| A |
| Adenine or adenosine. |
| 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 |
| 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. |
| 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
depending on whether non-homologous (adjacent-1) or homologous (adjacent-2)
centromeres segregate together. Adjacent-1 segregation is the usual
type of adjacent segregation, adjacent-2 segregation is rare. |
| 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. |
| 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 reciprocal translocation heterozygote such that
genetically balanced gametes are produced. |
| 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. |
| 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 of the spindle. |
| aneuploid cell |
| A cell having a chromosome number that differs
from the normal chromosome number for the species by a small number
of chromosomes. |
| aneuploid |
| Individuals or cells exhibiting aneuploidy.
|
| 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. |
| anti-oncogene |
| A gene that prevents malignant (cancerous)
growth and whose absence, by mutation, results in malignancy (eg
retinoblastoma). |
| antibody |
| A protein produced by a B lymphocyte that
protects the organism against an antigen. A protein (immunoglobulin)
molecule, produced by the immune system, that recognizes a particular
foreign antigen and binds to it; if the antigen is on the surface
of a cell, this binding leads to cell aggregation and subsequent
destruction. |
| anticoding strand |
| The DNA strand that forms the template for
both the transcribed mRNA and the coding DNA strand. |
| anticodon |
| The three-base sequence in tRNA complementary
to a codon on mRNA. 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 added to
a growing protein chain. |
| antigen |
| A molecule whose shape triggers the production
of antibodies (immunoglobulins) that will bind to the antigen. A
foreign substance capable of triggering an immune response in an
organism. |
| antimorph |
| A mutant expressing some agent that antagonizes
a normal gene product. |
| antimutator mutation
|
| mutation of DNA polymerase that decreases
the overall mutation rate. |
| 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. |
| antisense RNA |
| RNA product of mic (mRNA-interfering complementary
RNA) genes that regulates another gene by base pairing with and
thus preventing translation from its mRNA. |
| antisense strand |
| See anticoding strand. |
| AP endonuclease |
| endonuclease that initiates excision repair
at apurinic and apyrimidinic sites on DNA. |
| AP site |
| Apurinic or apyrimidinic site resulting
from the loss of a purine or pyrimidine residue from the DNA. |
| Arg |
| Arginine (an amino acid). |
| ascospore |
| A sexual spore from certain fungus species
in which spores are found in a sac called an ascus. Haploid spores
found in the asci of Ascomycete fungi. |
| ascus |
| In fungi, a sac that encloses a tetrad or
an octad of ascospores. The sac in Ascomycete fungi that holds the
ascospores. |
| Asn |
| Asparagine (an amino acid). |
| Asp |
| Aspartate (an amino acid). |
| assignment test |
| A test that determines whether a locus is
on a specific human chromosome by observation of the concordance
of the locus and the specific chromosome in a panel of human-mouse
hybrid cell lines containing only one or a few of the normal set
(22 autosomes, X and Y) of human chromosomes. |
| assortative mating |
| The mating of individuals with similar phenotypes.
|
| ataxia-telangiectasia
|
| A disease (loss of muscle control, and reddening
of the skin) in human beings caused by a defect in DNA repair mechanisms
induced by ionising radiation (X-rays, beta and alpha particles,
gamma rays). |
| ATP |
| (adenosine triphosphate) The energy molecule
of cells, synthesized mainly in mitochondria and chloroplasts; energy
from the breakdown of ATP drives many important reactions in the
cell. |
| attached X |
| A pair of Drosophila X chromosomes joined
at one end and inherited as a single unit. |
| attenuator |
| A control region at the promoter end of
repressible amino acid operons that exerts transcriptional control
based on the translation of a small leader peptide gene. A region
adjacent to the structural genes of the trp (tryptophan) operon;
in the presence of tryptophan this region acts to reduce the rate
of transcription from the structural genes. |
| attenuator stem |
| A configuration of the leader transcript
that signals transcription termination in attenuator-controlled
amino acid operons. |
| autogamy |
| nuclear reorganization in a single Paramecium
cell similar to the changes that occur during conjugation. |
| autonomous replication
sequence (ARS) |
| A segment of a DNA molecule necessary for
the initiation of its replication; generally a site recognized and
bound by the proteins of the replication system. |
| autopolyploid |
| A polyploid formed from the doubling of
a single genome. Polyploidy in which all the chromosomes come from
the same species. |
| autoradiography |
| A process by which radioactive materials,
often though not exclusively incorporated into cell structures,
are located by exposure to a photographic emulsion forming a pattern
on the film corresponding to the location of the radioactive compounds
within the cell. A technique in which radioactive molecules make
their location known by exposing photographic films or emulsions.
|
| autoregulation |
| The control of the transcription of a gene
by its own gene product. |
| autosomal set |
| The non-sex chromosomes consisting of one
from each homologous pair in a diploid species. |
| autosome |
| Any chromosome that is not a sex chromosome.
|
| autotroph |
| Organism that is able to utilize carbon
dioxide as a carbon source. |
| autozygosity |
| homozygosity in which the two alleles are
identical by descent (ie they are copies of an ancestral gene) .
|
| auxotroph |
| An organism with specific nutritional requirements.
|
| auxotrophic mutant |
| A mutant strain of microorganism that will
proliferate only when the medium is supplemented with some specific
substance not required by wild-type organisms. |
| axoneme |
| A bundle of microtubules and other proteins
forming the core of each cilium or flagellum. |
