Glossary Plant Breeding

Expressivity. The extent of manifestation of a genetic character. It is the degree to which
a particular genotype is expressed in the phenotype. For instance, some leaves of lima
beans show partial chlorophyll content, while others may show only at the margins,
still others develop it only at the tip of the leaves, thus exhibiting variable expressivity
for this trait. Therefore, a gene may have variable penetrance and variable
expressivity.
Extranuclear. Of structures or processes, which are located outside the nucleus in
contrast to intranuclear structures or processes.
Extranuclear Genes. Genes located outside the cell nucleus, that is, on cytoplasmic
organelles (mitochondria, chloroplast, etc). Inheritance of such genes is called
extranuclear inheritance. The inheritance pattern is quite different from those of the
nuclear genes (Mendelian genes). Reciprocal differences (as observed for sex-linked
genes and genes conditioning sporophytic incompatibility) are encountered in the
extranuclear inheritance. Interactions among loci of nuclear and the chloroplast
genomes have been reported in various wild species. Therefore, intergenomic
interactions should also be considered as the rational approach while conducting
breeding operations.
Extranuclear Mutation. A heritable change occurring in the extranuclear genes. It is
also referred to as cytoplasmic mutation because it is carried in the cytoplasmic
organelles like chloroplast and mitochondria.
Eye Ball Index. The visual judgement of plant breeders based on their experience and
knowledge of plant traits. This constitutes the art of plant breeding. It is used for
evaluation and selection of plants, plant progenies, or even families in early
segregating generations (F 2 /F 3 /F 4 ).
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