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Modern Genetic Analysis Glossary
E
ecdysone A molting hormone in insects.
ectopic expression The occurrence of gene expression in a tissue in which it is normally not expressed. Such ectopic expression can be caused by the juxtaposition of novel enhancer elements to a gene.
ectopic integration In a transgenic organism, the insertion of an introduced gene at a site other than its usual locus.
electrophoresis A technique for separating the components of a mixture of molecules (proteins, DNAs, or RNAs) in an electric field within a gel.
embryonic (or tissue) polarity The production of axes of asymmetry in a developing embryo or tissue primordium.
embryonic stem cells Cultured cell lines that are established from very early embryos and that are essentially totipotent. That is, these cells can be implanted into a host embryo and populate many or all tissues of the developing animal, most importantly including the germ line. Manipulations of these embryonic stem cells (ES cells) are used extensively in mouse genetics to produce targeted gene knockouts.
endocrine signal The ligand secreted into the circulatory system from a gland (that is, endocrine organ). This ligand will bind to receptors within or on the surface of target cells.
endocrine system The organs in the body that secrete hormones into the circulatory system.
endogenote See merozygote.
endonuclease An enzyme that cleaves the phosphodiester bond within a nucleotide chain.
endopolyploidy An increase in the number of chromosome sets caused by replication without cell division.
endosperm Triploid tissue in a seed, formed from the fusion of two haploid female nuclei and one haploid male nucleus.
enforced outbreeding Deliberate avoidance of mating between relatives.
enhancer A cis-regulatory sequence that can elevate levels of transcription from an adjacent promoter. Many tissuespecific enhancers can determine spatial patterns of gene expression in higher eukaryotes. Enhancers can act on promoters over many tens of kilobases of DNA and can be 5 or 3 to the promoter that they regulate.
enhancer trap A transgenic construction inserted in a chromosome and used to identify tissue-specific enhancers in the genome. In such a construct, a promoter sensitive to enhancer regulation is fused to a reporter gene, such that expression patterns of the reporter gene identify the spatial regulation conferred by nearby enhancers.
enucleate cell A cell having no nucleus.
environment The combination of all the conditions external to an organism that potentially affect the expression of its genome.
environmental variance The variance due to environmental variation.
enzyme A protein that functions as a catalyst.
epigenetic inheritance Processes by which heritable modifications in gene function occur but are not due to changes in the base sequence of the DNA of the organism. Examples of epigenetic inheritance are paramutation, X chromosome inactivation, and parental imprinting.
episome A genetic element in bacteria that can replicate free in the cytoplasm or can be inserted into the main bacterial chromosome and replicate with the chromosome.
epistasis A situation in which the differential phenotypic expression of a genotype at one locus depends on the genotype at another locus.
epitope The part of an antigen molecule that is recognized by a specific immunoglobulin.
equational division A nuclear division that maintains the same ploidy level of the cell.
equivalence group A set of immature cells that all have the same developmental potential. In many cases, cells of an equivalence group end up adopting different fates from one another.
ES cells See embryonic stem cells.
EST (expressed sequence tag) A sequence-tagged site derived from a cDNA clone; used to position and identify genes in genomic analysis.
ethidium A molecule that can intercalate into DNA double helices when the helix is under torsional stress.
euchromatin A chromosomal region that stains normally; thought to contain the normally functioning genes.
eugenics State-controlled human breeding in an attempt to improve future generations.
eukaryote An organism having eukaryotic cells.
eukaryotic cell A cell containing a nucleus.
euploid A cell having any number of complete chromosome sets or an individual composed of such cells.
excision repair The repair of a DNA lesion by removal of the faulty DNA segment and its replacement with a wild-type segment.
exconjugant A female bacterial cell that has just been in conjugation with a male and that contains a fragment of male DNA.
exogenote See merozygote.
exon Any nonintron section of the coding sequence of a gene; together, the exons constitute the mRNA and are translated into protein.
exonuclease An enzyme that cleaves nucleotides one at a time from an end of a polynucleotide chain.
expression library A library in which the vector carries transcriptional signals to allow any cloned insert to produce mRNA and ultimately a protein product.
expression vector A vector with the appropriate bacterial regulatory regions located 5 to the insertion site, allowing transcription and translation of a foreign protein in bacteria.
expressivity The degree to which a particular genotype is expressed in the phenotype.

© 1999 by W. H. Freeman and Company. All rights reserved.