Glossary Plant Breeding

always proceeds in the opposite direction (5’ to 3’ direction). This is why lagging
strand is synthesized in fragments (while the leading strand continuously).
Oligogene. Major gene.
Oligogenic. Of characters controlled by one or a few genes.
Oligonucleotide. A short segment of synthetic DNA.
Oncogene. The cancer-causing gene; a gene whose expression leads to the production of
a cancer. Oncogenes are usually mutated forms of normal cellular genes.
OneGene–OneEnzymeHypothesis(Beadle and Tatum 1941). A hypothesis stating
that each gene controls the synthesis or the activity of only a single protein with
catalytic activity (enzyme). With the developments in biochemical genetics, the
principle has been verified and modified. The modified version is one cistron – one
polypeptide.
Ontogeny. The course of development of an individual organism, organ, or an organelle.
On-Types. Plants that conform to the standard type in question with respect to a
characteristic or group of characters. For example, several on-type progenies are
bulked to form the pure-seed lot during purification of a variety through mass
selection.
Opal Codon. One of three stop codons, UGA.
Open-Pollinated Corn. Corn propagated from seed produced by uncontrolled
pollination.
Open-Pollinated Progeny Test. A test for GCA by evaluating progenies derived from
seed produced on selected plants outcrossed with other plants of the same population.
Usually used in forage species.
Operator. A DNA region at one end of an operon that acts as the binding site for
repressor protein.
Operon. A set of adjacent structural genes (whose mRNA is synthesized in one unit) and
the adjacent regulatory signals that affect transcription of the structural genes.
Oppositional Factor Hypothesis (East and Mangelsdorf, 1925). A hypothesis put
forward to account for gametophytic incompatibility in Nicotiana spp. It states that
166