Glossary Plant Breeding

Second Site Mutation. The second mutation of a double mutation within a gene. In many
cases, the second-site mutation suppresses the first mutation resulting in wild-type
phenotype of the double mutant.
Secondary Structure (of protein). A spiral or zigzag arrangement of the polypeptide
chain.
Secondary Trisomics. Trisomics with a secondary or isochromosome as the additional
one (2n + one isochromosome). Total number of secondary trisomics is equal to the
diploid chromosome number of the species. Only a few trisomics has been reported in
barley.
Secondary Triticales. See Triticales.
Sector. An area of tissue with a distinct phenotype from that of the rest of the tissue.
Sectorial. Of a chimera in which the genetically distinct tissues are cross-sectionally
arranged as sectors of a circle.
Sedimentation. The sinking of a molecule under the opposing forces of gravitation and
buoyancy.
Seed. A mature ovule with normal coverings. A seed consists of the seed coat, embryo,
and in certain plants, an endosperm.
Seedling Resistance. A category of resistance that is expressed in the early stage of
plants and may persist even in the later growth stages.
Segment. A portion of a chromosome taken as a unit under given circumstances.
Segregation. Separation of paternal from maternal chromosomes at meiosis and
consequent separation of different alleles from each other leading to the possibility of
recombination in the offspring. It may also be the production of two separate
phenotypes, corresponding to two alleles of a gene, either in different individuals
(meiotic segregation) or in different tissues of the same individual (mitotic
segregation).
Selection. A process in which individuals with certain characteristics are favoured in
reproduction. It is the discrimination among individuals in the number of offspring
contributed to the next generation. It is thus a non-random process that causes
individuals with different genotypes to be represented unequally in subsequent
212