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Modern Genetic Analysis
Glossary
A
A Adenine or adenosine.
abortive transduction The failure of a transducing DNA segment to be incorporated into the recipient chromosome.
acentric chromosome A chromosome having no centromere.
achondroplasia A type of dwarfism in humans, inherited as an autosomal dominant phenotype.
acrocentric chromosome A chromosome having the centromere located slightly nearer one end than the other.
activator A protein that, when bound to a cis-regulatory DNA element such as an operator or an enhancer, activates transcription from an adjacent promoter.
active site The part of a protein that must be maintained in a specific shape if the protein is to be functionalfor example, in an enzyme, the part to which the substrate binds.
adaptation In the evolutionary sense, some heritable feature of an individual's phenotype that improves its chances of survival and reproduction in the existing environment.
adaptive landscape The surface plotted in a three-dimensional graph, with all possible combinations of allele frequencies for different loci plotted in the plane and with mean fitness for each combination plotted in the third dimension.
adaptive peak A high point (perhaps one of several) on an adaptive landscape. Selection tends to drive the genotype composition of the population toward a genotype corresponding to an adaptive peak.
adaptive surface See adaptive landscape.
adaptor protein A protein that binds to certain specific phosphorylated amino acid sequences on a second protein, often a transmembrane receptor, and associates with still other proteins, thereby allowing a complex of proteins to "dock" to the receptor. This docking brings proteins of this complex into proximity to one another and, by doing so, permits propagation of an intracellular signal in a signal transduction pathway.
additive genetic variance Genetic variance associated with the average effects of substituting one allele for another.
adenine A purine base that pairs with thymine in the DNA double helix.
adenosine The nucleoside containing adenine as its base.
adenosine triphosphate See ATP.
adjacent segregation In a reciprocal translocation, the passage of a translocated and a normal chromosome to each of the poles.
ADP Adenosine diphosphate.
Ala Alanine (an amino acid).
albino A pigmentless "white" phenotype, determined by a mutation in a gene coding for a pigment-synthesizing enzyme.
alkylating agent A chemical agent that can add alkyl groups (for example, ethyl or methyl groups) to another molecule; many mutagens act through alkylation.
allele One of the different forms of a gene that can exist at a single locus.
allele frequency A measure of the commonness of an allele in a population; the proportion of all alleles of that gene in the population that are of this specific type.
allopolyploid See amphidiploid.
allosteric effector The small molecule that binds to an allosteric site.
allosteric site The site on a protein to which a small molecule binds, causing a change in the conformation of the protein that modifies the activity of its active site.
allosteric transition A change from one conformation of a protein to another.
alternate segregation In a reciprocal translocation, the passage of both normal chromosomes to one pole and both translocated chromosomes to the other pole.
alternation of generations The alternation of gametophyte and sporophyte stages in the life cycle of a plant.
alternative splicing The process by which different mRNAs are produced from the same primary transcript, through variations in the splicing pattern of the transcript. Multiple mRNA "isoforms" can be produced in a single cell or the different isoforms can display different tissue-specific patterns of expression. If the alternative exons fall within the open reading frames of the mRNA isoforms, different proteins will be produced by the alternative mRNAs.
amber codon The codon UAG, a nonsense codon.
amber suppressor A mutant allele eccoding a tRNA whose anticodon is altered in such a way that the tRNA inserts an amino acid at an amber codon in translation.
Ames test A widely used test to detect possible chemical carcinogens; based on mutagenicity in the bacterium Salmonella.
amino acid A peptide; the basic building block of proteins (or polypeptides).
amniocentesis A technique for testing the genotype of an embryo or fetus in utero with minimal risk to the mother or the child.
AMP Adenosine monophosphate.
amphidiploid An allopolyploid; a polyploid formed from the union of two separate chromosome sets and their subsequent doubling.
amplification The production of many DNA copies from one master region of DNA.
anaphase An intermediate stage of nuclear division during which chromosomes are pulled to the poles of the cell.
aneuploid cell A cell having a chromosome number that differs from the normal chromosome number for the species by a small number of chromosomes.
animal breeding The practical application of genetic analysis for development of lines of domestic animals suited to human purposes.
annealing Spontaneous alignment of two single DNA strands to form a double helix.
antibody A protein (immunoglobulin) molecule, produced by the immune system, that recognizes a particular substance (antigen) and binds to it.
anticodon A nucleotide triplet in a tRNA molecule that aligns with a particular codon in mRNA under the influence of the ribosome, so that the amino acid carried by the tRNA is inserted into a growing protein chain.
antigen A molecule that is recognized by antibody (immunoglobulin) molecules. Generally, multiple antibody molecules can recognize a given antigen.
antiparallel A term used to describe the opposite orientations of the two strands of a DNA double helix; the 5 end of one strand aligns with the 3 end of the other strand.
AP sites Apurinic or apyrimidinic sites resulting from the loss of a purine or pyrimidine reside from the DNA.
apoptosis The cellular pathways responsible for cell death and subsequent removal of the remains of the dead cell. Cell death may be induced by intracellular damage or by signals from neighboring cells.
Arg Arginine (an amino acid).
ascospore A sexual spore from certain fungus species in which spores are found in a sac called an ascus.
ascus In fungi, a sac that encloses a tetrad or an octad of ascospores.
asexual spores See spore.
Asn Asparagine (an amino acid).
Asp Aspartate (an amino acid).
ATP (adenosine triphosphate) The "energy molecule" of cells, synthesized mainly in mitochondria and chloroplasts; energy from the breakdown of ATP drives many important cell reactions.
attached X A pair of Drosphila X chromosomes joined at one end and inherited as a single unit.
attenuator A region adjacent to the structural genes of the trp operon; this region acts in the presence of tryptophan to reduce the rate of transcription from the structural genes.
autofeedback loop See autoregulatory loop.
autonomous controlling element A controlling element that seems to have both regulator and receptor functions combined in a single unit, which enters a gene and causes an unstable mutation.
autonomous phenotype A genetic trait in multicellular organisms in which only genotypically mutant cells exhibit the mutant phenotype. Conversely, a nonautonomous trait is one in which genotypically mutant cells cause other cells (regardless of their genotype) to exhibit a mutant phenotype.
autonomous replication sequence (ARS) A segment of a DNA molecule needed for the initiation of its replication; generally a site recognized and bound by the proteins of the replication system.
autophosphorylation The process by which a protein kinase phosphorylates specific amino acid residues on itself.
autopolyploid A polyploid formed from the doubling of a single genome.
autoradiogram A pattern of dark spots in a developed photographic film or emulsion, in the technique of autoradiography.
autoradiography A process in which radioactive materials are incorporated into cell structures, which are then placed next to a film or photographic emulsion, thus forming a pattern on the film corresponding to the location of the radioactive compounds within the cell.
autoregulatory loop The process by which the expression of a gene is controlled by it's own gene product.
autosome Any chromosome that is not a sex chromosome.
auxotroph A strain of microorganisms that will proliferate only when the medium is supplemented with some specific substance not required by wild-type organisms.

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