| identity by descent |
| The state of two alleles when they are identical
copies of the same ancestral allele (autozygous). |
| idiogram |
| A photograph or diagram of the chromosomes
of a cell arranged in an orderly fashion. See karyotype. |
| idiotypic variation |
| Variation unique to an individual immunoglobulin
molecule produced by the variable region of immunoglobulin genes.
|
| idling reaction |
| The production of guanosine tetraphosphate
(3' ppGpp 5', magic spot) by the stringent factor when a ribosome
encounters an uncharged tRNA in the A site. |
| Ile |
| Isoleucine (an amino acid). |
| imago |
| An adult insect. |
| immune system |
| The cells and tissues involved in recognizing
and attacking foreign substances within the body of an animal. |
| immunity |
| The ability of an organism to resist infection.
|
| immunoglobulin (Ig) |
| A general term for the kind of globular
blood proteins that constitute antibodies. A tetrameric protein
composed of two identical light chains and two identical heavy chains.
Specific proteins produced by derivatives of B lymphocytes that
interact with and help protect an organism from specific antigens.
|
| immunohistochemistry
|
| The use of antibodies or antisera as histological
tools for identifying patterns of antigen distribution within a
tissue or an organism. An antibody (or mixture of antibodies) that
binds to a specific protein or other antigen is tagged with a fluorescent
chemical or an enzyme that can convert a substrate to a visible
dye. The tagged antibody is incubated with the tissue and after
washing unbound antibody away the bound antibody distribution is
revealed by fluorescence microscopy or incubation with a chromogenic
substrate. It is assumed that antibody distribution reflects antigen
distribution. |
| in situ |
| In place |
| in vitro |
| In an experimental situation outside the
organism. Biological or chemical work done in the test tube (literally
in glass) rather than in living systems. |
| in vitro mutagenesis
|
| The production of either random or specific
mutations in a piece of cloned DNA. Typically, the DNA will then
be reintroduced into a cell or an organism to assess the results
of the mutagenesis. |
| in vivo |
| In a living cell or organism. |
| inbreeding |
| The mating of genetically related individuals.
Mating between relatives. |
| inbreeding coefficient
(F) |
| The probability of homozygosity by descent
(having common ancestors). The probability that a zygote obtains
copies of the same ancestral gene from both its parents because
they are related. The probability of autozygosity. |
| inbreeding depression
|
| A depression of vigour or yield due to inbreeding.
|
| incestuous |
| A mating between blood relatives who are
more closely related than the law of the land allows. |
| inclusive fitness |
| The expansion of the concept of the fitness
of a genotype to include benefits accrued to relatives of an individual
since relatives share parts of their genomes. Hence an apparently
altruistic act toward a relative may in fact enhance the fitness
of the individual performing the act. |
| incomplete dominance
|
| The situation in which both alleles of a
heterozygote influence the phenotype. The phenotype is usually intermediate
between the two homozygous phenotypes. The situation in which a
heterozygote shows a phenotype somewhere (but not exactly half-way)
intermediate between the corresponding homozygote phenotypes. (Exact
intermediacy is no dominance.) See also dominance, codominance and
recessivity. |
| independent assortment
|
| Mendel's second rule describing the independent
segregation of alleles of different unlinked or remotely linked
loci. |
| inducer |
| An environmental agent that triggers transcription
from an operon. |
| inducible operon |
| A gene system, often encoding a coordinated
group of enzymes involved in a catabolic pathway, is inducible if
an early metabolite in the pathway causes activation, usually by
interaction with and inactivation of a repressor, of transcription
of the genes encoding the enzymes. |
| induction |
| 1. The relief of repression for a gene or
set of genes under negative control by a repressor. |
| 2. Regarding temperate phages, the process
causing a prophage to become virulent. |
| 3. In development, an interaction between
two cell lineages to alter the developmental fate of one or both
of them. |
| inductive interaction
|
| The interaction between two groups of cells
in which a signal passed from one group of cells causes the other
group of cells to change their developmental state (or fate). |
| industrial melanism |
| The darkening of several species of moths
during the recent period of industrialization in many countries
by the spread of a gene controlling elevated melanin synthesis.
|
| infectious transfer |
| The rapid transmission of free episomes
(plus any host chromosomal genes they may carry) from donor to recipient
cells in a bacterial population. |
| initiation codon |
| The mRNA sequence AUG, which specifies methionine,
the first amino acid used in the translation process. (Occasionally
GUG, valine, is recognized as an initiation codon). |
| initiation complex |
| The complex formed for initiation of translation.
It consists of the 30S ribosomal subunit; mRNA; N-formyl-methionine
tRNA; and three initiation factors . |
| initiation factors (IF1,
IF2, IF3) |
| Proteins (prokaryotic with eukaryotic analogues)
required for the proper initiation of translation. |
| initiator protein |
| Protein that recognizes the origin of replication
on a replicon and takes part in primosome construction. |
| inosine |
| A rare base that is important at the wobble
(3rd) position of some tRNA anticodons. |
| insertion mutagenesis
|
| Change in gene activity due to an insertion
(of DNA) event that either changes the gene directly or disrupts
control mechanisms. |
| insertion sequence (IS)
|
| A mobile piece of bacterial DNA (several
hundred nucleotide pairs in length) that is capable of inactivating
a gene into which it inserts. Small simple transposons. See transposable
genetic element. |
| insertional translocation
|
| The insertion of a segment from one chromosome
into another non-homologous chromosome. |
| inside marker |
| The middle locus of three linked loci. |
| intercalary heterochromatin
|
| Heterochromatin, other than centromeric
heterochromatin, dispersed throughout eukaryotic chromosomes. |
| intercalating agent |
| A chemical that can insert itself between
the stacked bases at the centre of the DNA double helix, possibly
causing a frameshift mutation. |
| interchromosomal recombination
|
| Recombination resulting from independent
assortment. |
| interference |
| A measure of the independence of crossover
events from each other, calculated by subtracting the coefficient
of coincidence from 1. |
| interference competition
|
| A form of competition that involves a fight
or other active interaction among organisms. |
| intergenic suppression
|
| A mutation at a second locus that apparently
restores the wild-type phenotype to a mutation at a first locus.
|
| interkinesis |
| The abbreviated interphase that occurs between
meiosis I and II. No DNA replication occurs here. |
| interphase |
| The cell cycle stage between nuclear divisions,
when chromosomes are extended and functionally active. The metabolically
active non-dividing stage of the cell cycle. |
| interrupted mating |
| A technique used to map bacterial genes
by determining the sequence in which donor genes enter recipient
cells. A gene mapping technique in which bacterial conjugation is
disrupted after specified time intervals. |
| intersex |
| An organism with external sexual characteristics
that have attributes of both sexes. |
| interstitial region |
| The chromosomal region between the centromere
and the site of a rearrangement. |
| intra-allelic complementation
|
| The restoration of activity or partial activity
(complementation) to an enzyme made of identical subunits (polypeptides
encoded by one gene) in a heterozygote of two different mutant alleles
of that gene. |
| intrachromosomal recombination
|
| Recombination resulting from crossing over
between two linked gene pairs. |
| intragenic suppression
|
| A second mutation within a mutant gene that
results in an apparent restoration of the original phenotype. |
| intron (intervening sequence)
|
| A DNA segment of largely unknown function
within a gene that specifically interupts the coding (exon) sequences
of that gene. Introns are transcribed as part of the normal gene
primary transcript, but intron sequences are not found in the functional
mRNA. Intron sequences are removed from the primary transcript by
a splicing mechanism. |
| inversion |
| A chromosomal mutation involving the removal
of a chromosome segment, its rotation through l80 degrees, and its
reinsertion in the same location. The replacement of a section of
a chromosome in the reverse orientation. |
| inverted repeat (IR)
sequence |
| A sequence found in identical (but inverted)
form, for example, at the opposite ends of a transposon. A nucleotide
sequence found at two sites on the same double helix but in opposite
orientation. |
| iojap |
| A locus in maize that produces variegation.
|
| ionizing radiation |
| Radiation, such as X-rays and gamma rays
(high energy photons), that causes atoms to release electrons and
become ions. |
| isoaccepting tRNA |
| The various types of tRNA molecule carrying
a specific amino acid. |
| isochromosome |
| A chromosome with two genetically and morphologically
identical arms. |
| isoschizomer |
| restriction endonuclease that recognizes
the same target DNA sequence and cleaves it in the same way. |
| isotope |
| One of several forms of an element having
the same atomic number but differing atomic masses. |
| isozyme |
| A different electrophoretic form of the
same multi-subunit enzyme. Unlike allozymes isozymes are due to
differing subunit configurations rather than allelic differences.
|
