic forms; and a unit that recombines with other such units. Two classes are GeneCards a database of human genes, their prod- now recognized: (1) genes that are transcribed into ucts, and their involvement in diseases assembled by mRNAs, which enter ribosomes and are translated M. Rebhan and three colleagues at the Weizmann into polypeptide chains, and (2) genes whose tran- Institute of Science, Rehovot, Israel. http://bioinfor- scripts (tRNAs, rRNAs, snRNAs, etc.) are used di- matics.weizmann.ac.il/cards. rectly. Class 1 genes are called structural genes or cis- gene cloning creation of a line of genetically iden- trons in the earlier literature. As shown in the table tical organisms, containing recombinant DNA mole- on page 172, structural genes vary greatly in size. cules, which can be propagated and grown in bulk, Some genes that are transcribed into mRNAs can thus amplifying the recombinant molecules. undergo alternative splicing and generate a series of structurally related proteins. There are also shorter gene cloning vehicle See lambda cloning vector, DNA segments that are not transcribed but serve as plasmid cloning vector. recognition sites for enzymes and other proteins that gene cluster See multigene family. function during transcription or replication. Some of these elements (i.e., operators) fulfill the classical gene conversion a situation in which the products definition of genes, but now they are generally called of meiosis from an AA′ individual are 3A and 1A′ or regulatory sequences. These should not be confused 1A and 3A′, not 2A and 2A′ as is usually the case. with regulatory genes, which encode (1) proteins that Thus, one gets the impression that one A gene has bind to regulatory sequences in other parts of the ge- been converted to an A′ gene (or vice versa). Gene nome or (2) RNAs that inactivate entire chromo- conversion is thought to involve a rare error in DNA somes. The i gene of the lac operon is an example of repair that occurs while recombination is going on a regulatory gene of the first type. The Xist gene is an during meiotic prophase. A double-strand break in example of the second type. It encodes an RNA that one bivalent is enlarged to eliminate one allele of the inactivates an entire X chromosome and is responsi- sister strand. When the gap is repaired, a non-sister ble for dosage compensation in female mammals. See strand carrying the alternate allele is used as a tem- Appendix C, 1909, Johannsen; 1933, Morgan; 1955, plate, with the result that the tetrad comes to con- Benzer; 1961, Jacob and Monod; 1975, King and tain three copies of one allele and one of the other. Wilson; dosage compensation, isoform, lac operon, Therefore gene conversion leads to the unequal re- replicon, selector gene, transcription unit, Xist. covery of alleles from DNA molecules that each carry a pair of alleles, one normal and one defective. gene activation See genetic induction. The human Y chromosome contains several palin- genealogy a record of the descent of a family, dromes (q.v.) in which are imbedded structural group, or person from an ancestor or ancestors; lin- genes that function in spermatogenesis. Within each eage; pedigree. palindrome, multiple copies of structural genes function as templates for repair of mutated genes. In gene amplification any process by which specific this way gene conversion prevents the Y from accu- DNA sequences are replicated to a disproportion- mulating sterility mutations. See Appendix C, 1935, ately greater degree than their representation in the Lindgren; 2003, Skaletsky et al. parent molecules. During development, some genes gene dosage the number of times a given gene is become amplified in specific tissues; e.g., rRNA present in the nucleus of a cell. genes amplify and become active during oogenesis, especially in some amphibian oocytes (see rDNA am- gene duplication the production of a tandem re- plification, Xenopus). Genes encoding Drosophila peat of a DNA sequence by unequal crossing over chorion proteins are also amplified in ovarian follicle (q.v.) or by an accident of replication. Duplicated cells. Gene amplification can be induced by treating genes created in these ways may subsequently evolve cultured cells with drugs like methotrexate (q.v.). new functions. See hemoglobin genes, repeat-in- Gene amplification is a common and critically im- duced point mutation (RIP). portant defect in cancer cells. For example, a specific set of autosomal genes is overexpressed in oral squa- gene expression the display of genetic activity by the synthesis of gene products that affect the pheno-mous cell carcinomas. See Appendix C, 1968, Gall, Brown, and Dawid; 1978, Schimke et al.; 2002, Hu- type. Some genes are active throughout the life of 174 gene family the cell or organism. Such genes, which are contin- duction of a particular phenotype. Such interactions often occur when the products of the nonallelicually transcribed, show constitutive expression. Other genes are transcribed only under certain environmen- genes under study function at steps in a sequence of reactions that result in compounds which generatetal conditions or at specific times during develop- ment. These genes show conditional expression. Most the phenotype in question. These interactions can produce variations from the classical genetic ratios.genes are expressed in direct proportion to their ploidy levels in the nucleus. However, there is a An example would be the inheritance of aleurone color in Zea mays. In order for the corn kernel tosmall number of genes whose transcription rates are increased disproportionally, or even decreased, as possess colored aleurone, at least one A and one C gene must be present. Given A and C in the hetero-polyploidy levels rise. See Appendix C, 1999, Galitski et al.; constitutive mutation, derepression, DNA meth- zygous or homozygous condition and, in addition, R in the heterozygous or homozygous condition, thenylation, inducible system, operon, parental imprinting, repressible system, selector gene, selfish operon. a red pigment is produced. Purple pigment is syn- thesized if P is present in addition to A, C, and R. gene family See multigene family. All four genes reside on different chromosomes. Thus, if a plant of genotype AaCCRRPp is self-polli- gene flow the exchange of genes between differ- nated, the offspring will contain the following aleu- ent populations of the same species produced by mi- rone classes: purple, red, and white in a 9 : 3 : 4 ratio. grants, and commonly resulting in simultaneous Here the 9 : 3 :3:1 ratio has been converted into a changes in gene frequencies at many loci in the re- 9:3:4 ratio, because the P gene cannot be ex- cipient gene pool. pressed in the absence of A. gene-for-gene hypothesis the proposal that dur- gene knockout See knockout. ing their evolution a host and its parasite develop complementary genetic systems, with each gene that gene library See genomic library. provides the host with resistance matched by a gene in the parasite that confers susceptibility. The inter- gene machine an automated DNA synthesizer for acting genes from the two species are called corre- producing short DNA probes (generally 15–30 base sponding genes, since for each gene that conditions pairs long) or primer DNA (q.v.) for use in a poly- resistance in the host there is a corresponding gene merase chain reaction (q.v.). that conditions avirulence in the parasite, and the gene manipulation the formation of new combi- products of the two genes interact. The product of nations of genes in vitro by joining DNA fragments the resistance gene serves as a receptor for a ligand of interest to vectors so as to allow their incorpora- produced by the parasite, directly or indirectly tion into a host organism where they can be propa- through expression of an avirulence gene. The bind- gated. See DNA vector, genetic engineering. ing of receptor and ligand is the recognition event that elicits through cellular signal transduction gene mapping assignment of a locus to a specific (q.v.), a cascade of defense responses that constitute chromosome and/or determining the sequence of the resistant phenotype. See Appendix C, 1955, Flor; genes and their relative distances from one another coevolution, Linum usitatissimum, Melampsora lini. on a specific chromosome. gene frequency the percentage of all alleles at a gene networking the concept that there exist func- given locus in a population represented by a specific tional networks of genes which program early devel- allele. Also referred to as allelic frequency (q.v.). opment, and that genes which encode proteins with multiple conserved domains serve to cross-link such gene fusion the union by recombinant DNA tech- networks. Thus, a set of genes containing domain A niques of two or more genes that code for different and a set containing domain B are linked by genes products, so that they are subject to control by the containing both domains. The segmentation gene same regulatory systems. paired (prd)ofDrososphila illustrates the theory. It gene gun See particle-mediated gene transfer. contains a homeobox (q.v.) and a histidine-proline repeat domain. This prd-specific repeat occurs in at gene insertion any technique that inserts into a least 12 other genes, while the homeobox defines a cell a specific gene or genes from an outside source, second gene set. Presumably, the prd product can in- including cell fusion, gene splicing, transduction, teract with products of genes containing only the and transformation. homeobox sequence or the histidine-proline repeat, or both. The conserved domains are thought to servegene interaction interaction between different genes residing within the same genome in the pro- as the sites to which the proteins bind to specific genetic assimilation 175 chromosomal regions to regulate neighboring genes. gene substitution the replacement of one gene by its allele, all the other genes (or all other relevantSee Appendix C, 1986, Noll et al.; eyeless. genes) remaining unchanged. Genentech, Inc. the first company to specialize in gene superfamily a collection of genes that are all recombinant DNA technology. It is located in South products of gene duplication and have diverged from San Francisco, California, and its name is a contrac- one another to a considerable degree. The repeated tion of Genetic Engineering Technology. See Appen- copies of an ancestral gene can follow three evolu- dix C, 1976, Boyer and Swanson; 1982, Eli Lilly. tionary pathways: (1) they can be inactivated by mu- gene pair in a diploid cell, the two representative tation, (2) they can acquire new functions, or (3) genes (either identical or nonidentical alleles) at a they can retain their original function. The globin given locus on homologous chromosomes. superfamily of genes provides examples of genes that (1) mutated to pseudogenes (q.v.), (2) acquired gene pool the total genetic information possessed new functions (the gene for myoglobin [q.v.] versus by the reproductive members of a population of sex- the alpha chain gene of hemoglobin), and (3) re- ually reproducing organisms. tained their original functions (the Gγ and Aγ genes). See hemoglobin genes. gene probe See probe. gene targeting a technique for inserting into labo- gene product for most genes, the polypeptide ratory mice genetic loci modified in desired ways. chain translated from an mRNA molecule, which in Standard recombinant DNA techniques are used to turn is transcribed from a gene; if the RNA tran- introduce desired chemical changes into cloned script is not translated (e.g., rRNA, tRNA), the DNA sequences of a chosen locus. The mutated se- RNA molecule represents the gene product. quence is then transferred into an embryo-derived, gene 32 protein the first DNA unwinding protein stem-cell genome, where it is allowed to undergo (q.v.), to be isolated. It is the product of gene 32 of homologous recombination (q.v.). Microinjection of phage T4 and is essential for its replication. The pro- mutant stem cells into mouse blastocysts is then per- tein has a molecular weight of 35,000 daltons and formed to generate chimeras. The stem cells come binds to a stretch of DNA about 10 nucleotides from a black mouse line, and the recipient embryos long. See Appendix C, 1970, Alberts and Frey. are from a white strain. Therefore, chimeras can be identified by their variegated coat colors. Large generalized in evolution theory, an unspecialized numbers of these chimeras are mated together, and condition or trait, usually considered to have a in the F 1 rare black progeny are observed. Some of greater potential for evolving into a variety of alter- these will be homozygous for the targeted gene. If native conditions than that possessed by a highly the mutation represents a null allele (q.v.), the func- specialized one. Primitive traits tend to be general- tions of the normal allele can be inferred from the ized; derived or advanced traits tend to be more spe- abnormal phenotypes shown by the homozygotes. cialized. See Appendix C, 1988, Mansour et al.; knockout. generalized transduction See transduction. gene therapy addition of a functional gene or group of genes to a cell by gene insertion (q.v.)to generation time (Tg) the time required for a cell correct a hereditary disease. See Appendix C, 1983, to complete one growth cycle. See doubling time. Mann, Mulligan, and Baltimore; 1990, Anderson; gene redundancy the presence in a chromosome 1996, Penny et al.; ex vivo. of many copies of a gene. For example, the nucleolus genetic anticipation the occurrence of a heredi- organizer of Drosophila melanogaster contains hun- tary disease with a progressively earlier age of onset dreds of duplicate copies of the cistrons that code in successive generations. In those diseases caused by for the 18S and 28S rRNA molecules. expansion of trinucleotide repeats (q.v.) anticipation gene silencing a phenomenon in which genes results from an intergenerational increase in repeat near certain chromosomal regions, such as centro- lengths. However, reversion of the triplet repeat meres or telomeres, are rendered transcriptionally lengths to the normal size range can also occur, al- inactive. See antisense RNA, DNA methylation, het- though this happens less often. erochromatin, position effect, RNA interference, so- genetic assimilation the process by which a phe- matoclonal variation, telomeric silencing. notypic character initially produced only in response to some environmental influence becomes, throughgene splicing See recombinant DNA technology. [...]... two pairs, a pair of alpha chains and a pair of beta chains Each alpha chain contains 141 amino acids, and each beta chain contains 146 amino acids Normal human adults also have a minor hemoglobin component (2%) called A2 This hemoglobin has two alpha chains and two delta peptide chains The delta chains have the same number of amino acids as the beta chains, and 95% of their amino acids are in sequences... half the mass of a radioactive substance to decay into another substance Each radionuclide has a unique half-life half-sib mating mating between half brother and half sister Such individuals have one parent in common half-tetrad analysis recombinational analysis where two of the four chromatids of a given tetrad can be recovered, as in the case of attached X chromosomes in Drosophila half-value layer... chains and a pair of abnormal chains, each chain made up of 146 amino acids Each abnormal chain appears to be a hybrid molecule consisting of an Nterminal end containing amino acids in a sequence characteristic of the left end of the delta chain and a C-terminal end containing amino acids in a sequence characteristic of the right end of a beta chain Hemoglobin Lepore is presumed to have arisen by un- 5′... properties of the cell walls of the two groups of bacteria Agrobacterium, Escherichia, Haemophilus, Salmonella, Serratia, Shigella, and Vibrio are all Gram-negative; Bacillus, Mycobacterium, Staphylococcus, and Streptococcus are examples of Gram-positive bacterial genera See Appendix C, 18 84, Gram grana (singular granum) long columns of dense discs found in chloroplasts (q.v.) Each disc contains a double layer... identical to those of the beta chains The hemoglobin of the fetus (HbF) is made up of two alpha chains and two gamma chains Each gamma chain also contains 146 amino acids There are two types, G gamma and A gamma, which differ only in the presence of glycine or alanine, respectively, at position 136 The earliest embryonic hemoglobin tetramer consists of two zeta (alphalike) and two epsilon (betalike) chains... the case of relatively giant cells, cytoplasmic regions of characteristic morphology can be transplanted Grafts between species of Acetabularia (q.v.) are examples See allograft, autograft, heterograft, homograft, scion, stock, transplantation, xenograft graft hybrid a plant made up of two genetically distinct tissues due to fusion of host and donor tissues after grafting graft rejection a cell-mediated... guanine-7-methyl transferase See methylated cap guanine quartet model a three-dimensional arrangement of guanine molecules which explains the interactions occurring within DNA strands that contain repeating units rich in guanine, such as telomeric repeats One, two, or four DNA strands can fold into a compact unit containing a planar array of four guanines, as shown above Each guanine serves both as a. .. artificial chromosomes (BACs), cosmid, DNA vector, lambda phage vector, plasmid cloning vector, P1 179 artificial chromosomes (PACs), restriction endonuclease, transformation, yeast artificial chromosomes (YACs) genomic RNA the genetic material of all viruses that do not use DNA as genetic material All cells and the vast majority of viruses use DNA as genetic material, but some bacteriophages and a few plant... radiation or drugs); synonymous with allogeneic disease, runt disease (q.v.) gram atomic weight the quantity of an element that has a mass in grams numerically equal to its atomic weight group-transfer reactions gram equivalent weight the mass of an acid or a base that will release or neutralize one gram molecule (mole) of hydrogen ion A 1-mole solution of H2SO4 contains 2 gram equivalents A thousandth... digestive tract and is often a cause of diarrhea in hikers Giardia was first described about 1681 by van Leeuwenhoek The parasite is tear-drop shaped, about 12 µm long, and is binucleate The stationary phase trophozoites arrest in the G2 phase with a ploidy of 8N (2 nuclei, each with 4N ploidy) The haploid number of chromosomes is 5 and the C value is 10–12 Mb Each cell has 4 pairs of flagella, but lacks conventional . et al. ga/gigaannum one billion years. The age of the earth is 4. 6 ga. See Appendix C, 1953, Patterson. galactosidase See alpha galactosidase, beta galac- tosidase. gain of function mutation a. one. The apparatus demonstrates how the compounding of random gametoclonal variation the appearance of new events will generate a family of bell-shaped curves. traits in haploid plants that grow. protein Galton apparatus fraction of the blood. See Appendix C, 1939, Tiselius and Kabat. gametangium an organ in which gametes are gamma ray an electromagnetic radiation of short formed. See antheridium,