Glossary Plant Breeding

Gene Family. A set of genes in one genome all descended from the same ancestral gene.
Gene Frequency. Also called allele frequency; the proportion in which different alleles
of a gene occur in a population. The term ‘allelic’ frequency is most appropriate,
although used less frequently. One should properly use the term “allele frequency”;
however, the usage of the term “gene frequency” is too widespread to enable an easy
change.
Gene Fusion. The accidental joining of DNA of two genes (as can occur in a
translocation). It can give rise to hybrid proteins or to the misregulation of the
transcription unit of one gene by the cis-regulatory elements (enhancers)ofanother.
Genetic Gain. The expected genetic superiority of the progeny of the selected
individuals compared to the base population. If the original population consisted of a
mixture of pure lines/clones, the genetic advance predicted on the basis of broad
sense heritability amounts to genetic gain. If, however, the base population consisted
of a mixture of segregating materials, then genetic gain amounts to the genetic
advance based on narrow sense heritability. Under such a situation, genetic gain is: Gs
= k x σ P x h 2 , whereas, h 2 = narrow sense heritability.
Genetic Homeostasis. The stability of a group of genotypes (plants) that exceeds that of
its individual members; also called population buffering. Heterogeneous cultivars
generally have more stability on the average than do homogeneous ones. In genetics,
it is the maintenance of genetic variability in a population in the face of all the forces
acting to reduce it. Over the long period of evolution in a particular environment, a
population has built-up a particular array of gene frequencies, which confers a very
high degree of fitness upon the population. Therefore, any departure from these
frequencies may reduce fitness, and thus the population opposes any change and
maintains such frequencies in it. According to Lerner (1954), homeostasis may also
arise from the necessity for the maintenance of certain level of heterozygosity to
ensure normal development.
Genetic Load. The proportion by which fitness of a population at the given locus is
decreased owing to the presence of deleterious (lethal, semilethal, sub vital) genes. It
is expressed as the average number of potential genetic deaths per individual. At a
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