Glossary Plant Breeding

interest and those influencing other traits. The transfer that occurs during back
crossing involves a block of closely linked genes instead of a single one.
Isolation Distance. The minimum distance required to separate seed-producing field
from other varieties of the same or related species in order to prevent outcrossing or
undesired mating. This is an indispensable endeavour to maintain the genetic purity of
a variety during the process of its multiplication.
Isolate. A group of individuals (population) or of populations, who because of their
isolation, choose mating partners from that group. Because of their isolation, one
isolate is completely separated from other such groups.
Isolation. The separation of one group from another so that mating between or amongst
them is prevented. In genetic term, it is the restriction to exchange of genetic
information between distinct populations due to barriers to interbreeding. If the gene
flow is prevented through differences in reproductive mechanisms, it is called
reproductive isolation.
Isomeric Genes. Genes which can each produce the same phenotype. If a number of such
genes are present in one individual, they may be cumulative or non-cumulative in
their action.
Isotope. One of several forms of an atom having the same atomic number but differing
atomic masses ( 12 C 6 and 14 C 6 , for example).
Isozymes. Multiple molecular forms of a single enzyme that differ by minor variations in
amino acid composition and sometimes in regulation. Lactic dehydrogenase, which
catalyses the conversion of pyruvate to lactate, is a tetramer, and have five isozymes
(isoenzymes) derived from all the combinations. They differ in their electrophoretic
mobility in starch gels. The relative proportions of the isoenzymes are characteristic
for each tissue and for each stage of development.
Iteration. A procedure that makes use of repeated trials to find the best-fitting value of a
parameter from observed data.
Japonica Rice. The temperate ecotypes of rice, characterized by more leaves, fewer
tillers, relatively shattering resistance, and having broad and short grains with lower
amylose content.
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