Glossary

Genetics glossary

914 definitions · page 4 of 16.

chi-square distribution: The sampling distribution of the chi-square statistic. A family of curves whose shapes depend on degrees of freedom.
chi-square test: A statistical test used to determine the probability of obtaining the observed results by chance, under a specific hypothesis.
chiasma (plural chiasmata): A cross-shaped structure commonly observed between nonsister chromatids during meiosis; the site of crossing-over. X-shaped configuration seen in tetrads during the latter stages of prophase 1 of meiosis. They represent physical crossovers between DNA…
chimeric plasmid: Hybrid or genetically mixed plasmid used in DNA cloning.
chloroplast: The organelle that carries out photosynthesis and starch grain formation. A chlorophyll-containing organelle in plants that is the site of photosynthesis.
chromatid: One of the two side by side replicas produced by chromosome replication in mitosis or meiosis. Subunit of a chromosome after replication and prior to anaphase of meiosis II or mitosis. At anaphase of meiosis II or mitosis when the centromeres divide and the…
chromatid conversion: 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 interference: 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. The nucleoprotein material of the eukaryotic chromosome.
chromocenter: The point at which polytene chromosomes appear to be attached together.
chromomere: A small beadlike structure visible on a chromosome during prophase of meiosis and mitosis. Dark regions of chromatin condensation in eukaryotic chromosomes at meiosis or mitosis.
chromosome: A linear end-to-end arrangement of genes and other DNA, sometimes with associated protein and RNA. The form of the genetic material in viruses and cells. A circle of DNA in prokaryotes; a DNA or an RNA molecule in viruses; a linear nucleoprotein complex in…
chromosome aberration: Any type of change in the chromosome structure or number.
chromosome jumping: A technique of isolating clones from a genomic library that are not contiguous by skipping a region between known points on the chromosome. Done usually to bypass regions that are difficult or impossible to walk through or regions known not to be of interest.
chromosome loss: Failure of a chromosome to become incorporated into a daughter nucleus at cell division.
chromosome marker: Any distinct and heritable feature of chromosome structure that can be used to follow (usually by microscopy) that chromosome or chromosome region in breeding experiments.
chromosome mutation: Any type of change in the chromosome structure or number.
chromosome painting: The use of fluorescent-tagged chromosome-specific dispersed repeat DNA sequences to visualize specific chromosomes or chromosome segments by in situ DNA hybridization and fluorescence microscopy.
chromosome puff: A swelling at a site along the length of a polytene chromosome; the site of active transcription. A diffuse uncoiled region in a polytene chromosome where transcription is actively taking place.
chromosome rearrangement: A chromosome mutation 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 theory that chromosomes are linear sequences of genes. The unifying theory stating that inheritance patterns may be generally explained by assuming that genes are located in specific sites on chromosomes.
chromosome walking: A technique which produces sets of overlapping DNA clones for studying segments of DNA larger than can be cloned individually. A method for the analysis of large regions of DNA, in which a each end of a large single cloned DNA fragment is used separately to…
circular permutation: Some phages with linear genomes (eg T2) are both terminally redundant and circularly permuted. If the genetic information is represented by ABCDEFGH then a circular permutation would generate molecules ABCDEFGH BCDEFGHA CDEFGHAB DEFGHABC EFGHABCD and so on.
cis: Meaning on the near side of; refers to geometric configurations of atoms or mutations on the same molecule or chromosome.
cis conformation: In a heterozygote involving two mutant sites (a,b) within a gene or within a gene cluster, the arrangement ab/AB.
cis dominance: The ability of a gene to affect genes next to it on the same chromosome.
cis-dominant: mutations (eg of an operator) that alter the functioning of genes on that same piece of DNA.
cis-trans complementation test: A mating test to determine whether two different recessive mutations (a1;a2) on opposite chromosomes (trans, a1+/+a2) of a diploid or partial diploid will not complement (ie have a mutant phenotype) each other; but the same two recessive mutations on the…
cistron: Term coined by Benzer for the smallest genetic unit that does NOT show genetic complementation when two different mutations are in trans during a cis-trans complementation test; but shows wild-type phenotype when the same mutations are in cis. Synonymous…
cladogenesis: The evolutionary process whereby one species splits into two or more species. See anagenesis.
ClB method: A technique devised by Muller to rapidly screen fruit flies for recessive X chromosome lethal mutations. The ClB chromosome carries a recessive lethal (l) a dominant marker (B) and an inversion (crossover suppressor C).
clinal selection: Selection that changes gradually along a geographic gradient.
clone: (1) A group of genetically identical cells or individuals derived by asexual division from a common ancestor. (2) (colloquial), An individual formed by some asexual process so that it is genetically identical to its parent. (3) See also DNA clone.
code dictionary: A listing of the 64 possible codons and their translational meanings (the corresponding amino acids).
coding strand: The DNA strand with the same sequence as the transcribed mRNA (given U in RNA and T in DNA) and containing the linear array of codons which interact with anticodons of tRNA during translation to give the primary sequence of a protein. Compare with anticoding…
codominance: The situation in which a heterozygote shows the phenotypic effects of both alleles equally, (eg blood group antigens).
codon: A section of DNA (three nucleotide pairs in length) or RNA (three nucleotides in length) that codes for a single amino acid. A sequence of three RNA or DNA nucleotides that specifies (codes for) either an amino acid or the termination of translation.
codon preference: The idea that for amino acids with several codons one or a few are preferred and are used disproportionately. They would correspond with tRNAs that are abundant.
coefficient of coincidence: In a three point cross the number of observed double crossovers divided by the number expected based on the observed occurrence of single crossovers. The ratio of the observed number of double recombinants to the expected number.
coefficient of relationship (r): The proportion of alleles held in common by two related individuals.
cohesive end: A single-stranded end to a linear duplex DNA molecule which can hydrogen-bond with a complementary single-strand base sequence from the end of the same or another DNA molecule.
cointegrate: A fusion of two elements. An intermediate structure in replicative transposition. The product of the fusion of two circular elements to form a single, larger circle.
col plasmid: plasmid that produces an antibiotic (colicin) used by the host to kill other strains of bacteria.
colinearity: The one to one linear correspondence between the order of codons in a coding sequence and the order of amino acids in the protein encoded. A linear map of mutation sites within a gene corresponds to the linear location of amino acid substitutions within the…
colony: A visible clone of cells.
common ancestry: The state of two individuals when they are blood relatives. When two parents have a common ancestor their offspring will be inbred.
compartment: The existence of boundaries within the organism beyond which a specific clone of cells will never extend during development.
competence factor: A surface protein that binds extracellular DNA and enables the cell to be transformed.
competent: Able to take up exogenous DNA and thereby be transformed.
complementarity: The correspondence of DNA bases in the double helix such that adenine in one strand is opposite thymine in the other strand and cytosine in one strand is opposite guanine in the other. This relationship explains Chargaff's rule.
complementary DNA (cDNA): Synthetic DNA reverse transcribed from a specific RNA through the action of the enzyme reverse transcriptase. DNA synthesized by reverse transcriptase using RNA as a template.
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. The production of the wild-type phenotype by a cell or an organism that contains two mutant genes. If complementation occurs the mutations are…
complementation group: A group of mutant genes which do not complement each other. A cistron (determined by the cis-trans complementation test).
complementation test: A mating test to determine whether two different recessive mutations (a1;a2) on opposite chromosomes (trans, a1+/+a2) of a diploid or partial diploid will not complement (ie have a mutant phenotype) each other; but the same two recessive mutations on the…
complete linkage: The state in which two loci are so close together that alleles of these loci are virtually never separated by crossing over.
complete medium: A culture medium that is enriched to contain all of the growth requirements of a strain of organisms.
component of fitness: A particular aspect in the life cycle of an organism upon which natural selection acts.
composite transposon: A transposon constructed of two IS (insertion sequence) elements flanking a control region that frequently contains host genes.