Glossary Plant Breeding

used to compensate for defects by choosing contrasting parents each of which offset
weaknesses of the other.
Phenotypic Sex Determination. Sex determination by nongenetic means.
Phenotypic Variance. The total variance observed in a character.
Philadelphia Chromosome. A translocation between the long arms of chromosomes 9
and 22, often found in the white blood cells of patients with chronic myeloid
leukaemia.
Phosphodiester Bond. A bond between a sugar group and a phosphate group. Such
bonds form the sugar-phosphate backbone of DNA.
Phosphorylation. The addition of a phosphate group to ADP for producing ATP.
Photophosphorylation. ATP synthesis by phosphorylation of ADP using the light energy
in the chloroplasts of plants.
Phyletic Evolution. Gradual transformation of one species into another without
branching.
Phylogeny. The evolutionary history of an organism or a taxonomic group.
Physiological Drought. See drought.
Ph Gene (Pairing homoeologous gene). The gene(s) that suppresses homeologous
pairing and thus ensures homologous pairing in bread wheat. It results in diploid-like
genetics for this basically hexaploid species. The gene is situated on the long arm of
5B chromosome. Two such genes are known: Ph 1 and Ph 2 . However, Ph 1 is the
genetic system that determines exclusive homologous pairing in modern wheat.
Ph 1 Suppressor. The species (T. searsii) known to suppress the activity of Ph 1 in hybrids
with bread wheat, resulting in high homoeologous pairing.
ph Mutant (Wall et al. 1971). A mutant that carries a mutation in the Ph gene, and thus
permits homeologous pairing. The level of pairing in this mutant, however, is not as
high as in the nullisomic for 5B. Intercalary deficiency for Ph 1 is designated ph 1 b
mutant, which permits high level of homeologous pairing. The terminal deficiency of
3D is termed ph 2 a and is characterized by an intermediate level of homeologous
pairing. Further, ph1c mutant has been found in durum wheat and is used in
promoting homeologous pairing in intergenomic hybrids.
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