Glossary Plant Breeding

Dihaploid. Ahaploid(n=2x) of tetraploid. It contains two basic sets of chromosomes.
For example, the haploid of S. tuberosum, which is a tetraploid species (2n = 4x),
would be called a dihaploid (n=2x).
Dihybrid. Heterozygous with respect to two loci.
DIMBOA. A biochemical (2,4-Dihydroxy-7-methoxy-1,4 benzoxazin-3-one). Its
presence in the leaves of inbred lines of maize has been shown to confer resistance to
first brood larvae of stem borer. It has also been found as a primary chemical factor in
the non-preference mechanism of resistant dent maize. As the feeding habits of the
second brood larvae are different, the above correlation does not hold true in this case.
Dimorphism. The occurrence of two forms (genotypes) within a population; the
occurrence of two types of flowers on one plant.
Dioecy. The state of being dioecious. It is a condition in which male and female flowers
occur on different plants of the same species (e.g., papaya, asparagus, spinach, hemp,
hops, etc). This mating system is genetically controlled, and prevents self-pollination
and thus enforces cross-pollination. Although more frequent in animals, it is only
sporadic in higher plants, probably because it is wasteful of gametes in non-mobile
organisms.
Dioecious. A mating system wherein staminate and pistillate flowers are borne on
different individuals of the same species (papaya, for example).
Diploid. An organism with two chromosomes of each kind. Such individuals are
designated by 2n.
Diplospory.
Diplotene. The stage of meiosis which follows pachytene and during which chromatids
move apart in pairs but still united in the region of chiasmata.
Directed Mutagenesis. Altering some specific part of a cloned gene and reintroducing
the modified gene back into the organism.
Directional Selection. Selection favouring an extreme phenotype in a given direction. It
changes the frequency of an allele in a constant direction, either towards or away from
fixation for that allele. It pushes a population towards homozygosity (fixation of the
allele). If practiced in the opposite direction, it may eliminate the allele from the
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