Glossary Plant Breeding

Population Theory (Malthus 1798). A theory related to human population growth in
response to means of subsistence. It states that the increase in human population will
be controlled by hunger in absence of wars or any other forms of disaster. Modern
Malthusians maintain that the catastrophe has only been delayed, and that the misery
associated over population is still the fate of modern civilization. They believe that
population growth may proceed at a faster rate (geometric progression) than means of
subsistence (which may increase only at a arithmetic rate).
Population. A term originally coined to denote a group of people. However, it has been
broadened to include groups of individuals of any one kind of organism. The term as
used in Genetics & Plant Breeding includes a reproductive community of sexual and
cross-fertilizing organisms, which share in a common gene pool. Since Mendel’s laws
of inheritance still apply to the transmission of genes among individuals of such a
group, it is also referred to as Mendelian population. As a consequence of its
reproductive systems and its previous evolutionary history, each population in this
category is believed to possess an integrated genetic structure, which may be
described in terms of systems of gene and genotypic frequencies. In autogamous
crops, it refers to a group of segregating individuals, pure lines and/or families. It
tends to maintain their individual integrity by virtue of special reproductive
mechanism that precludes (within limits) free interbreeding among themselves. In
statistics, population is a hypothetical and infinitely large series of potential
observations among which observations actually made constitute a sample.
Position Effect. A term used to describe a situation in which the phenotypic influence of
a gene is altered by changes in the position of that gene within the genome.
Position-Effect Variegation. Variegation caused by the inactivation of a gene in some
cells through its abnormal juxtaposition with heterochromatin.
Positive Assortative Mating. A situation in which like individuals mate more commonly
than expected by chance. If the mating-individuals are related by descent/ancestry (AA
× AA), it is called genetic assortative mating. If mating of like-to-like (tall × tall) is
based on appearance, it is referred to as phenotypic assortative mating. The ultimate
effect of assortative mating is an increase in the homozygosity of the population.
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