I
identical twins -- See monozygotic twins.
Ig -- See immunoglobulin.
imaginal disc -- Discrete groups of cells set aside during embryogenesis in holometabolous insects which are determined to form the external body parts of the adult.
immunoglobulin -- The class of serum proteins having the properties of antibodies.
inborn error of metabolism -- A biochemical disorder that is genetically controlled; usually an enzyme defect that produces a clinical syndrome.
inbreeding -- Mating between closely related organisms.
inbreeding depression -- A decrease in viability, vigor, or growth in progeny following several rounds of inbreeding.
incomplete dominance -- Expression of heterozygous phenotype which is distinct from, and often intermediate to, that of either parent.
incomplete linkage -- Occasional separation of two genes on the same chromosome by a recombinational event.
independent assortment -- The independent behavior of each pair of homologous chromosomes during their segregation in meiosis I. The random distribution of maternal and paternal homologs into gametes.
inducer -- An effector molecule that activates transcription.
inducible enzyme system -- An enzyme system under the control of a regulatory molecule, or inducer, which acts to block a repressor and allow transcription.
Back to top
initiation codon -- The triplet of nucleotides (AUG) in an mRNA molecule that codes for the insertion of the amino acid methionine as the first amino acid in a polypeptide chain.
insertion sequence -- See IS element.
in situ hybridization -- A technique for the cytological localization of DNA sequences complementary to a given nucleic acid or polynucleotide.
intercalary deletion -- A form of chromosome deletion where material is lost from within the chromosome. Deletions that involve the end of the chromosome are called terminal deletions.
intercalating agent -- A compound that inserts between bases in a DNA molecule, disrupting the alignments and pairing of bases in the complementary strands (e.g., acridine dyes).
interference -- A measure of the degree to which one crossover affects the incidence of another crossover in an adjacent region of the same chromatid. Negative interference increases the chances of another crossover; positive interference reduces the probability of a second crossover event.
interphase -- That portion of the cell cycle between divisions.
intervening sequence -- See intron.
intron -- A portion of DNA between coding regions in a gene which is transcribed, but which does not appear in the mRNA product.
inversion -- A chromosomal aberration in which the order of a chromosomal segment has been reversed.
inversion loop -- The chromosomal configuration resulting from the synapsis of homologous chromosomes, one of which carries an inversion.
Back to top
in vitro-- Literally, in glass; outside the living organism; occurring in an artificial environment.
in vivo-- Literally, in the living; occurring within the living body of an organism.
IS element -- A mobile DNA segment that is transposable to any of a number of sites in the genome.
isoagglutinogen -- An antigenic factor or substance present on the surface of cells that is capable of inducing the formation of an antibody.
isochromosome -- An aberrant chromosome with two identical arms and homologous loci.
isolating mechanism -- Any barrier to the exchange of genes between different populations of a group of organisms. In general, isolation can be classified as spatial, environmental, or reproductive.
