Glossary Plant Breeding

facing in breeding for drought tolerance is how to evaluate the trait. Drought tolerance
has a low heritability caused by high genotype-environment interaction. Even when
consistently selected, it is difficult to demonstrate drought tolerance consistently. This
probably reflects the fact that it has a complex inheritance and is governed by
polygenes.
Drought. A condition of moisture deficit. It refers to a situation wherein available soil
moisture is not sufficient to meet the demands of potential evapo-transpiration. The
kind of drought that is of our interest is the agricultural drought. It occurs where soil
moisture and rainfall are inadequate during the growing season to support healthy
crop growth to maturity and cause extreme crop stress and wilt. It should carefully be
distinguished with physiological drought, which refers to non-availability of water to
the plant due to unfavourable physiological conditions (such as soil salinity, low
temperature, and the like). Water is present in the real sense, but plants are unable to
use it.
Duplex. A polyploid containing recessive alleles in all chromosomes except two with
respect to a particular locus or a polyploid having two dominant alleles at a given
genetic locus, e.g., AAa, AAaa, AAaaaa, etc.
Duplicate Base Collections. Duplicates of the base collections that are housed in
geographically different locations for security purposes. The objectives and methods
of storage are essentially the same as for the base collections.
Duplicate Genes. Two gene pairs that produce identical effects, whether alone or
together. Non-floating habit of rice is governed by duplicate genes. (Floating habit is
present only when the two gene pairs are recessive). The two gene pairs are located in
different chromosomes. Duplicate genes are of frequent occurrence, and are probably
due to secondary polyploidy. A heterozygote for duplicate genes under the situation of
complete dominance gives a 15:1 F 2 ratio. If, however, dominant genes act in
additive/cumulative manner, the F 2 segregation gets modified to a 9:6:1 ratio.
Duplication. The presence of a segment twice in a chromosome. Adjacent duplication
may occur in tandem sequence with respect to each other – abcbcd – or in reverse
order – abccbd. Obviously, the pairing patterns obtained in these two cases are
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