V(D)J recombination the process by which most vertebrates assemble immunoglobin (Ig) and T cell receptor (TCR) genes during the development of lymphoid cells. In the germ cells, the genes that en- code the variable portions of the Ig and T cell recep- tor heterodimers are split into V (variable), J (join- ing), and sometimes D (diversity) segments. In immature lymphoid cells, segments of each type are joined together to make a V-J or a V-D-J fusion product. V(D)J recombination involves DNA cleav- age catalyzed by an element that behaves like a transposase (q.v.).
The element is the product of two genes, RAG-1 and RAG-2 (q.v.).
See Appendix C, 1990, Oettinger et al.; allelic exclusion, B lympho- cyte, immunoglobulin chains, immunoglobulin genes, somatic recombination, T cell receptor genes, T lym- phocyte. VDR vitamin D receptor (q.v.). vector an organism (such as the malaria mosquito) that transfers a parasite from one host to another.
See DNA vector, plasmid cloning vector, RNA vector, shuttle vector. vegetal hemisphere the surface of the amphibian egg farthest from the nucleus, the yolk-rich hemi- sphere of the egg. vegetative designating a stage or form of growth, especially in a plant, distinguished from that con- nected with reproduction. vegetative cell an actively growing cell, as op- posed to one forming spores. vegetative nucleus 1. the macronucleus of a cili- ate. 2. the tube-nucleus of a pollen grain. vegetative petites See petites. vegetative reproduction in plants, the formation of a new individual from a group of cells, without the production of an embryo or seed.
More gener- ally, asexual reproduction. See agamospermy, apo- mixis. vegetative state the noninfective state during which a phage genome multiplies actively and con- trols the synthesis by the host of the materials neces- sary for the production of infective particles. vehicle a plasmid or bacteriophage possessing a functional replicator site, and containing a genetic marker to facilitate its selective recognition, used to transport foreign genes into recipient cells during recombinant DNA experiments; also called a vector. vermilion (v) a sex-linked, recessive eye-color mu- tation in Drosophila melanogaster.
This was the first Drosophila mutation to be understood biochemi- cally.
The vermilion gene encodes tryptophan oxy- genase, an enzyme that converts tryptophan to for- mylkynurenine (q.v.). This is the first step in the reaction chain that leads to xanthomatin, the brown pigment in the Drosophila eye. If formylkynurenine is supplied in the diet of larvae containing mutant alleles of v, the adults that develop show normal eye color.
The first vermilion mutation to be isolated re- sulted from a transposon (q.v.) insertion.
See Appen- dix C, 1935, Beadle and Ephrussi; Drosophila eye pigments. vermilion plus substance formylkynurenine; so called because the synthesis of this compound in Drosophila melanogaster is controlled by the plus or wild-type allele of the gene vermilion (q.v.). vernalization the treatment of germinating seeds with low temperatures to effect their flowering. Winter varieties of certain cereals, if vernalized, can be sown in the spring and harvested in the summer.
Veronica a large genus of hardy herbs belonging to the family Scrophulariaceae.
Classic studies on the genetic control of self-sterility were performed on this species. vertical classification a system of classification that recognizes taxa corresponding to clades and groups transitional forms with their descendants rather than with their ancestors; the opposite of horizontal classification (q.v.). vertical evolution the process whereby an ances- tral species changes through time (without splitting) to become distinctively different, and therefore rec- ognized as a new species; phyletic evolution. See anagenesis, speciation. vertical transmission 1. passage of genetic infor- mation from one cell or individual organism to its progeny by conventional heredity mechanisms (mi- tosis, meiosis), in contrast to horizontal transmission (q.v.). 2. transmission of a parasite from parent to offspring via the egg or in utero. vervet monkey another name for the African green monkey, Cercopithecus aethiops (q.v.). V gene one of many (perhaps hundreds) of genes coding for the variable (N-terminus) region of an immunoglobulin chain. viability a measure of the number of individuals surviving in one phenotypic class relative to another class, taken as standard, under specified environmen- tal conditions.