Glossary Plant Breeding

Tetrad. A group of four. It may be four homologous chromatids in a bundle visible
during meiotic prophase and metaphase. It may also be the four haploid product cells
of a single meiosis. Dyad and octad mean a group of two and eight cells or
chromosomes, respectively.
Tetramer. A protein consisting of four polypeptide subunits, for example, haemoglobin.
Tetraploid. A cell with four basic (x) sets of chromosomes. An organism composed of
such cells is also referred to as a tetraploid. If all the sets are identical, the individual
is said to be an autotetraploid (AAAA). Allotetraploids (AABB) have dissimilar
genomes or sets of chromosomes. They are also sometimes called amphidiploids.
Tetrasomic. An otherwise diploid cell having a particular chromosome present four
times, thus having 2n+2 chromosomes; also the organism composed of such cells.
Tetrasomics often behave more regularly than the other aneuploids with odd
chromosome number. The pairing possibilities among the four homologs and gametic
output are almost similar to those of tetraploids (4x).
Textbook. A book giving a systematic presentation of the principles and vocabulary of a
particular subject.
Three-line System. A widely used system for producing hybrid varieties involving A, B,
and R lines.
Three-point Testcross. A testcross in which the tester parent has three homozygous gene
pairs, for example a cross between AaBbCc × aabbcc.
Three-way Cross. A cross between a single cross and an inbred line [(A × B) ×C]. The
number of possible three-way hybrids involving n inbreds will be [n. (n-1). (n-2) / 2].
Threshold Effect. A term usually applied to traits with a polygenic basis that develop
when the dosage of contributory alleles exceed a critical value in particular
environments (sometimes used to explain all-or-none phenomena based on
polygenically inherited characters such as resistance vs. susceptibility to some
diseases); the environmental condition which obscures expression of a trait. It allows
character expression below/above a particular environmental condition. It is discerned
that a character develops through a series of physico-chemical reactions, which in turn
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