Glossary Plant Breeding

pairing only between homologous chromosomes, disomic genetics, homozygosity at
each locus, and the chance for heterozygosity and its consequent heterosis from
interactions between homeologous loci. In contrast, a polysomic polyploid such as the
tetraploid potato can have meiotic pairing between any two or more of the
homologous chromosomes, tetrasomic genetics and again the most important, the
opportunity for up to four alleles per locus available for intra- and interlocus
interactions in optimising heterotic combinations. In general autoploids have not been
very important in natural evolution due to aberrant chromosome behaviour during
meiosis and associated high degree of sterility. However, from breeding point of
view, they bear significance in crop species that are propagated by asexual means,
such as potato, banana, and the like. In contrast, hybrid polyploidy has played
important roles in both natural evolution and plant breeding.
Polyploidy. A situation or condition describing occurrence of individuals with more than
two sets of chromosomes. It leads to a great increase in allelic diversity, and provides
an opportunity to utilize intra-locus interactions among alleles at the specific locus.
This intra-locus interactions for most loci is analogous to true overdominance. It also
provides even greater opportunity for exploitation of favourable epistatic interactions
among alleles of different loci (pseudo-overdominance). Polyploidy breeding seems
to be more successful in asexually propagated crops, crop species with low
chromosome number, and cross-pollinated crops.
Polysaccharide. A biological polymer composed of several monosaccharides (such as
glucose, fructose or galactose) or sugar subunits, for example, starch or cellulose.
Population Density. The number of individuals of a population per unit area in the
habitat.
Population Genetics. The study of inherited variation, and thus the behaviour of genes in
a population of organisms. The problem of population genetics is to relate the
heritable changes in population of organisms to the underlying individual processes of
inheritance and development. It is the translation of Darwin’s three principles (the
principle of variation, of heredity, and of selection) into precise genetic terms.
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