Glossary Plant Breeding

Pedigree Breeding. A system of breeding in which individual plants are selected in the
segregating generations from a cross on the basis of their desirability judged
individually and on the basis of a pedigree record. It has been a widely used method in
breeding self-pollinated crops.
Pedigree. A record of ancestry of an individual, family, or a strain. In pedigree method
of handling segregating generations, it makes it possible for the elimination of all
except one member of the closely related families at the final stage. It is useful
primarily in avoiding selection of closely related lines whose probable worth is nearly
identical. However, it does not establish retraceable route that can be followed in
developing the same variety by repeating the same cross.
PEG. Polyethylene glycol. A chemical that acts as fusogenic agent for the fusion of
protoplasts. It is non-specific in that it induces fusion between protoplasts of the same
as well as of different species. The fused product is a somatic hybrid (amphidiploid).
Penetrance. The frequency with which an allele of a single locus or alleles of several loci
are manifested on the phenotype of the carriers. Its value ranges from zero to hundred
per cent. Suppose, a gene is expressed in 10 individuals out of 100. This implies a 10
per cent penetrance of the gene in question. Thus penetrance is also defined as the
proportion of individuals with a particular genotype expressing a phenotype
associated with that specific genotype. Penetrance of a gene depends upon the
position of other genes in the genome, the presence of modifiers and the external
environment. Incomplete penetrance complicates the task of breeders working with
the specific gene.
Peptide. An amino acid.
Peptide Bond. A bond joining two amino acids.
Perfect Population Size. The minimum size of a population that permits expression of
each genotype expected in an F 2 population from a cross differing at one, two, etc.
loci. It is represented by 4 n , where ‘n’ is the number of segregating loci in a hybrid.
According to Comstock and Robinson (1940), the number of individuals in an F 2
population for a monogenic trait should be at least 40. However, according to Allard
(1999), it should be 12.
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