Genetics A - C
Terms
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- 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
- acrocentric chromosome
- a chromosome having the centromere located slightly nearer one end than the other
- active site
- the part of a protein that must be maintained in a specific shape if the protein is to be functional--for example, in an enzyme, the part to which the substrate binds
- 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 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
- adenine
- a purine base that pairs with thymine in the DNA double helix
- adenosine
- the nucleoside containing adenine as its base
- ATP
- adenosine triphosphate
- 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)
- alkylating agent
- a chemical agent that can add alkyl groups to another molecule; many mutagens act through alkylation
- allele
- one of two or more forms that can exist at a single gene 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
- 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
- amber codon
- the codon UAG, a nonsense codon
- amber suppressor
- a mutant allele coding for 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 of 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
- Ã…
- angstrom
- angstrom (Ã…)
- a unit of length equal to 10 to the negative 10th meter; many scientists now prefer to use the nanometer
- 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 foreign antigen and binds to if; if the antigen is on the surface of the cell, this binding leads to cell aggregation and subsequent destruction
- 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 peptide carried by the tRNA is inserted in a growing protein chain
- antigen
- a molecule (typically found on the surface of a cell) whose shape triggers the production of antibodies that will bind to the 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 residue from the DNA
- 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
- 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 reactions in the cell
- attached X
- a pair of Drosophila 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 form the structural genes
- autonomous controlling element
- a controlling element that apparently has both regulator and receptor functions combined in the single unit, which enters a gene and causes an unstable mutation
- ARS
- autonomous replication sequence
- 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
- autoradiography
- a process in which radioactive materials are incorporated into cell structures, which are then placed next to a film of photographic emulsion, thus forming a pattern on the film corresponding to the location of the radioactive compounds within the cell
- 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
- B chromosomes
- small plant chromosomes of variable number between individuals of a species, having no known phenotypic role
- bacteriophage (phage)
- a virus that infects bacteria
- balanced polymorphism
- stable genetic polymorphism maintained by natural selection
- Balbiani ring
- a large chromosome puff
- Barr body
- a densely staining mass that represents an X chromosome inactivated by dosage compensation
- base analog
- a chemical whose molecular structure mimics that of a DNA base; because of the mimicry, the analog may act as a mutagen
- bead theory
- the disproved hypothesis that genes are arranged on the chromosome like beads on a necklace, indivisible into smaller units of mutation and recombination
- bimodal distribution
- a statistical distribution having two modes
- binary fission
- the process in which a parent cell splits into two daughter cells of approximately equal size
- biparental zygote
- a Chlamydomonas zygote that contains cpDNA from both parents; such cells generally are rare
- blastoderm
- in an insect embryo, the layer of cells that completely surrounds an internal mass of yolk
- blending inheritance
- a discredited model of inheritance suggesting that the characteristics of an individual result from the smooth blending of fluidlike influences from its parents
- branch migration
- the process by which a single "invading" DNA strand extends its partial pairing with its complementary strand as it displaces the resident strand
- bridging cross
- a cross made to transfer alleles between two sexually isolated species by first transferring the alleles to an intermediate species that is sexually compatible with both
- broad heritability (H²)
- the proportion of total phenotypic variance at the population level that is contributed by genetic variance
- bud
- the daughter cell that forms a new cell wall; (the other daughter cell retains the cell wall of the parent)
- buoyant density
- measured by density-gradient ultracentrifugation
- C
- cytosine, or cytidine
- callus
- an undifferentiated clone of plant cells
- cAMP
- cyclic adenosine monophosphate
- cyclic adenosine monophosphate (cAMP)
- a molecule that plays a key role in the regulation of various processes within the cell
- canalized character
- a character whose phenotype is kept within narrow boundaries even in the presence of disturbing environments or mutations
- cancer
- a syndrome that involves the uncontrolled and abnormal division of eukaryotic cells
- CAP
- catabolite activator protein
- catabolite activator protein (CAP)
- a protein whose presence is necessary for the activation of the lac operon
- carbon source
- a nutrient (such as sugar) that provides carbon "skeletons" needed in the organism's synthesis of organic molecules
- carcinogen
- a substance that causes cancer
- carrier
- an individual who possesses a mutant allele but does not express it in the phenotype because of a dominant allele partner; thus, an individual of genotype Aa is a carrier of a if there is complete dominance of A over a
- cassette model
- a model to explain mating-type interconversion in yeast; a copy of either type of cassette may be transposed to the mating-type locus, where it is "played" (transcribed)
- catabolite repression
- the inactivation of an operon caused by the presence of large amounts of the metabolic end product of the operon
- cation
- a positively charged ion
- cDNA
- complementary DNA
- cell division
- the process by which two cells are formed from one
- cell lineage
- a pedigree of cells related through asexual division
- cM
- centimorgan
- central dogma
- the hypothesis that information flows only from DNA to RNA to protein; although some exceptions are now known, the rule is generally valid
- centromere
- a kinetochore; the constricted region of a nuclear chromosome to which the spindle fibers attach during division
- character
- some attribute of individuals within a species for which various heritable differences can be defined
- character difference
- alternative forms of the same attribute within a species
- chiasma (plural, chiasmata)
- a cross-shaped structure commonly observed between nonsister chromatids during meiosis; the site of crossing-over
- chi-square (X²) test
- a statistical procedure used to determine whether differences between sets of observed frequencies exceed those expected by chance if the sets were randomly selected from a single large population
- chloroplast
- a chlorophyll-containing organelle in plants that is the site of photosynthesis
- chromatid
- one of the two side-by-side replicas produced by chromosome division
- chromatid conversion
- a type of gene conversion that is inferred from the existence of identical sister-spore pairs in a fungal octad that shows a non-Mendelian allele ratio
- chromatid inference
- a situation in which the occurrence of a crossover between any two nonsister chromatids can be shown to affect the probability of those chromatids being involved in other crossovers in the same meiosis
- chromatin
- the substance of chromosomes; now known to include DNA, chromosomal proteins, and chromosomal RNA
- chromocenter
- the point at which the polytene chromosomes appear to be attached together
- chromomere
- a small beadlike structure visible on a chromosome during prophase of meiosis and mitosis
- chromosome
- a linear end-to-end arrangement of genes and other DNA, sometimes with associated protein and RNA
- chromosome aberration
- any type of change in the chromosome structure or number
- chromosome loss
- failure of a chromosome to become incorporated into a daughter nucleus at cell division
- chromosome puff
- a swelling at a site along the length of a polytene chromosome; the site of active transcription
- chromosome rearrangement
- a chromosome aberration involving new juxtapositions of chromosome parts
- chromosome set
- the group of different chromosomes that carries the basic set of genetic information for a particular species
- chromosome theory of inheritance
- the unifying theory stating that inheritance patterns may be generally explained by assuming that genes are located in specific sites on chromosomes
- cis conformation
- in a heterozygote involving two mutant sites within a gene or gene cluster
- cis dominance
- the ability of a gene to affect genes next to it on the same chromosome
- cis-trans test
- a test to determine whether two mutant sites of a gene are in the same functional unit or gene
- cistron
- originally defined as a functional genetic unit within which two mutations cannot complement. Now equated with the term gene as the region of DNA that encodes a single polypeptide (or functional RNA molecule such as tRNA or rRNA)
- clone
- a group of genetically identical cells or individuals derived by asexual division from a common ancestor; (colloquial) an individual formed by some asexual process so that it is genetically identical to its "parent"
- code dictionary
- a listing of the 64 possible codons and their translational meanings (the corresponding amino acids)
- codominance
- the situation in which a heterozygote shows the phenotypic effects of both alleles equally
- codon
- a section of DNA (three nucleotide pairs in length) that codes for a single amino acid
- coefficient of coincidence
- the ratio of the observed number of double recombinants to the expected number
- colinearity
- the correspondence between the location of a mutant site within a gene and the location of an amino-acid substitution within the polypeptide translated from that gene
- colony
- a visible clone of cells
- compartmentalization
- the existence of boundaries within the organism beyond which a specific clone of cells will never extend during development
- complementary DNA (cDNA)
- synthetic DNA transcribed from a specific RNA through the action of the enzyme reverse transcriptase
- complementary gene action
- intergenic complementation between mutant alleles at different loci to give wild-type phenotype
- complementary RNA (cRNA)
- synthetic RNA produced by transcription from a specific DNA single-stranded template
- complementation
- the production of a wild-type phenotype when two different mutations are combined in a diploid or a heterokaryon
- conditional mutation
- a mutation that has the wild-type phenotype under certain (permissive) environmental conditions and a mutant phenotype under other (restrictive)conditions
- conjugation
- the union of two bacterial cells, during which chromosomal material is transferred from the donor to the recipient cell
- conservative replication
- a disproved model of DNA synthesis suggesting that one-half of the daughter DNA molecules should have both strands composed of newly polymerized nucleotides
- constant region
- a region of an antibody molecule that is nearly identical with the corresponding regions of antibodies of different specificities
- constitutive heterochromatin
- specific regions of heterochromatin always present and in both homologs of a chromosome
- controlling element
- a mobile genetic element capable of producing an unstable mutant target gene; two types exist, the regulator and the receptor elements
- copy-choice model
- a model of the mechanism for crossing-over, suggesting that crossing-over occurs during chromosome division and can occur only between two supposedly "new" nonsister chromatids; the experimental evidence does not support this model
- correction
- the production (possibly by excision and repair) of a properly paired nucleotide pair from a sequence of hybrid DNA that contains an illegitimate pair
- correlation coefficient
- a statistical measure of the extent to which variations in one variable are related to variations in another
- cosegregation
- in Chlamydomonas, parallel behavior of different chloroplast markers in a cross, due to their close linkage on cpDNA
- cotransduction
- the simultaneous transduction of two bacterial marker genes
- cotransformation
- the simultaneous transformation of two bacterial marker genes
- coupling conformation
- linked heterozygous gene pairs in the arrangement A B/ a b
- covariance
- a statistical measure used in computing the correlation coefficient between two variables
- cpDNA
- chloroplast DNA
- cri-du-chat syndrome
- an abnormal human condition caused by deletion of part of one homolog of chromosome 5
- crisscross inheritance
- transmission of a gene from male parent to female child to male grandchild -- for example, X-linked inheritance
- cross
- the deliberate mating of two parental types of organisms in genetic analysis
- crossing-over
- the exchange of corresponding chromosome parts between homologs by breakage and reunion
- crossover suppressor
- an inversion (usually complex) that makes pairing and crossing-over impossible
- cruciform configuration
- a region of DNA with palindromic sequences in both strands, so that each strand pairs with itself to form a helix extending sideways from the main helix
- CSAR
- cytoplasmic segregation and recombination
- cytoplasmic segregation and recombination (CSAR)
- an acronym used to describe the process whereby organelle-based genes assort and recombine in a cytohet
- culture
- tissue or cells multiplying by asexual division, grown for experimentation
- Cys
- cysteine (an amino acid)
- cytidine
- the nucleoside containing cytosine as its base
- cytochromes
- a class of proteins, found in mitochondrial membranes, whose main function is oxidative phosphorylation of ADP to form ATP
- cytogenetics
- the cytological approach to genetics, mainly involving microscopic studies of chromosomes
- cytohet
- a cell containing two genetically distinct types of a specific organelle
- cytoplasm
- includes fluid (cytosol), organelles, and various membranes
- cytoplasmic inheritance
- inheritance via genes found in cytoplasmic organelles
- cytosine
- a pyrimidine base that pairs with guanine
- cytosol
- the fluid portion of the cytoplasm (outside the organelles)