Glossary Plant Breeding

Sister Chromatid Exchange. An event similar to crossing-over that can take place
between sister chromatids at mitosis or meiosis. It has been detected in harlequin
chromosomes.
Site. The smallest unit in a gene capable of independent mutation. The entity is a single
nucleotide. Also called mutational site.
Site Directed Mutagenesis. A process of introducing specific mutations into the
sequences of a cloned gene in order to assay the resulting phenotype to make it
possible to ascertain the function of the gene. Site directed mutagenesis based on
polymerase chain reaction is the most popular.
Site Specific Recombination. Recombination between two specific sequences that need
not be homologous. It is mediated by a specific recombination system.
Skewness. The departure from symmetrical spread of the frequency curve around the
central value. Positive skewness enhances value of the constant of selection intensity
(> 2.06 at 5% SI) whereas the negative skewness retards it. Therefore, the data must
be tested for normal distribution while making prediction for genetic advance. In case
of skewness, genetic advance is overestimated (or underestimated).
Soma. The body of an organism aside from the germ line cells.
Somaclonal Variation (Larkin and Scowcroft 1983). Tissue culture induced variation
occurring within a clone of cells. Such variations may be developmental or heritable
(stable). If stable, it may provide for broadening the genetic base and thus obtaining
incremental improvement in the commercial cultivars, more particularly in clonally
propagated crops. In India, Pusa Jaikisan (a somaclone of Varuna variety of Indian
mustard) and B-13 (a somaclone of a medicinal plant, Citronella java) havealready
been released to farmers for general commercial cultivation.
Somatic. Referring to diploid body cells (2n), normally with one set of chromosomes
coming from the male parent and the other from the female one.
Somatic Cell. A “body cell” that is not destined to become a gamete. Its genes will not be
passedontofuturegenerations.
Somatic Cell Genetics. The genetics of somatic or body cells. It is the asexual genetics,
including the study of somatic mutation, assortment, crossing-over, and cell fusion.
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