Glossary Plant Breeding

Modified Single Cross. The progeny of a cross between a single cross, derived from two
related inbred lines, and an unrelated inbred line, (A 1 × A 2 )×B.
Modifier. A gene(s) that affects the expression of other gene(s) primarily in a quantitative
manner. It can act as enhancer or inhibitor depending upon the situation. Its effect
may be dominant or recessive. The degree of dominance or recessiveness of a gene
may also be influenced by modifying genes. Modifiers may have a large or small
effect (but the mode of action is almost always quantitative). Some of them may have
their own primary function and modifying actions appear as secondary effects, while
others have solely enhancing or inhibitory effects on the expression of other genes.
The well-documented example is that of spotting in mice. Similar cases in plants do
exist. Actually further improvement in any character (wherein major genes have
already been exploited) will depend upon manipulation of modifiers.
Molecular Biology. A branch of modern biology in which biological phenomena are
studied by physical, chemical and biochemical investigations at the molecular level.
Molecular Cytogenetics. A correlated study of cytology and genetics at molecular level.
It may provide clues about the stability and expression of trans genes.
Molecular Genetics. The study of genetic systems that can be described at the molecular
level. It is, therefore, the study of molecular processes underlying gene structure and
function.
Molecular Markers. Agents that mark genetic variation at DNA level. There are
numerous molecular markers, for example, RFLP, RAPDs, AFLP, STS, EST, SSRs
(also called microsatellites), SCARs, and the like. Except RFLP (that is southern
hybridisation based molecular marker), all are PCR-based markers. AFLP combines
the properties of RFLP (restriction fragment length polymorphism) and PCR
(polymerase chain reaction). However, STS (sequence tagged sites) is more desirable
because of high degree of reproducibility and large-scale automation, which is
essential for handling a large number of samples. These are numerous, permanent and
uninfluenced by environments/developmental stage of plants. Molecular markers can
be reliably used in the selection of superior genotypes. However, these are expensive
compared to morphological or biochemical ones. These can be used to: (a) develop
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