QUALITATIVE AND QUANTITATIVE CHARACTERS Plant breeding K Vanangamudi TNPSC AO, HO, ADH, AAO, AHO EXAMS ICAR AIEEA JRF & SRF for PG admissions exams ICAR NET, ARS & STO (T-6) exams IBPS – AFO exams Qualitative characters or oligogenic characters Quantitative characters or polygenic characters Environment and quantitative variation Polygenic inheritance Gene action Additive gene, Dominance gene, Epistatic gene and Over dominance gene action Concept of heritability

1. PLANT BREEDING
QUALITATIVE AND QUANTITATIVE CHARACTERS
Dr. K. Vanangamudi
Formerly Dean (Agriculture), AC & RI, Coimbatore,
Dean, Adhiparashakthi Agricultural College, Kalavai,
Professor and Head - Seed Science and Technology,
Tamil Nadu Agricultural University, Coimbatore.
Qualitative characters or oligogenic characters
 Character under simple genetic control i. e., governed by one or few
genes (Oligogenic characters).
 Eg: Colour of stem, flower, pollen and their shapes
Quantitative characters or polygenic characters
 Character under complex genetic control i. e., governed by many genes
(Polygeneic characters).
 Term polygene was introduced by Mather (1941)
 Eg: Yield per plant, days to flower, days to maturity, seed size, drought
tolerance, etc.
Economically important characters
Qualitative characters
(Oligogenic characters)
Quantitative characters
(Polygenic characters)
Nature of traits Traits that have Mendelian
inheritance and described
according to kind.
Degree of expression of the trait
Scale of variability Discrete or discontinuous
phenotypic variation
Continuous variation
Number of genes Single genes are readily
detectable
Traits are under polygenic control
Mating pattern Individual mating and
their progenies
Population of individuals that may
comprise a diversity of mating kind
Statistical analysis Based on frequencies and
ratios
Estimates of population parameters
namely mean, variance
Environmental effect Less More

2. Environment and quantitative variation
 All genes are expressed in an environment (Phenotype = Genotype x
Environmental effect).
 However, quantitative traits tend to be influenced to a greater degree
than qualitative traits.
 In polygenic inheritance, segregation occurs at a large number of loci
affecting a trait.
 Phenotypic expression of polygenic traits is depending on variation in
environmental factors to which plants in the population are subjected.
 Polygenic variation cannot be classified into discrete groups i.e.,
continuous variation.
 This is because of the large number of segregating loci, each with so small
effects that is not possible to identify of individual gene effects in the
segregating population.
 Biometrics is used to describe the population in terms of means and
variances.
 Continuous variation is caused by environmental and genetic variations
due to the simultaneous segregation of many genes affecting the trait.
Polygenic inheritance
 Nilsson-Ehle (1910) provided a demonstration of polygenic inheritance.
 Defined as quantitative inheritance, where multiple independent genes
have an additive or similar effect on a single quantitative trait.
 Each polygenic character is controlled by several independent genes and
each gene has cumulative effect.
Assumptions of polygenic characters
 Six important assumptions
1. Many genes determine the quantitative trait, each produces an
equal effect.
2. These genes lack dominance, intermediate expression between
two parents.
3. Action of the genes are either cumulative or additive effect.
4. There is no epistasis among genes at different loci.
5. The linkage is in equilibrium, means there is no linkage.
6. Environmental effects are absent or may be ignored.

3.  Two types of alleles or genes in the polygenic inheritance
1. Contributing alleles: Those alleles which contribute to
continuous variation
2. Non-contributing alleles: Those which do not contribute to
continuous variation
Gene action
 Refers to the behaviour or mode of expression of genes in a genetic
population.
 Helps in the selection of parents for use in the hybridization programmes
and also in the choice of appropriate breeding procedure for the genetic
improvement of various quantitative characters.
Main features of gene action
 Measured in terms of components of genetic variance or combining ability
variance and effects.
 Studied using various biometrical techniques such as diallel analysis,
partial diallel cross, triallel analysis, quadriallel analysis, line x tester
analysis, generation mean analysis, biparental cross and triple test
cross analysis.
Four types of gene action
 Additive, dominance, epistatic, and over dominance.
1. Additive gene action
 Effect of a gene is said to be additive when each additional gene enhances
the expression of the trait by equal increments.
 If one gene adds one unit to a trait, the effect of aabb = 0, Aabb = 1,
AABb = 3, and AABB = 4.
 For a single locus (A, a), the heterozygote would be exactly intermediate
between the parents i.e., AA = 2, Aa =1, aa = 0.
 That is, the performance of an allele is the same irrespective of other alleles
at the same locus.
 A superior phenotype will breed true in the next generation, making
selection for the trait more effective to conduct.
 Selection is most effective for additive variance; it can be fixed in plant
breeding i.e., develop a cultivar that is homozygous.

4. 2. Dominance gene action
 Dominance effects are deviations from additivity that make the
heterozygote resemble one parent more than the other.
 When dominance is complete, the heterozygote is equal to the homozygote
in effects i.e., Aa = AA.
 Breeder cannot distinguish between the heterozygous and homozygous
phenotypes.
 Consequently, both kinds of plants will be selected, homozygotes will
breed true, while heterozygotes will not breed true in the next generation.
3. Epistatic gene action
 In quantitative inheritance, epistasis is described as non-allelic gene
interaction.
 When two genes interact, an effect can be produced where there was none
(e.g., Aabb = 0, aaBB = 0, but A–B– = 4).
 Estimation of gene action or genetic variance requires the use of large
populations and a mating design.
 But, effect of the environment on polygenes makes estimations more
challenging.
4. Over dominance gene action
 Exists when each allele at a locus produces a separate effect on the
phenotype, and their combined effect exceeds the independent effect of the
alleles i.e., aa =1, AA =1, and Aa= 2.
 From breeding standpoint, breeder can fix over dominance effects only in
the first generation i.e., F1 hybrid through apomixis or through
chromosome doubling of the product of a wide cross.

5. Components of genetic variance
 Genetic properties of a population are determined by the relative
magnitudes of the components of variance.
 By knowing the components of variance, one may estimate the relative
importance of the various determinants of phenotype.
 Phenotypic value of quantitative traits:
 P (phenotype) = G (genotype) = E (environment)
 Phenotypic value is variable because it depends on genetic differences
among individuals, as well as environmental factors and the interaction
between genotypes and the environment; called G = E interaction .
 Total variance of a quantitative trait may be mathematically expressed
as follows:
o VP = VG +VE +VGE
o Where
o VP = total phenotypic variance of the segregating population.
o VG = genetic variance.
o VE = environmental variance.
o VGE = variance associated with the genetic and environmental
interaction.
 Genetic component of variance may be further partitioned into three
components as follows:
o VG = VA + VD + VI
o Where VA = additive variance (variance from additive gene
effects).
o VD = dominance variance (variance from dominance gene
action).
o VI = interaction (variance from interaction between genes).

6.  Additive genetic variance is the variance of breeding values and is the
primary cause of resemblance between relatives.
 Hence, VA is the primary determinant of the observable genetic properties
of the population, and of the response of the population to selection.
 Further, VA is the only component that the researcher can most readily
estimate from observations made on the population.
 Consequently, it is common to partition genetic variance into two –
additive versus all other kinds of variance.
 This ratio, VA/VP, gives what is called the heritability of a trait, an
estimate that is of practical importance in plant breeding.
 Total phenotypic variance may then be rewritten as:
o VP = VA + VD + VI + VE + VGE
 In sum, variances from additive, dominant, and environmental effects may
be obtained as follows:
o VP1 = E; VP2 = E; VF1 = E;
o VF2 = 1/2A + 1/4D + E
o VB1 = 1/4A + 1/4D + E
o VB2 = 1/4A + 1/4D + E
o VB1 + VB2 = 1/2A + 1/2D +2E
Concept of heritability
 Defined as a fraction: it is the ratio of genetically caused variation to
total variation (including both environmental and genetic variation).
 Concept of the reliability of the phenotypic value of a plant as a guide to
the breeding value (additive genotype variance) is called the heritability of
the metric trait.
 Heritability is the proportion of the observed variation in a progeny that is
inherited.
 Heritability measures this degree of correspondence.
 It does not measure genetic control or trait.
Types of heritability
 1. Broad sense heritability: Heritability estimated using the total genetic
variance (VG).
 It is expressed mathematically as: H = VG/VP
 2. Narrow sense heritability: Because the additive component of genetic
variance determines the response to selection, the narrow sense heritability
estimate is more useful to plant breeders than the broad sense estimate.
 It is estimated as: H2 = VA/VP

7.  Magnitude of narrow sense heritability cannot exceed, and is usually less
than, the corresponding broad sense heritability estimate.