🎃 Quantitative and Qualitative

👉🏻 Basic requirements of Plant breeding

  • Variation in a character = It is must for the improvement in the character. Variation may be created by hybridization, mutation or polyploidy.
  • Selection means identification and isolation of desirable plants. Selection from a population depends on the appearance i.e. phenotype. Phenotype may be heritable or may not be. Heritable character of phenotype is due to genotype i.e. genes present in the plants.
  • Genes do not produce characters directly. Genes produce different proteins which often acts as enzymes. Enzymes catalyze a specific biochemical reaction which finally leads to development of various characters i.e. phenotype.
  • Some characters are little affected by other genes i.e. the genetic background, or the environment. Such characters are generally governed by one or few genes with large easily detectable effects. These genes are called oligogenes.
  • Oligogenes produce the characters having distinct classes. These characters are called qualitative characters.
  • In other words Qualitative characters are such characters which show distinct classes, are little affected by the environment (both external and internal), and are governed by one or few genes with large distinct effects i.e. oligogenes.
  • Whereas most of the characters are governed by several genes with individual effects and very much affected by the genetic background and more particularly by the environment. These genes are called polygenes. The characters produced by polygenes do not show clear-cut classes and have to be studied by measurement, are called Quantitative Characters.
  • In other words, the quantitative characters show continuous distribution, are generally influenced by the environment and are controlled by several genes with small and cumulative effects i.e. polygenes.
  • The small effect of each gene is usually cumulative. Example: yield, plant height, days to flower, days to maturity, protein content, seed size etc.
  • Inheritance of both characters i.e. qualitative and quantitative follows the laws of Mendel but the effects of individual genes are totally different.
  • Mendel proposed his laws of inheritance based on his studies with qualitative characters.

Pleiotropy

  • A single major gene i.e. oligogene generally governs a single character but there are many instances where an oligogene affects, more than one character. Such phenomenon of a single major gene affecting more than one character is known as pleiotropy and such a gene action is called pleiotropic gene action.
  • Clear-cut examples of pleiotropy are limited mainly because of difficulty in demonstrating pleiotropic effect of a gene. To investigate pleiotropic effect of a gene, two lines must be developed which are identical with respect to all other genes except for the gene under investigation. Such lines are called isogenic lines.

Threshold characters

  • Certain genes require a specific environment for their expression, such characters are called threshold characters. E.g. a mutant gene in barley (Hordeum vulgare) produces albino seedlings at temperatures below 8°C.

Modifying genes

  • The genes modify the effects of other genes are called modifying genes. Such genes have no any effect of their own but they increase or decrease the expression of other oligogenes. The effects of such genes are quantitative in nature.
  • E.g. spotting in mice is affected by modifying genes.

Quantitative Characters

  • In 1906, Yule suggested that many genes with small, similar effects could produce continuous variation. He proposed that these genes (responsible for quantitative characters) were transmitted according to the laws of Mendel.
  • In 1908, Nilsson-Ehle presented experimental evidence to support the hypothesis of Yule i.e. seed colour in wheat and oat. The F2 generations from various crosses had red & white grains in the 2 ratios of 3 : 1, 15 : 1 or 63:1. It means that the seed colour in these crosses were governed by one, two and three genes respectively. The closer study of seed colour in 15:1 ratio revealed that the seed classified as red differed in the intensity of colour. Thus it was assumed that the seed colour was governed by two genes with similar, small and additive effects and Nilsson-Ehle showed certain characters were governed by genes which have small & cumulative effects.
  • This is known as multiple factor hypothesis or polygenic inheritance.
  • East in 1916 presented conclusive evidence by studying inheritance of corolla length in Nicotiana Iongiflora that quantitative characters are governed by many genes with small & cumulative effects.
  • To analyse his data, East used statistical parameters such as frequency distribution, mean & coefficient of variability. Quantitative characters are considerably affected by the environment i.e. 10-50%. The chief effect of environment is to mask the differences between different genotypes and to produce a continuous variation even if the no. of governing genes is very small or even one.
  • Assuming the effect of environment is zero, the increase in no. of governing genes produces a similar continuous variation. It means polygenic inheritance produces continuous variation.
  • Due to the environmental effect or polygenic inheritance, phenotype does not reveal the genotype. The phenotype may be described in a mathematical model:

X̄ = μ + g + e + ge

  • Where
    • X̄ = phenotypic mean
    • μ = general population mean (i.e. phenotype of all possible genotypes grown under all possible environments)
    • g = effect of genotype
    • e = effect of environment
    • ge = interaction between genotype & environment
  • genotype X environment: signifies the various genotypes affected by environment.
  • Variance due to environment may be estimated from a replicated trial consisting of several genotypes. But the interaction component may be estimated only when the trial is conducted more than one environment, preferably at different locations and during different years. From such a trial the genetic variance and phenotypic variance can be competed which may be used to estimate heritability.
  • Phenotypic variance (σ²p) = genotypic variance (σ² g) + environmental variance (σ² e)
  • In crop improvement, only the genetic component of variation is important. The ratio of genetic variance to the total variance i.e. phenotypic variance is called heritability.

Heritability (H) = Vg/Vp = (Genetic Variance) /(Phenotypic Variance)

Vg/(Vg + Ve)

  • Where, Ve environmental variance

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