Glossary
Genetics glossary
914 definitions · page 1 of 16.
- A (aminoacyl) site
- The site on the ribosome occupied by an aminoacyl-tRNA just prior to peptide bond formation.
- A form DNA
- The form of DNA at high humidity; it has tilted base pairs and more base pairs per turn than does B DNA.
- abortive transduction
- The failure of a transducing DNA segment to be incorporated into the recipient chromosome.
- acentric chromosome
- A chromosome having no centromere.
- acentric fragment
- A chromosomal piece without a centromere.
- achondroplasia
- A type of dwarfism in humans inherited as an autosomal dominant phenotype.
- acrocentric chromosome
- A chromosome whose centromere lies very near one end. See also metacentric chromosome
- activation energy (deltaG)
- Energy needed to initiate a chemical reaction.
- active site
- The part of a protein that must be maintained in a specific shape if the protein is to be functional, for example, the part to which the substrate binds in an enzyme. The part of an enzyme where the actual enzymatic function is performed.
- 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 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 combination corresponding to an adaptive peak.
- adaptive surface
- See adaptive landscape.
- adaptive value
- See fitness.
- additive genetic variance
- Genetic variance associated with the average effects of substituting one allele for another.
- additive model
- A mechanism of quantitative inheritance in which alleles at different loci either add a fixed amount to the phenotype or add nothing.
- adenine
- A purine base that pairs with thymine in the DNA double helix. See purines.
- adenosine
- The nucleoside containing adenine as its base.
- adjacent segregation
- In a reciprocal translocation heterozygote during meiosis the segregation of a translocated and a normal chromosome together, giving unbalanced gametes with duplications and deficiencies leading to non-viable zygotes. Adjacent segregation is of two kinds…
- adjacent-1 segregation
- segregation of non-homologous centromeres during meiosis in a reciprocal translocation heterozygote such that unbalanced gametes with duplications and deficiencies are produced. See alternate segregation and adjacent-2 segregation.
- adjacent-2 segregation
- segregation of homologous centromeres during meiosis in a translocation heterozygote such that unbalanced gametes with duplications and deficiencies are produced. See alternate segregation and adjacent-1 segregation.
- ADP
- Adenosine diphosphate.
- affected
- Individuals in a pedigree that exhibit the specific phenotype under study.
- 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
- Alternative form of a gene. One of the different forms of a gene that can exist at a single locus.
- allele frequency
- Often called gene frequency. A measure of how common an allele is in a population; the proportion of all alleles at one gene locus that are of one specific type in a population.
- allelic exclusion
- A process whereby only one immunoglobulin light chain and one heavy chain gene are transcribed in any one cell; the other genes are repressed.
- allopatric speciation
- speciation in which the evolution of reproductive isolating mechanisms occurs during physical separation of the populations.
- allopolyploid
- polyploid produced by the hybridization of two species. See amphidiploid.
- allosteric protein
- A protein whose shape is changed when it binds a particular molecule. In the new shape the protein's ability to react to a second molecule is altered.
- allosteric transition
- A change from one conformation of a protein to another.
- allotype
- The protein product (or the result of its activity) of an allele which may be detected as an antigen in another member of the same species.(eg histocompatibility antigens, immunoglobulins), obeying the rules of simple Mendelian inheritance.
- allozygosity
- homozygosity in which the two alleles are alike but unrelated. See autozygosity.
- allozyme
- Form of an enzyme that differs in amino acid sequence, as shown by electrophoretic mobility or some other property, from other forms of the same enzyme and is encoded by one allele at a single locus. See isozymes.
- alternate segregation
- At meiosis in a reciprocal translocation heterozygote, the passage (segregation) of both normal chromosomes to one pole and both translocated chromosomes to the other pole, giving genetically balanced gametes. Segregation of centromeres during meiosis in a…
- alternation of generations
- The alternation of gametophyte and sporophyte stages in the life cycle of a plant.
- alternative splicing
- Various ways of splicing out introns in eukaryotic pre-mRNAs resulting in one gene producing several different mRNAs and protein products.
- altruism
- A form of behaviour in which an individual risks lowering its fitness for the benefit of another.
- Alu family
- A dispersed intermediately repetitive DNA sequence found in the human genome in about three hundred thousand copies. The sequence is about 300 bp long. The name Alu comes from the restriction endonuclease AluI that cleaves it.
- amber codon
- The codon UAG, a nonsense codon.
- amber suppressor
- A mutant allele coding for a tRNA whose anticodon is altered in such a way that the suppressor tRNA inserts an amino acid at an amber codon in translation suppressing (preventing) termination.
- Ames test
- A widely used test to detect possible chemical carcinogens; based on mutagenicity in the bacterium Salmonella.
- amino acid
- The basic building block of proteins (or polypeptides). Containing a basic amino (NH2) group, an acidic carboxyl (COOH) group and a side chain (R - of a number of different kinds) attached to an alpha carbon atom. Thus: R | NH2-C-COOH | H
- aminoacyl-tRNA
- Transfer RNA (tRNA) molecule with its cognate amino acid attached.
- aminoacyl-tRNA synthetase
- enzyme that attaches an amino acid to its cognate tRNA(s).
- amniocentesis
- A technique for testing the genotype of an embryo or fetus in utero with minimal risk to the mother or the child.
- amorph
- A mutant showing the complete lack of some normal substance or structure.
- AMP
- Adenosine monophosphate.
- amphidiploid
- An allopolyploid; a polyploid formed from the union of two separate chromosome sets and their subsequent doubling. An organism produced by hybridization of two species followed by chromosome doubling. An allotetraploid that appears to be a normal diploid.
- amplification of DNA
- The production of many DNA copies from one or a few copies.
- anagenesis
- The evolutionary process whereby one species evolves into another without any splitting of the phylogenetic tree. See cladogenesis.
- anaphase
- The stage of meiosis and mitosis in which sister chromatids (meiosisII) or homologous chromosomes (mitosis and meiosisI) are separated by spindle fibers. An intermediate stage of nuclear division during which chromosomes or chromatids are pulled to the poles…
- aneuploid
- Individuals or cells exhibiting aneuploidy.
- aneuploid cell
- A cell having a chromosome number that differs from the normal chromosome number for the species by a small number of chromosomes.
- aneuploidy
- The condition of a cell or of an organism that has additions or deletions of a small number of whole chromosomes from the expected balanced diploid number of chromosomes.
- angiosperm
- Plant whose seeds are enclosed within an ovary. Flowering plants.
- animal breeding
- The practical application of genetic analysis for development of lines of domestic animals suited to human purposes.
- annealing
- Spontaneous alignment of two complementary single polynucleotide (RNA, or DNA, or RNA and DNA) strands to form a double helix.