Glossary Plant Breeding

is dependent upon genetically controlled threshold - the point at which a stimulus
(gene) produces a response (character), e.g., temperature sensitive albinism.
Thymidine. The nucleoside having thymine as its base.
Thymine Dimer. A pair of chemically bonded adjacent thymine bases in DNA. If
present, this lesion is repaired by cellular processes. However, errors may be left
during DNA repair that ultimately results in mutation.
Thymine. A pyrimidine base that pairs with adenine in a DNA double helix. It has a
single ring structure compared to its pairing partner, which has two.
Ti Plasmid. Tumour Inducing Plasmid; a vector for gene transfer to plant cells. It is a
circular plasmid that enables the bacterium to infect plant cells. It is contained outside
genome of Agrobacterium tumefaciens and causes crown gall of plants. It has a vir
region (responsible for virulence towards the host) and a T-DNA region that is
transferred to the host along with the gene intended to be transferred. It has selectable
marker genes that help in selecting transformed cells.
Tift 23A (Burton 1965). A cytoplasmic male sterile line containing A 1 cytoplasm. The
male-sterile condition was found due to homozygous recessive alleles of a nuclear
gene pair ‘ms 1 ms 1 ’, besides sterile cytoplasm. It was used previously as a female
parent to develop hybrid bajra (HB series). Its use stopped after 1970 when it gave
way to downy mildew.
Tissue. A mass of cells woven together with their intercellular substances.
Tissue Culture. In vitro culture of cells, tissues, organs, etc on an artificial medium
under aseptic condition.
Tissue Specific Expression. Theexpressionofageneinahighereukaryoteinaspecific
and reproducible subset of tissues and cells during development.
Tolerance (Insect Pests). The ability of the host to grow and reproduce or to repair insect
injury to a marked degree in spite of supporting an insect population nearly equal to
that damaging a susceptible host. Tolerance, which is measured through yield
depression being minor, is polygenically determined. A tolerant and non-tolerant
cultivar would be indistinguishable when rated for the number of insects present, but
they would less affect the tolerant one. Tolerance differs from non-preference and
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