Glossary
Genetics glossary
914 definitions · page 15 of 16.
- oogenesis
- The process of ovum formation in female animals.
- oogonia
- Cells in females that produce primary oocytes by mitosis.
- opal codon
- The codon UGA, a nonsense codon.
- operator
- A DNA region at one end of an operon that acts as the binding site for repressor protein. A DNA sequence that is recognized by a repressor protein or repressor-corepressor complex. When the operator is complexed with the repressor, transcription is prevented.
- operon
- A set of adjacent structural genes in bacteria whose mRNA is synthesized in one piece, together with the adjacent regulatory signals that affect transcription of the structural genes. A sequence of adjacent bacterial genes all under the transcriptional…
- ORF (open reading frame)
- A section of a sequenced piece of DNA that begins with an initiation (methionine ATG) codon and ends with a nonsense codon. ORFs all have the potential to encode a protein or polypeptide, however many may not actually do so.
- organelle
- A subcellular structure having a specialized function for example the mitochondrion, the chloroplast, or the spindle apparatus.
- organogenesis
- The production of organ systems during animal embryogenesis.
- origin of replication
- The point of specific sequence at which DNA replication is initiated.
- outbreeding
- The mating of genetically unrelated individuals.
- outside markers
- loci on either side of another locus or specified region.
- overdominance
- A relationship in which the phenotypic expression of the heterozygote is greater than that of either homozygote.
- ovum
- Egg. The one functional product of each meiosis in female animals.
- P (peptidyl) site
- The site on the ribosome occupied by the peptidyl-tRNA just before peptide bond formation.
- P element
- A Drosophila transposable element that has been used as a tool for insertion mutagenesis and for germline transformation.
- P1
- Parental generation.
- P53
- A 53KDa protein the product of a tumour suppressor gene. The loss of this protein due to mutation is a primary event in the formation of many types of cancer (breast, colon, lung, leukaemia, liver). P53 is involved in regulating the activity of some other…
- pachynema (pachytene stage)
- The stage of prophase 1 of meiosis in which chromatids are first distinctly visible.
- palindrome
- A sequence of letters, words, phrases, or nucleotides that reads the same regardless of direction, for instance what Adam said to Eve "Madam I'm Adam". The DNA sequence of type II restriction endonuclease recognition sites (eg EcoRI 5'GAATTC3') and the…
- panmictic
- Referring to unstructured (random-mating) populations.
- paracentric inversion
- An inversion not involving the centromere. A chromosomal inversion that does not include the centromere.
- paralogous genes
- Two genes or clusters of genes at different chromosomal locations in the same organism that have structural similarities indicating that they derived from a common ancestral gene.
- paramecin
- A toxin liberated by a killer Paramecium.
- parameters
- Measurements of attributes of a population.
- parapatric speciation
- speciation in which the evolution of reproductive isolating mechanisms occurs when a population enters a new niche or habitat within the range of the parent species.
- parental ditype (PD)
- A tetrad type containing two different genotypes, both of which are parental. A spore arrangement in Ascomycetes that contains only the two non-recombinant-type ascospores.
- parthenogenesis
- The production of offspring by a female with no genetic contribution from a male. The development of an individual from an unfertilized egg that did not arise by meiotic chromosome reduction .
- partial digest
- A restriction digest that has not been allowed to go to completion and thus contains pieces of DNA with some restriction endonuclease sites that have not yet been cleaved.
- particulate inheritance
- The model proposing that genetic information is transmitted from one generation to the next in discrete units (particles) so that the character of the offspring is not a smooth blend of essences from the parents (See blending inheritance).
- Pascal s triangle
- A triangular array of numbers made up of the coefficients of the binomial expansion.
- path diagram
- A modified pedigree showing only the direct line of descent from common ancestors.
- pathogen
- An organism that causes disease in another organism.
- patroclinous inheritance
- Inheritance in which all offspring have the nucleus-based phenotype of the father.
- pattern formation
- The developmental processes by which the complex shape and structure of higher organisms occurs.
- pedigree
- A family tree drawn with standard genetic symbols, showing inheritance patterns for specific phenotypic characters. A representation of the ancestry of an individual or family; a family tree.
- penetrance
- The proportion of individuals with a specific genotype who manifest that genotype at the phenotype level.
- peptide bond
- A bond joining two amino acids.
- peptidyl transferase
- The enzymatic centre in the ribosome responsible for peptide bond formation during translation.
- pericentric inversion
- An inversion that involves the centromere. A chromosomal inversion that includes the centromere.
- permissive condition
- Environmental condition under which a conditional mutation shows the wild-type phenotype.
- permissive temperature
- A temperature at which temperature-sensitive mutants are normal.
- PEST hypothesis
- The short life-time of a protein is signaled by a region rich in the amino acids proline (P); glutamic acid (E); serine (S); or threonine (T).
- petite
- A yeast mutation producing small colonies and altered mitochondrial functions. In cytoplasmic (neutral and suppressive) petites the mutation is a deletion in mitochondrial DNA, while in segregational petites the mutation occurs in nuclear DNA.
- petite mutation
- A mutation of yeast that produces small (petite) anaerobic-like colonies.
- Phe
- Phenylalanine (an amino acid).
- phenocopy
- A phenotype that is not genetically controlled but looks like a genetically controlled phenotype. An environmentally induced phenotype that resembles the phenotype produced by a mutation.
- phenotype
- (1) The form taken by some character (or group of characters) in a specific individual. (2) The detectable outward manifestations of a specific genotype. (3) The observable attributes of an organism.
- phenotypic sex determination
- Sex determination by non-genetic means.
- phenotypic variance
- Variance of phenotype due to genotypic and environmental factors combined.
- phenylketonuria(pku)
- A human metabolic disease caused by a mutation in a gene coding for a phenylalanine processing enzyme (phenylalanine hydroxylase), which leads to accumulation of phenylalanine and mental retardation if not treated; inherited as an autosomal recessive…
- pheromone
- A chemical signal, analogous to a hormone, that passes information between individuals.
- Philadelphia chromosome
- A translocation between the long arms of chromosomes 9 and 22, often found in the white blood cells of patients with chronic myeloid leukemia.
- phosphodiester bond
- A bond between a two sugar groups and a phosphate group; such bonds form the sugar-phosphate-sugar backbone of DNA and RNA. A diester bond (between phosphoric acid and two sugar molecules) linking two nucleotides together to form the nucleotide polymers DNA…
- photoreactivation
- The process whereby dimerized pyrimidines (usually thymines) in DNA are restored by an enzyme (deoxyribodipyrimidine photolyase) that requires light energy.
- phyletic gradualism
- The process of gradual evolutionary change over time.
- phylogenetic tree
- A diagram showing evolutionary lineages of organisms.
- piebald
- A mammalian phenotype in which patches of skin are unpigmented because of lack of melanocytes; generally inherited as an autosomal dominant.
- pilus (plural pili)
- A conjugation tube; a hollow hair-like appendage of a donor Escherichia coli cell that acts as a bridge for transmission of donor DNA to the recipient cell during conjugation.
- plant breeding
- The application of genetic analysis to development of plant lines better suited for human purposes.
- plaque
- A clear area on a bacterial lawn, left by lysis of the bacteria through progressive infections by a phage and its descendants. Clear area on a bacterial lawn caused by cell lysis due to viral attack.