Glossary Plant Breeding

Non-Mendelian Ratio. An unusual ratio of progeny phenotypes that does not conform to
the simple operation of Mendel’s laws of inheritance. For example, mutant: wild
ratios of 3: 5, 5: 3, 6: 2 or 2: 6 in tetrads indicate that gene conversion has occurred.
Non-Preference. An insect response to characteristics of plants that make them
undesirable for use by insects as a site for reproduction, food, shelter, or any
combination of these three. Two types of non-preferences have been reported : (a) one
that is manifested only in the presence of the preferred host, and (b) the second even
in the absence of the preferred host. Such a mechanism is strong enough to force the
insects to starve, particularly in early instar stages. Therefore, the extreme nonpreference
may be mistaken for antibiosis and vice-versa. The plant characters that
influence non-preferences include colour, light reflection, type of pubescence, leaf
angle, odour, taste, etc. It has been observed that nectariless cultivars of cotton are
resistant to cotton ball worm. Similarly, yellow-green varieties of peas are less
desirable to the pea aphid than are blue-green cultivars. Solid stem has been
associated with resistance to stem saw fly and stem borer in wheat and maize,
respectively. However, solid stem is often associated with low yielding capacity, and
precisely for this reason, resistant cultivars are not grown where these insects are
problems in the respective crops. It seems likely that non-preference is polygenically
determined.
Non-Random Mating. Deviations from random mating based either on mating of like to
like or on mating of unlikes. Both assortative and disassortative mating are nonrandom
mating systems.
Nonsense Codon. A codon for which no formal tRNA molecule exists, and the presence
of which causes termination of translation of genetic message in the form of ending of
the polypeptide chain. Three such codons are UAG, UGA and UAA. Though these
bear the sense of termination of polypeptide chain, this misnomer still exists. (The use
of the term such as stop codon instead of calling it nonsense one seems to be the most
appropriate).
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