Terminologies in Genetics and Plant Breeding Part -17

U’s Triangle (U 1935). A triangle that depicts three basic diploid species (one each at each end) and three amphidiploid species (one each at the middle of each side) of Brassicas showing phylogenetic relationship of each alloploid species with the parental diploid species. The three basic species are B. nigra (n = 8), B. oleracea (n =9), and B. campestris (n = 10). It is obvious that haploid number increases or decreases by one chromosome at a time. Such changes in chromosome number could have been brought about in nature by duplication or loss of kinetochore, combined with a series of translocations.

Ultracentrifugation: Centrifugation carried out at high rotor speeds (<100,000rpm) and therefore under high centrifugal forces (<750,000 g)

Underdominance. A situation wherein the hybrid falls outside the limit of the parentwith lower phenotypic value, for example, early-maturing hybrids derived from across between parents having medium maturity.

Unequal crossing-over: A crossing-over after improper pairing betweenchromosome homologues that are not perfectly aligned; the result is, for example, one crossing-over chromatid with one copy of the segment and another with threecopies

Unequal Crossover. A crossover between homologs that are not perfectly aligned(absence of point-to-point pairing).

Uniformity One of the DUS criteria for PBR protection; the new variety must be sufficiently uniform in appearance, particularly for the traits used to establish its distinctness, on plant-by-plant basis under the specified environment of its adaptation.

Uniformity Trial. A trial that involves planting an experimental site with a single cropvariety and applying all cultural and management practices as uniformly as possible.It consists of measuring differences among several plot means treated alike. In such atrial, all sources of variability except that due to native soil differences are keptconstant. It facilitates measurement of soil heterogeneity, and thus providesinformation concerning presence of fertility gradient in an experimental field. Soilheterogeneity can be minimized through proper choice of plot size and shape, blocksize and shape, increase in the number of replications, etc.

Unigenes: Unique gene sequences; a set of non-redundant EST sequences for a species, each of which has a unique identity and genomic position.

Unilateral Incompatibility (Anderson and de Winton 1931). A unique phenomenon inwhich one of the reciprocal cross combinations succeeds whereas the other fails. It isvery closely related to self-incompatibility, for it occurs in matings of self-fertile toself-incompatible species and is controlled by the same physiological mechanism(inhibition of pollen tube growth). Although it occurs most frequently betweenspecies, it may be found within species and the term unilateral interspecificincompatibility describes the same mechanism. It was first described in a cross ofself-fertile Nicotiana langsdorfii with self-incompatible N. alata. UI may be attributedto the involvement of S-locus, S-allele specific ribonucleases, non-S-RNase factorsand HT proteins. It can be overcome by the use of mixed and mentor pollen,manipulation of environmental conditions and of style and ovary, chemical treatment,etc. There has been observed a clear linkage between SI and UI, which offersopportunities to unravel the molecular mechanisms of pollen rejection.

Unineme Theory. A theory that states that each chromatid represents a single linearDNA molecule with its associated proteins.

Union for Protection of New Varieties of Plants (UPOV): An intergovernmental organisation with headquarters in Geneva, Switzerland; it is based on theInternational Convention for the Protection of New Varieties of Plants, as revisedsince its signature in Paris on December 2, 1961; on April 16, 1993, the unionconsisted of 23-member States; the objective of the convention is the protection ofnew varieties of plants by an intellectual property right

Uniparental Inheritance. A kind of inheritance in which the transmission of certainphenotypes occurs from one parental type to all the progeny. Such inheritance isgenerally produced by organelle genes (mit-genes, plastomes).

Uniparental inheritance: A pattern of inheritance in which only one parentprovides genes to the progeny; a phenomenon, usually exhibited by extranucleargenes, in which all progeny have the phenotype of only one parent

Uniparental. Having only a single parent – parthenogenetic.

Unique DNA sequence: A DNA sequence that is present only once per genome, with no repetitive nucleotide sequence

Unisexual. Of organisms having separate sexes (dioecious) and producing only one kindof gametes. Such organisms are thus dimorphic.

Unit Character. A character difference which is transmitted as a unit in heredity; acharacter difference controlled by one pair of alleles.

Unit Membrane (Robertson 1959). The basic unit of the membrane system of any cell.The plasma membrane represents a single unit membrane. All other membranousstructures of the cell are also unit membrane.

Univalent shift: A spontaneous change in monosomy from one chromosome toanother; it is caused by partial asynapsis or desynapsis during meiosis

Univalent. An unpaired chromosome in meiosis; also called an asynaptic chromosome.Univalents either lack a homologue or result from asynapsis (due to asynaptic genesor influence of environmental factors). Bivalents, trivalents, quadrivalents, etc. areassociations of 2, 3, 4, etc. homologous chromosomes held together by chiasmata.

Univalent: A single chromosome observed during meiosis when bivalents arealso present; it has no pairing mate

Unpredictable Legume. A legume yield potential of which cannot be assessed visuallyowing to underground formation of economic products. Peanut sometimes is calledunpredictable legume; however, there are other legumes also (yam bean, for example)which form underground economic parts.

Unreduced gametes: Gametes not resulting from common meiosis, and soshowing the number of chromosomes per cell that is characteristic of a sporophyte; they spontaneously arise as consequence of irregular division in anaphase I ofmeiosis; they may contribute to spontaneous (meiotic) polyploidisation; in rye, they were used for production of tetraploids via valence crosses

Unstable Mutation. A mutation having high frequency of reversion. Such mutation maybe caused by the insertion of a controlling element, whose subsequent exit produces areversion.

Unstable QTL: The phenotypic effect of such a QTL is markedly affected by the environment so that it is detected in only some of the environments.

UPOV (1961). Union Pour La Protection Des Obtention Vegetables. It is an acronymderived from the fresh translation of above words meaning “International Union forthe Protection of New Varieties of Plants”. The objective of the UPOV convention(also called Paris convention, 1961) was to allow the breeder to acquire the sole rightto commercialise the newly developed variety and/or multiply and commercialiseunder license. Many countries in the developed world have adhered to the UPOV249convention, and a number of other countries have laws relating to plant varietyprotection at an advanced stage. The system has worked well and led to significantincrease in plant breeding and seed industry investment in UPOV member states.

Uracil. A pyrimidine base that appears in RNA instead of thymine found in DNA.

Uracil: A nitrogenous base normally found in RNA but not DNA; uracil iscapable of forming a base pair with adenine.

URF. Unassigned Reading Frame; an open reading frame (ORF) whose function is still undetermined. It is a sequence of nucleotides with proper start and stop codons but with no known function. URFs occurring within introns appear to be involved in specifying proteins important in splicing out of the introns themselves at the RNA level.

Uridine. The nucleoside having uracil as its base.

Useful LD: The level of LD that is useful for association mapping