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Modern Genetic Analysis Glossary
R
R plasmid A plasmid containing one or several transposons that bear resistance genes.
radiation hybrid A type of humanmouse hybrid cell in which human chromosomes have been fragmented by radiation to determine which markers are inherited together and therefore linked.
random genetic drift Changes in allele frequency that result because the genes appearing in offspring are not a perfectly representative sampling of the parental genes.
random mating Mating, between individuals, in which the choice of a partner is not influenced by the genotypes (with respect to specific genes under study).
RAPD (randomly amplified polymorphic DNA) A set of several genomic fragments amplified by single PCR primer; somewhat variable from individual to individual; + / heterozygotes for individual fragments can act as markers in genome mapping.
reading frame The codon sequence that is determined by reading nucleotides in groups of three from some specific start codon.
realized heritability 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 that had been denatured.
receptor See ligandreceptor interaction.
receptor element A controlling element that can insert into a gene (making it a mutant) and can also excise (thus making the mutation unstable); both of these functions are nonautonomous, being under the influence of the regulator element.
receptor tyrosine kinase (RTK) A transmembrane receptor whose cytoplasmic domain includes a tyrosine kinase enzymatic activity. In normal situations, the kinase is activated only on binding of the appropriate ligand to the receptor.
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 crosses A pair of crosses of the type genotype A × genotype B and genotype B × genotype A .
reciprocal translocation A translocation in which part of one chromosome is exchanged with a part of a separate nonhomologous chromosome.
recombinant An individual or cell with a genotype produced by recombination.
recombinant DNA A novel DNA sequence formed by the combination of two nonhomologous DNA molecules.
recombinant frequency (RF) The proportion (or percentage) of recombinant cells or individuals.
recombination (1) 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. (2) 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 meiotic diploid.
recombinational repair The repair of a DNA lesion through a process, similar to recombination, that uses recombination enzymes.
recon A region of a gene within which there can be no crossing-over; now known to be a nucleotide pair.
reduction division A nuclear division that produces daughter nuclei each having one-half as many centromeres as the parental nucleus.
redundant DNA See repetitive DNA.
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 dominance, gene interaction, and nongenetic influences on traits.
regression coefficient The slope of the straight line that most closely relates two correlated variables.
regulator element See receptor element.
regulatory genes Genes that are involved in turning on or off the transcription of structural genes.
regulatory region Upstream (5) end of a gene to which bind various proteins that cause transcription of the gene at the correct time and place.
repetitive DNA Redundant DNA; DNA sequences that are present in many copies per chromosome set.
replication DNA synthesis.
replication fork The point at which the two strands of DNA are separated to allow replication of each strand.
replicon A chromosomal region under the influence of one adjacent replication-initiation locus.
reporter gene A gene whose phenotypic expression is easy to monitor; used to study tissue-specific promoter and enhancer activities in transgenes.
repressor A protein that binds to a cis-acting element such as an operator or a silencer, thereby preventing transcription from an adjacent promoter.
repressor protein A molecule that binds to the operator and prevents transcription of an operon.
repulsion conformation Two linked heterozygous gene pairs in the arrangement of Ab / aB.
resolving power The ability of an experimental technique to distinguish between two genetic conditions (typically discussed when one condition is rare and of particular interest).
restriction enzyme An endonuclease that will recognize specific target nucleotide sequences in DNA and break the DNA chain at those points; a variety of these enzymes are known, and they are extensively used in genetic engineering.
restriction map A map of a chromosomal region showing the positions of target sites of one or more restriction enzymes.
restrictive conditions Environmental conditions under which a conditional mutant shows the mutant phenotype.
retinoblastoma A childhood cancer of the human retina.
retrotransposon A transposable element that utilizes reverse transcriptase to transpose through an RNA intermediate.
retrovirus An RNA virus that replicates by first being converted into double-stranded DNA.
reverse genetics An experimental procedure that begins with a cloned segment of DNA or a protein sequence and uses it (through directed mutagenesis) to introduce programmed mutations back into the genome to investigate function.
reverse transcriptase An enzyme that catalyzes the synthesis of a DNA strand from an RNA template.
reversion The production of a wild-type gene from a mutant gene.
RTF See recombinant frequency.
RFLP (restriction fragment length polymorphism) At some chromosomal locations, a probe sometimes detects different sizes or different numbers of restriction fragments (often as a result of presence or absence of restriction sites), and this situation is an RFLP. If an individual is heterozygous for such a chromosomal difference, that region can be used as a marker in chromosome mapping.
RFLP mapping A technique in which DNA restriction fragment length polymorphisms are used as reference loci for mapping in relation to known genes or other RFLP loci.
rho A protein factor required to recognize certain transcription termination signals in E. coli.
ribonucleic acid See RNA.
ribosomal RNA See rRNA.
ribosome A complex organelle that catalyzes the translation of messenger RNA into an amino acid sequence. Composed of proteins plus rRNA.
ribozymes RNAs with enzymatic activitiesfor instance, the 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 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 DNA probe is labeled, either radioactivity or by chemically attaching an enzyme that can convert a substrate into a visible dye. A tissue or organism is soaked in a solution of single-stranded labeled DNA under conditions that allow the DNA to hybridize to complementary RNA sequences in the cells; unhybridized DNA is then removed. Radioactive probe is detected by autoradiography. 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 polymerase An enzyme that catalyzes the synthesis of an RNA strand from a DNA template. In eukaryotes, there are several classes of RNA polymerase; structural genes for proteins are transcribed by RNA polymerase II.
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.
RTK See receptor tyrosine kinase.

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