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An Introduction to Genetic Analysis Glossary-P

P element A Drosophila transposable element that has been used as a tool for insertional mutagenesis and for germ-line transformation.

PAC (P1-based artificial chromosome) A derivative of phage P1 engineered as a cloning vector for carrying large inserts.

pair-rule gene In Drosophila, a member of a class of zygotically expressed genes that act at an intermediary stage in the process of the establishment of the correct numbers of body segments. Pair-rule mutations have half of the normal number of segments, owing to the loss of every other segment.

paracentric inversion An inversion not involving the centromere.

paracrine signaling The process by which a secreted molecule binds to receptors on or within nearby cells, thereby inducing a signal transduction pathway within the receiving cell.

paralogous genes Two genes in the same species that have evolved by gene duplication.

paramutation An epigenetic phenomenon in plants, in which the genetic activity of a normal allele is heritably reduced by virtue of that allele having been heterozygous with a special "paramutagenic" allele.

parental ditype (PD) A tetrad type containing two different genotypes, both of which are parental.

parental imprinting An epigenetic phenomenon in which the activity of a gene is dependent on whether the gene was inherited from the father or the mother. Some genes are maternally imprinted, others paternally.

parthenogenesis The production of offspring by a female with no genetic contribution from a male.

partial diploid See merozygote.

particulate inheritance The model proposing that genetic information is transmitted from one generation to the next in discrete units ("particles"), so the character of the offspring is not a smooth blend of essences from the parents (compare blending inheritance).

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 processes by which the complex shape and structure of higher organisms develop.

PD See parental ditype.

pedigree A "family tree," drawn with standard genetic symbols, showing inheritance patterns for specific phenotypic characters.

penetrance The proportion of individuals with a specific genotype who manifest that genotype at the phenotype level.

peptide See amino acid.

peptide bond A bond joining two amino acids.

pericentric inversion An inversion that involves the centromere.

permissive conditions Those environmental conditions under which a conditional mutant shows the wild-type phenotype.

petite A yeast mutation producing small colonies and altered mitochondrial functions. In cytoplasmic petites (neutral and suppressive petites), the mutation is a deletion in mitochondrial DNA; in segregational petites, the mutation is in nuclear DNA.

phage See bacteriophage.

Phe Phenylalanine (an amino acid).

phenocopy 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.

phenotypic sex determination Sex determination by nongenetic means.

phenylketonuria (PKU) A human metabolic disease caused by a mutation in a gene encoding a phenylalanine-processing enzyme, which leads to mental retardation if not treated; inherited as an autosomal recessive phenotype.

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 sugar group and a phosphate group; such bonds form the sugar-phosphate backbone of DNA.

phyletic evolution Heritable change over time in a continuous line of descent.

physical mapping Mapping the positions of cloned genomic fragments.

piebald A mammalian phenotype in which patches of skin are unpigmented because of lack of melanocytes; generally inherited as an autosomal dominant.

pilus (pili) A conjugation tube; a hollow hairlike appendage of a donor E. 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 the 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.

plasmid Autonomously replicating extrachromosomal DNA molecule.

plate (1) A flat dish used to culture microbes. (2) To spread cells over the surface of solid medium in a plate.

pleiotropic mutation A mutation that has effects on several different characters.

ploidy The number of chromosome sets.

point mutation A mutation that can be mapped to one specific locus.

Poisson distribution A mathematical expression giving the probability of observing various numbers of a particular event in a sample when the mean probability of an event on any one trial is very small.

poky A slow-growing mitochondrial mutant in Neurospora.

polar gene conversion A gradient of conversion frequency along the length of a gene.

polar granules Cytoplasmic granules localized at the posterior end of a Drosophila oocyte and early embryo. These granules are associated with the germ-line and posterior determinants.

polar mutation A mutation that affects the transcription or translation of the part of the gene or operon on only one side of the mutant site for example, nonsense mutations, frame-shift mutations, and IS-induced mutations.

polyacrylamide A material used to make electrophoretic gels for the separation of mixtures of macromolecules.

poly (A) tail A string of adenine nucleotides added to mRNA after transcription.

polycistronic mRNA An mRNA that encodes more than one protein.

polydactyly More than five fingers or toes or both. Inherited as an autosomal dominant phenotype.

polygenes See multiple-factor hypothesis.

polylinker A vector DNA sequence containing multiple unique restriction-enzyme-cut sites, convenient for inserting foreign DNA.

polymerase chain reaction (PCR) A method for amplifying specific DNA segments that exploits certain features of DNA replication.

polymorphism The occurrence in a population (or among populations) of several phenotypic forms associated with alleles of one gene or homologs of one chromosome.

polypeptide A chain of linked amino acids; a protein.

polyploid A cell having three or more chromosome sets or an organism composed of such cells.

polysaccharide A biological polymer composed of sugar subunits for example, starch or cellulose.

polytene chromosome A giant chromosome produced by an endomitotic process in which the multiple DNA sets remain bound in a haploid number of chromosomes.

position effect Used to describe a situation in which the phenotypic influence of a gene is altered by changes in the position of the gene within the genome.

position-effect variegation Variegation caused by the inactivation of a gene in some cells through its abnormal juxtaposition with heterochromatin.

positional information The process by which chemical cues that establish cell fate along a geographic axis are established in a developing embryo or tissue primordium.

positive assortative mating A situation in which like phenotypes mate more commonly than expected by chance.

positive control Regulation mediated by a protein that is required for the activation of a transcription unit.

postzygotic isolation Failure to exchange genes between species because zygotes that are formed either cannot develop to adulthood or, if they develop, are sterile in one or both sexes.

prezygotic isolation Failure to exchange genes between species because zygotes cannot be formed. The inability to form zygotes can be a consequence of temporal or ecological differences between the species, behavioral barriers to mating, mechanical lack of fit between the genitalia of the two sexes, or incompatibility of gametes.

primary structure of a protein The sequence of amino acids in the polypeptide chain.

primase An enzyme that makes RNA primers during DNA replication.

primer A short single-stranded RNA or DNA that can act as a start site for 3 chain growth when bound to a single-stranded template.

primer walking The use of a primer based on a sequenced area of the genome to sequence into a flanking unsequenced area.

prion Proteinaceous infectious particle that causes degenerative disorders of the central nervous system, such as "scrapie" in sheep and Creutzfeldt-Jacob disease in humans.

Pro Proline (an amino acid).

probe Defines a nucleic acid segment that can be used to identify specific DNA molecules bearing the complementary sequence, usually through autoradiography.

product of meiosis One of the (usually four) cells formed by the two meiotic divisions.

product rule The probability of two independent events occurring simultaneously is the product of the individual probabilities.

proflavin A mutagen that tends to produce frame-shift mutations.

prokaryote An organism composed of a prokaryotic cell, such as bacteria and blue-green algae.

prokaryotic cell A cell having no nuclear membrane and hence no separate nucleus.

promoter A regulator region a short distance from the 5 end of a gene that acts as the binding site for RNA polymerase.

prophage A phage "chromosome" inserted as part of the linear structure of the DNA chromosome of a bacterium.

prophase The early stage of nuclear division during which chromosomes condense and become visible.

propositus In a human pedigree, the person who first came to the attention of the geneticist.

protein kinase An enzyme that phosphorylates specific amino acid residues on specific target proteins. One major class of protein kinases phosphorylates tyrosines, and the other phosphorylates serines and threonines on target proteins.

proteome The complete set of protein-encoding genes in a genome.

proto-oncogene The normal cellular counterpart of a gene that can be mutated to become a dominant oncogene.

protoplast A plant cell whose wall has been removed.

prototroph A strain of organisms that will proliferate on minimal medium (compare auxotroph).

provirus A virus "chromosome" integrated into the DNA of the host cell.

pseudodominance The sudden appearance of a recessive phenotype in a pedigree, owing to the deletion of a masking dominant gene.

pseudogene An inactive gene derived from an ancestral active gene.

puff See chromosome puff.

pulse-chase experiment An experiment in which cells are grown in radioactive medium for a brief period (the pulse) and then transferred to nonradioactive medium for a longer period (the chase).

pulsed-field gel electrophoresis An electrophoretic technique in which the gel is subjected to electrical fields alternating between different angles, allowing very large DNA fragments to "snake" through the gel and hence permitting efficient separation of mixtures of such large fragments.

Punnett square A grid used as a graphic representation of the progeny zygotes resulting from different gamete fusions in a specific cross.

pure-breeding line or strain A group of identical individuals that always produce offspring of the same phenotype when intercrossed.

purine A type of nitrogen base; the purine bases in DNA are adenine and guanine.

pyrimidine A type of nitrogen base; the pyrimidine bases in DNA are cytosine and thymine.

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