POLYGENIC VARIATION
Quantitative genetics deals with the inheritance of quantitative or polygenic characters. Hence, it is essential to have knowledge of polygenic traits before dealing with polygenic variation and its assessment.
Any property of an individual showing heritable variation is known as character or trait. It includes morphological, physiological biochemical and behavioral properties. Plant characters are of two types viz., quantitative or polygenic and qualitative or oligogenic.
Quantitative characters are expressed in term of degree rather than kind and such characters show continuous variation from one extreme to the other.
Almost all plant parts and functions exhibit differences of quantitative nature. Examples of quantitative characters are the yield, days to flower, days to maturity, seed size, seed oil content, protein content, etc. Examples of qualitative traits are color of stem, flower, pollen etc. and their shapes.
In-plant breeding, both quantitative and qualitative characters have equal economic importance.
FEATURES OF POLYGENIC TRAITS
Polygenic characters have been described by various authors (Allard, 1960; Falconer, 1989; Mather and Jinks; 1971; Simmonds, 1979).
Variation: Polygenic characters exhibit continuous variation from one extreme to other, whereas the variation is discontinuous in case of qualitative characters.
Number of gene:
Quantitative characters are governed by several genes and, therefore, are also referred to as polygenic characters. On the other hand, qualitative characters are controlled by one or few genes and hence are also called monogenic or oligogenic traits.
Effect of single gene:
Effect of individual gene is large and easily detectable in case of qualitative characters, hence such characters are also called as major gene characters.
Classification:
Classification of quantitative characters into different clear space cut groups is not possible because of continuous variation from one extreme to the other.
For qualitative characters such grouping is possible because of discrete or discontinuous variation.
Gene Action:
Generally, quantitative traits are governed by additive gene action, but now cases are known where quantitative characters are governed by dominance and epistatic gene action. In case of qualitative traits the gene action is primarily of non-additive type (dominance and epistasis).
Effect of Environment:
Polygenic traits are highly sensitive to environmental changes, whereas oligogenic traits are little influenced by environmental variation. In order words, quantitative traits are more prone to genotype x environmental interactions. Thus quantitative characters are lesser stable to environmental changes than oligogenic traits.
Metric Measurement:
Various measurements such as height, length, width, weight, duration, etc. are possible for quantitative or polygenic characters, whereas such measurements are not possible in case quantitative characters, Qualitative characters can be grouped in different classes based on shape and color.
Transgression:
Transgression refers to the phenomenon through which we get variation in F2 or later generation outside the range of both the parents. Transgressive segregants are only possible from the crosses between two parents with mean values for a quantitative trait. Such segregants are not possible in case of qualitative traits.
Field of genetics:
Inheritance of quantitative traits is studied with the help of quantitative genetics or biometrical genetics, whereas the qualitative traits are studied by Mendelian genetics and population genetics.
Statistical Analysis:
SIMILARITIES POLYGENIC AND OLIGOGENIC TRAITS.
East (1916) demonstrated that polygenic characters were perfectly in agreement with Mendelian segregation and later on Fisher (1918) and Wright (1921, 1935) provided a mathematical basis for the genetic interpretation of such characters. Quantitative characters do not differ in any essential feature from the qualitative characters (Falconer, 1960, 1981; Mather, 1949; and Jinks, 1971.
SIGNIFICANCE OF POLYGENES
Polygenes are of prime importance to the plant breeder for the evolution of improved cultivars (Mather and Jinks, 1971; Simmonds, 1979). Polygenes have great evolutionary significance. They provide variation of fine adjustment and are systems of smooth adaptive change and of speciation.
Mather (1943) has nicely explained the mechanism of storage and release of polygenic variability.. Mather recognized two types of variability, viz., free variability and potential variability.
Free variability:
It refers to phenotypic differences between homozygotes with extreme phenotypes. Such variability is expressed and exposed to selection. Natural selection acts against extreme phenotypes.
Potential Variability:
It refers to hidden or bound variability in the heterozygote or in the homozygotes which do not have the extreme phenotype and, therefore, is not exposed to selection. It is of two types as given below.
- a) Heterozygotic Potential Variability:
AaBb. Such heterozygotes are phenotypically uniform and are very close to the population mean. However, they would produce extreme phenotypes in the next generation due to segregating and recombination.
- b) Homozygotic Potential Variability:
Homozygotes also function as stores of variability. For example, two gene homozygotes AAbb and aaBB may be expected to cluster around the mean of the population.
demands of immediate fitness and long term evolutionary requirements.
This mechanism of storage and release of genetic variability in the form of polygenic complexes gives the response to selection in a new direction.
The linkage among polygenes is useful. It reduces immediate response to selection but prolongs the response to selection due to the slow release of potential genetics variability in the segregating generations.
POLYGENIC VARIATION
Variability refers to the presence of differences among the individuals of the plant population.
The existence of variability is essential for resistance to biotic and abiotic factors as well as for wide adaptability. Selection is also effective when there is genetic variability among the individuals in a population.
Hence, insight into the magnitude of genetic variability present in a population is of paramount importance to a plant breeder for starting a judicious breeding program.
The large scale replacement of landraces, which have high genetic diversity, by modern high yielding cultivars has brought about a significant reduction in the genetic variability.
Plant breeders develop high yielding and uniform varieties reducing the genetic variability.
The uniform varieties have a narrow genetic base,
poor adaptability and are more prone to the new races of a pathogen than diversified genotypes, i.e., landraces or primitive cultivars (Simmonds, 1962).
Landraces have more genetic diversity high degree of tolerance to biotic and abiotic stresses and wide adaptability.
There are three main sources of maintaining genetic variability in nature viz.,
(1) spontaneous mutation,
(2) natural outcrossing, and
(3) Recombination’s. Besides these natural forces, there are three important measures for conserving the genetic variability, i.e.,
(1) maintenance of the global gene pool,
(2) the deliberate use of heterogeneous populations, and
(3) use of multiline varieties.
The polygenic variation present in the plant population is to three types, viz.,
(1) phenotypic,
(2) genotypic and
(3) environmental.
Phenotypic Variation
It is the total variability that is observable. It includes both genotypic and environmental variation and hence changes under different environmental conditions.
Genotypic Variation:
It is the inherent or genetic variability that remains unaltered by environmental conditions. This type of variability is more useful to a plant breeder for exploitation in selection or hybridization. Such variation is measured in terms of genotypic variance.
The genotypic variance consists of additive, dominance and epistatic components.
Environmental Variation:
It refers to non-heritable variation which is entirely due to environmental effects and varies under different environmental conditions.The variation in true-breeding parental lines and F1 is non – heritable.
