Glossary Plant Breeding

R Plasmid. A plasmid containing one or several transposons that bear resistance genes.
Race. A population or aggregate of populations with characteristic gene frequencies that
distinguishes it from any other such groups within formally recognizable subspecies
or species. It is essentially an intraspecific category. Differences between races are
only relative and not absolute. Any race is able to interbreed freely with any other race
of the same species. There may be geographical races (subspecies occupying a
geographical subdivision of the range of a species), ecological races (ecotypes),
physiological races (differences are physiological rather than morphological), etc.
Races (pathological). Pathogens of the same species with or without identical/similar
morphology but are characterized by differing pathogenic capabilities. This implies
that the two races may have the same genetic complements except for their
pathogenic capacity. Zadoks (1966) defined a race as a “taxon” within a pathogen
species characterized by a specific combination of virulence genes. Differential series
of cultivars used to identify races, in fact, characterize particular virulence genes. In
older classification, races were used to be subdivided into biotypes. However, in
modern classification, the two terms are used interchangeably; but the term, biotype,
is used more frequently by entomologists than by plant pathologists. Terms like
pathotype, physiological race, and strain are also used as synonyms for the race.
RAD. Radiation Absorbed Dose; a measure of radiation dose. It can be defined as the
absorbed dose of 100 ergs of energy per gram of tissue. The Roentgen (R), an older
unit which, strictly speaking, is to be used only for γ and χ-rays. One Roentgen unit
provides 93 ergs of energy per gram of tissue.
Random Chromatid Assortment (Haldane 1930). A type of partition and segregation
(or assortment) of chromatids wherein sister chromatids at a given locus may end up
in the same gamete (in a higher order even polyploids). For example, in an
autotetraploid, when quadrivalent form and crossing over takes place betwixt
kinetochore and the locus in question, sister chromatids at this locus can end up
attached to different kinetochores. Consequently, sister alleles may be included in the
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