Genetic Base in Breeding Populations | Term Paper | Crops | Plant Breeding

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Term Paper Contents:

1. Term Paper on the Definition of Genetic Base
2. Term Paper on the Types of Genetic Base
3. Term Paper on Broadening the Genetic Base of a Population
4. Term Paper on the Factors affecting Genetic Base of a Population
5. Term Paper on the Advantages of Genetic Base
6. Term Paper on the Disadvantages of Genetic Base

Term Paper # 1. Definition of Genetic Base:

The amount or quantum of genetic diversity available in a breeding population is referred to as genetic base of that population. In other words, genetic base represents spectrum of genetic variability in a plant breeding population.

Main points related to genetic base are listed below:

(i) The genetic base of a population is determined by the number of constituent genotypes.

(ii) The genetic base has positive association with the amount of genetic diversity present in a population.

(iii) The genetic base has also positive association with the buffering capacity of a population to environmental changes.

(iv) Genetic base of a population is influenced by various factors.

(v) Genetic base can be broadening by various breeding approaches.

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Term Paper # 2. Types of Genetic Base:

Depending upon the amount of genetic diversity present in a plant breeding population, genetic base is of two types, viz.:

(i) Broad genetic base and

(ii) Narrow genetic base.

These are discussed below:

(i) Broad Genetic Base:

Plant populations that are composed of several pure lines, inbred lines or heterozygotes and homozygotes are said to have broad genetic base. Examples of such populations are mass selected varieties, multiline varieties, synthetics, composites and land races. Such genotypes have wider adaptation to environmental changes.

(ii) Narrow Genetic Base:

Those plant populations that have been developed from single homozygote or heterozygote are-said to have narrow genetic base. Examples of such populations are pure line variety, varieties developed by backcross, pedigree method bulk method, and single seed decent method, clonal variety and hybrids between two inbred lines or pure lines. Such populations have danger of uniformity.

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Term Paper # 3. Broadening the Genetic Base of a Population.

The genetic base of plant breeding populations can be broaden in various ways as suggested/indicated below:

i. In self pollinated species, mass selected varieties, multiline varieties and varietal blends can be used for broadening the base of populations.

ii. In cross pollinated species, mass selected varieties, synthetics and composites can be used to develop populations with broad genetic base, Three-way, double and composite or multiple crosses can be made for broadening the genetic base of the populations.

iii. Population improvement approaches, viz. recurrent selection, disruptive selection, biparental mating and diallel selective mating should be used for developing populations with broad genetic base. All these approaches can be used for broadening of genetic base in self pollinated species. In cross pollinated species, the first three approaches are used.

iv. The biotechnology can be used by creating genetic diversity in plant breeding populations and thereby in broadening the genetic base.

v. The introgressive hybridization is a very potent tool for increasing genetic diversity in breeding populations. It can be used for broadening the genetic base of breeding populations.

vi. The gene pyramiding technique can be used for broadening the genetic base against biotic and abiotic stresses.

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Term Paper # 4. Factors affecting Genetic Base of a Population:

There are various factors that affect the genetic base of a population.

Such factors include:

(i) Mode of pollination,

(ii) Genetic diversity,

(iii) Breeding method,

(iv) Genetic polymorphism, and

(v) Genetic material.

These are briefly discussed below:

(i) Mode of Pollination:

Cross pollinated species have more genetic diversity (heterogeneity and heterozygosity) than self pollinated species. As a result cross pollinated species have broad genetic base which provides them more buffering capacity to environmental fluctuations.

(ii) Genetic Diversity:

Genetic diversity refers to presence of heritable variation in a population. Generally heterogeneous populations give more stable yield over several environments than pure lines, Such populations have broad genetic base which provides them higher degree of buffering capacity to environmental changes, Thus genotypes or population with high genetic diversity have broad genetic base and those with low genetic diversity have narrow genetic base.

(iii) Breeding Method:

The genetic base of a population is also affected by breeding method. For example, in self pollinated species, varieties developed by mass selection and multiline breeding have broad genetic base. While varieties developed by backcross, pedigree, bulk and single seed descent methods have narrow genetic base.

(iv) Genetic Polymorphism:

Genetic polymorphism refers to regular occurrence of several phenotypes in a population. It is maintained due to superiority of heterozygotes over homozygotes. It enhances diversity of genotypes in a population and thus provides more buffering capacity to a population. Thus polymorphic populations have broad genetic base.

(v) Genetic Material:

The genetic base is also influenced by genetic material. For example land races, multilines, synthetics and composites have broad genetic base, where as pure lines and inbred lines have narrow genetic base. Similarly F₂ populations and double cross hybrids are more stable than F₁ population, because F₂ population has more genetic diversity and broad genetic base.

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Term Paper # 5. Advantages of Genetic Base:

In plant breeding populations, genetic base has several advantages.

It provides protection from:

(i) Biotic stresses,

(ii) Abiotic stresses,

(iii) Wide adaptation,

(iv) Yield stability, and

(v) High heterosis.

These are briefly discussed below:

(i) Protection from Biotic Stresses:

Populations with broad genetic base are composed of several genotypes which provide them protection from both biotic (insects, diseases) and abiotic stresses (drought, soil salinity, frost, cold, heat etc.). The broad genetic base provides high buffering capacity which provides protection form biotic stresses because all the component lines of the population will never become susceptible to a particular disease or insect, Thus it provides insurance against yield losses caused by biotic stresses.

(ii) Protection from Abiotic Stresses:

Abiotic stress refers to adverse effects of environmental factors such as moisture stress, salinity, frost, heat etc. on growth and reproduction of crop plants. Populations with broad genetic base have several component lines in their genetic composition which provide them high buffering capacity to environmental changes.

(iii) Wide Adaptation:

Populations with broad genetic base have wider adaptation due to presence of more genetic diversity (heterogeneity and heterozygosity). Broad genetic base provides high buffering capacity against environmental changes, resulting in wider adaptability.

(iv) Yield Stability:

Populations with broad genetic base provide stable yield over regions and seasons due to their wide adaptation or high buffering capacity to environmental changes.

(v) High Heterosis:

It has been observed that genotypes with broad genetic base result in manifestation of high level of heterosis when used as a parent in development of hybrid.

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Term Paper # 6. Disadvantages of Genetic Base:

There are two main disadvantages of populations with genetic base, viz.:

(i) Less uniform, and

(ii) Low yield.

These are discussed below:

(i) Less Uniform:

The produce of heterogeneous population that have broad genetic base is lesser uniform and lesser attractive than those of pure lines, inbred lines and F₁ hybrids which are homogeneous and have narrow genetic base.

(ii) Low Yield:

The yield of heterogeneous populations is always lesser than that of most productive pure line cultivar under normal conditions. However, under adverse conditions the yield of heterogeneous population is higher than uniform pure line cultivars.

Thus populations with narrow genetic base have two main advantages, viz.:

(i) uniform produce, and

(ii) higher yield.

For example the produce of pure lines, inbred lines and F₁ hybrid between two inbred lines/pure lines is very attractive and uniform.

However, such populations have two main disadvantages, viz.:

(i) Danger of uniformity and

(ii) Poor adaptation.

These are briefly discussed below:

(i) Danger of Uniformity:

Populations with narrow genetic base have danger of uniformity. Such populations are derived from single homozygote (such as pure lines and inbred lines) or single heterozygote (such as F₁ hybrid). If such population is attacked by a disease or insect it may lead to crop failure due to lack of heterogeneity.

(ii) Poor Adaptation:

Uniform populations such as pure lines and inbred lines have poor adaptation due to lack of heterogeneity. Such population cannot withstand adverse effects of environment due to poor buffering capacity.