Glossary Plant Breeding

P. A shorthand to designate parental generations in crosses.
P 1 , P 2 , P 3 ,……. First, second, third, etc. generations from a parent; also used to designate
different parents used in making a hybrid or series of hybrids. P 1 also refers to the
immediate parents of a first filial generation or F 1 .
Pachytene. The four-strand stage of meiosis during which crossing over occurs between
non-sister chromatids of a pair of homologous chromosomes.
Paleolithic Age. Cultural period beginning with the earliest chipped stone tools
(~750,000 years ago) until the beginning of the Mesolithic Age, ca. 15,000 years ago.
Palindrome.
Pangenesis Theory (Darwin, 1809-1882). An obsolete theory of evolution. According to
it, pangenes (gemmules), which are very small, exact, but invisible copy of each body
organ and component, assemble in gametes and upon fertilization, they separate out to
different parts of the body during development to constitute a mixture of maternal and
paternal organs and tissues. Although a disproved idea, however, it suggested a
physical basis of heredity.
Panmictic Population. A random mating population.
Panmictic Index. A measure of the relative amount of hetrozygosity that is reduced by
inbreeding in a random mating population of limited size. It is a complementary
parameter to inbreeding index or coefficient. Mathematically, P = 1 – F; the initial
value of this parameter in a large population is taken as unity as F = 0 in such a case.
The loss of heterozygosity in a small sized population is equal to the factor 1/2N
(where N = population size = number of diploid individuals).
Panmixia. Random mating without restriction (usually extended to include random
mating under the restrictions of sex or incompatibility).
Paracentric Inversion. An inversion that excludes the centromere of the chromosome. It
does not change the morphology of the chromosome. It is very difficult to detect, as it
does not change the arm ratio. There is selective recovery of parental type gametes in
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