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R plasmid A plasmid containing one or several transposons with resistance genes. Plasmids carrying genes controlling resistance to various drugs. RAM mutation Ribosomal ambiguity mutation that allows incorrect tRNAs to be incorporated into the translation process. random genetic drift Changes in allelic frequency due to sampling error. Changes in allele frequency that result because the genes appearing in offspring are not a perfectly representative sampling of the parental genes. (eg. in small populations). See also founder effect. random mating Mating between individuals where the choice of partner is not influenced by the genotypes (with respect to specific genes under study). The mating of individuals in a population such that the union of individuals with the trait under study occurs according to the product rule of probability. random strand analysis Mapping studies in organisms that do not keep together all the products of meiosis. reading frame The succession of codons determined by reading nucleotides in groups of three from a specific initiation codon. readthrough transcription or translation beyond the normal termination signals in DNA or mRNA respectively. realized heritability Heritability measured by a response to selection. The ratio of the single-generation progress of selection to the selection differential of the parents. reannealing Spontaneous realignment of two single DNA strands to re-form a DNA double helix. rec system Several loci controlling genes (recA; recB; recC and others) involved in postreplicative DNA repair. receptor element A controlling element in maize that can insert into a gene (making it mutant) and that can also excise (thus making the mutation unstable); both of these functions are nonautonomous, being under the influence of a regulator element. recessive An allele that is not expressed in the heterozygous condition. Also the phenotype of the homozygote of a recessive allele. recessive allele An allele whose phenotypic effect is not expressed in a heterozygote. recessive phenotype The phenotype of a homozygote for the recessive allele; the parental phenotype that is not expressed in a heterozygote. reciprocal altruism An apparently altruistic behavior performed with the understanding that the recipient will reciprocate at some future date. reciprocal cross A cross, with the phenotype of each sex reversed as compared with the original cross, to test the role of parental sex on inheritance pattern. A pair of crosses of the type genotype A(female) X genotype B(male) and genotype B(female) X genotype A(male). reciprocal translocation A chromosomal configuration in which (usually) the ends of two non-homologous chromosomes have become exchanged. A translocation in which part of one chromosome is exchanged with a part of a separate non-homologous chromosome. recombinant In genetic mapping studies an offspring having a non-parental allele combination. An individual or cell with a genotype produced by recombination. recombinant DNA A novel DNA sequence formed by the joining, usually in vitro, of two non-homologous DNA molecules. recombinant DNA technology Techniques of gene cloning. Recombinant DNA refers to the molecule formed by joining a DNA of interest to vector DNA. See gene cloning. recombinant frequency (RF) The proportion (or percentage) of recombinant cells or individuals. recombination (1) The non-parental arrangement of alleles in progeny that can result from either independent assortment or crossing over. (2) In general, any process in a diploid or partially diploid cell that generates new gene or chromosomal combinations not found in that cell or in its progenitors. (3) At meiosis, the process that generates a haploid product of meiosis whose genotype is different from either of the two haploid genotypes that constituted the original meiotic diploid. recombinational repair The repair of a DNA lesion through a process, similar to recombination, that uses recombination enzymes. recon A term coined by Seymour Benzer for the smallest recombinable unit within a cistron. A region of a gene within which there can be no crossing-over; now known to be a single nucleotide pair. reduction division A nuclear division that produces daughter nuclei each having one-half as many centromeres as the parental nucleus. The first meiotic division reduces the number of chromosomes and centromeres to half that of the original cell. regression A term coined by Galton for the tendency of the quantitative traits of offspring to be closer to the population mean than are their parents' traits. It arises from a combination of factors - dominance, gene interactions, and environmental influences on traits. regression coefficient The slope of the straight line that most closely relates two correlated variables. regulator gene A gene primarily involved in controlling another (structural) gene. Genes that are involved in turning on or off the transcription of structural genes. relaxed mutant A mutant that does not exhibit the stringent response under amino acid starvation. release factors (RFI and RF2) Proteins in prokaryotes responsible for termination of translation and release of the newly synthesized polypeptide when a nonsense codon appears in the A site of the ribosome. Replaced by eRF in eukaryotes. Renner complex Specific gametic chromosome combination in Oenothera the evening primrose. repetitive DNA DNA made up of copies of the same or nearly the same nucleotide sequence. DNA sequences that are present in many copies per chromosome set. Repetitive DNA sequences may be closely linked (eg satellite DNA or VNTR loci) or dispersed throughout the genome or parts of the genome (eg alu family). replica plating A technique to rapidly transfer microorganism colonies from a master plate, in an exact spatial pattern, to a number of further plates. replication DNA synthesis. The process of copying. replication fork The point at which the two strands of DNA are separated to allow replication of each strand. replicative transposition The transposition of a transposable element to a new location without it being lost from the original location. replicon A chromosomal region whose replication is controlled by a single adjacent DNA replication initiation site. A genetic unit of replication including a length of DNA and its site for initiation of replication. replisome The DNA-replicating structure at the replication fork consisting of two DNA polymerase III enzymes and a primosome (primase and DNA helicase). reporter gene A gene whose phenotypic expression is easy to monitor; used to study promoter activity in different tissues or developmental stages. Recombinant DNA constructs are made in which the reporter gene is attached to a promoter region of particular interest and the construct transfected into a cell or organism. repressible operon Synthesis of a coordinated group of enzymes, involved in a single synthetic (anabolic) pathway, is repressible if excess quantities of (usually) the end product of the pathway leads to cessation of transcription of the genes encoding the enzymes of the pathway. repression The condition of an inactive gene. Lack of activity (transcription) due to presence of repressor. repressor The protein product of a regulator gene that acts to control transcription of inducible and repressible operons. A molecule that binds to the operator and prevents transcription of an operon. Generally any molecule that can reversibly inactivate a gene. reproductive isolating mechanism Any environmental; behavioural; mechanical; and physiological barrier that prevents two individuals of different populations producing viable progeny. reproductive success The relative production of offspring by a particular genotype. repulsion Allelic arrangement of two linked heterozygous loci, in which each homologous chromosome has one mutant (a or b) and one wild-type (A or B) allele (ie. Ab/aB). Two linked heterozygous gene pairs in the arrangement, Ab/aB. resolving power The ability of an experimental technique to distinguish between two genetic conditions (typically discussed when one condition is rare (eg. recombination between very closely linked loci) and of particular interest) . restriction digest The result of the action of a restriction endonuclease on a DNA sample. restriction endonuclease Endonuclease that recognizes a particular short DNA sequence which they cleave. They help to protect cells from viral infection and are used in work with DNA. restriction enzyme An endonuclease that will recognize a specific target nucleotide sequence in DNA and break the DNA chain at the target; a variety of these enzymes are known, and they are extensively used in genetic engineering. restriction fragment length polymorphism (RFLP) Variation in DNA fragment banding patterns of electrophoresed restriction digests of DNA from different individuals of a species. Often due to the presence of a restriction enzyme cleavage site at one place in the genome in one individual and the absence of that specific site in another individual. See also variable-number-of-tandem-repeats (VNTR) locus. restriction map A physical map of a piece of DNA showing recognition sites of specific restriction endonucleases separated by lengths marked in numbers of bases. restrictive condition An environmental condition under which a conditional mutant shows the mutant phenotype. restrictive temperature A temperature at which temperature-sensitive mutants display the mutant phenotype. retinoblastoma A cancer of the human retina. Predisposition to retinoblastoma is inherited as an autosomal dominant. A childhood cancer of retinoblast cells caused by the inactivation of an anti-oncogene. retrotransposon (retroposon) A class of genetic elements that includes retroviruses and transposons that have an intermediate RNA stage. A transposon that was created by reverse transcription of an RNA molecule. retrovirus An RNA virus that replicates by first being converted into double-stranded DNA by reverse transcriptase. reverse genetics The experimental procedure that begins with a cloned segment of DNA, or a protein sequence, and uses this knowledge to introduce programmed mutations (through directed mutagenesis) back into the genome in order to investigate gene and protein function. reverse transcriptase An enzyme, requiring a DNA primer, that catalyzes the synthesis of a DNA strand from an RNA template. An enzyme that can use RNA as a template to synthesize DNA. reversion The production of a wild-type gene from a mutant gene. The return of a mutant to the wild-type phenotype by way of a second mutational event. RFLP mapping A technique in which DNA restriction fragment length polymorphisms (RFLPs) are used as reference markers for mapping in relation to known genes or other RFLP loci. rho A protein factor required to recognize certain transcription termination signals in Escherichia coli. rho-dependent terminator A DNA sequence signaling the termination of transcription; termination requires the presence of the rho protein. rho-independent terminator A DNA sequence signaling the termination of transcription; the rho protein is not required for termination. ribosome A complex organelle (composed of proteins plus rRNA) that catalyzes translation of messenger RNA into an amino acid sequence. Ribosomes are made up of two non-identical subunits each consisting of a different rRNA and a different set of proteins. ribozyme Catalytic or autocatalytic RNA. RNA with enzymatic activity, for instance, self-splicing RNA molecules in Tetrahymena. RNA (ribonucleic acid) A single-stranded nucleic acid similar to DNA but having ribose sugar rather than deoxyribose sugar and uracil rather than thymine as one of the pyrimidine bases. RNA editing The insertion of uridines into mRNAs after transcription is completed; controlled by guide RNA (gRNA). May also sometimes involve insertion of cytidines and possible deletions of bases. RNA in situ hybridization A technique that is used to identify the spatial pattern of expression of a particular transcript (usually an mRNA). In this technique, the probe is labeled, either radioactively or by chemically attaching a fluorochrome visualised by fluorescence or an enzyme that can convert a substrate to a visible dye. A tissue or organism is soaked in a solution of single-stranded probe under conditions that allow the probe to hybridize to complementary RNA sequences in the cells; unhybridized probe is then removed. Radioactive probe is detected by autoradiography. Fluorochrome is detected by fluorescence microscopy. Enzyme labeled probe is detected by soaking the tissue in the substrate; the dye develops in sites where the transcript of interest was expressed. RNA phage Phage whose genetic material is RNA. They are the simplest known phages. RNA polymerase An enzyme that catalyzes the synthesis of an RNA strand from a DNA template. The enzyme that polymerizes RNA by using DNA as a template. It can also act as a primase initiating DNA replication. (Also known as transcriptase or RNA transcriptase). RNA replicase A polymerase enzyme that catalyzes the self-replication of single-stranded RNA. Robertsonian fusion Fusion of two acrocentric chromosomes at the centromere. rolling-circle replication A model of DNA replication that accounts for a circular DNA molecule producing linear daughter double helices. rRNA (ribosomal RNA) A class of RNA molecules, coded in the nucleolar organizer, that have an integral (but poorly understood) role in ribosome structure and function. RNA components of the subunits of the ribosomes.