Term Paper on Genetic Polymorphism | Genetics | Biology

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Term Paper on Genetic Polymorphism

Term Paper Contents:

1. Term Papers on the Definition of Genetic Polymorphism
2. Term Papers on the Types of Genetic Polymorphism
3. Term Papers on the Causes of Genetic Polymorphism
4. Term Papers on the Methods of Detecting Genetic Polymorphism
5. Term Papers on the Theories of Genetic Polymorphism
6. Term Papers on the Advantages of Genetic Polymorphism
7. Term Papers on the Disadvantages of Genetic Polymorphism

Term Paper # 1. Definition of Genetic Polymorphism:

Genetic polymorphism refers to the regular occurrence of several phenotypes in the genetic population. The term genetic polymorphism was coined by Ford in 1940. It has been reported that two third of the loci in a population exhibit polymorphism. The genetic polymorphism is usually maintained due to superiority of heterozygotes over both the homozygotes.

When polymorphism is maintained as a result of heterozygote advantage, it is known as balanced polymorphism. Polymorphism can be detected on the basis of morphological, biochemical and molecular traits or markers. 

Genetic polymorphism increases the buffering capacity of a population by providing increased diversity of genotypes in a population. Genetic polymorphism broadens the genetic base of a population and thus enhances the adaptability of the population.

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Term Paper # 2. Types of Genetic Polymorphism:

There are six types of genetic polymorphism, viz.:

i.  Balanced Polymorphism:

The genetic polymorphism which is maintained due to superiority of heterozygote over both the homozygotes is referred to as balanced polymorphism. This leads to regular occurrence of several phenotypes in a population. The term balanced polymorphism was first used by Ford in 1940.

Main features of balanced polymorphism are given below:

(a) The polymorphism is maintained due to heterozygote advantage.

(b) This was first reported by Ford in 1940.

(c) This is the most common type of polymorphism observed in plant breeding populations.

ii. Transient Polymorphism:

A genetic polymorphism that is limited to a particular ‘period is called transient polymorphism. Sometimes one allele undergoes replacement by a superior allele. The genetic polymorphism during such period is known as transient polymorphism. It is not a regular phenomenon like balanced polymorphism.

Thus there are three main features of transient polymorphism:

(a) It is for a limited period,

(b) It is not a regular feature, and

(c) It was also reported by Ford in 1940.

iii. Neutral Polymorphism:

It refers to the genetic polymorphism that is dependent on a gene action which is almost neutral in its effect on the survival of the genotype in which it is contained. In other words, the effect on the career genotype is neutral. It results due to neutral mutations.

The main features of neutral polymorphism are given below:

(a) It was coined by Ford in 1940.

(b) It results due to neutral mutation.

(c) Neutral mutations take long time in contributing to polymorphism.

iv. Regional Polymorphism:

This refers to occurrence of two or more phenotypes in a population in different regions of the habitat. It results due to adaptation of different individuals in different environment.

Main features of regional polymorphism are given below:

(a) It results due to adaptive variation of alleles.

(b) It is not related to superiority of heterozygotes.

(c) It is also known as geographical polymorphism.

v. Unisexual Polymorphism:

If refers to the genetic polymorphism that is confined to one sex only. It results due to sex limited manifestation of genes. However, such gene can recombine in both sexes.

vi. Cryptic Polymorphism:

It refers to genetic polymorphism in which the genetically different alleles cannot be identified on the basis of their phenotype. It may include chromosal polymorphism.

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Term Paper # 3. Causes of Genetic Polymorphism:

The possible causes of genetic polymorphism include:

i. Heterozygote Advantage:

The natural selection usually favours heterozygotes than homozygotes because heterozygotes are more adaptable than homozygotes. In other words, heterozygotes have more buffering capacity to environmental changes than homozygotes. The heterozygotes maintain genetic polymorphism in a population.

ii. Frequency Dependent Selection:

The frequency dependent selection also leads to maintenance of polymorphism in a population. Generally selection favours those alleles that have low frequency but produce rare phenotype. The selection goes against the alleles that have high frequency. This type of frequency dependent selection maintains balanced polymorphism in a population.

iii. Heterogeneous Environment:

The environment differs from region to region and season to season. The balanced polymorphism is maintained when one allele is advantageous in one environment and another in different environment. In such situation stable polymorphism can be maintained even without heterozygote advantage.

iv. Transition:

In the evolutionary process, sometimes one allele is replaced by another which is more advantageous for adaptation. This may lead to polymorphism in a population. However, such polymorphism is for a limited period and hence is called as transitional polymorphism.

v. Neutral Mutation:

In a population, mutations do arise. However, the majority of mutants are harmful and deleterious. Such mutants are lost only few mutants will survive and replace the original allele. The changes in gene frequency depend on chance. Thus spread of a mutant through the population is erratic.

The frequency of a mutant is sometimes increasing and sometimes decreasing. Only those mutants that have selective advantage will survive and contribute to polymorphism in a population. The surviving few mutants take a very long time to spread in the population but contribute to the polymorphism.

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Term Paper # 4. Methods of Detecting Genetic Polymorphism:

In a plant breeding population, the genetic polymorphism can be detected in three main ways, viz.:

(i) On the basis of phenotype

(ii) Biochemical markers and

(iii) Molecular markers.

There are briefly discussed below:

i. On the basis of Phenotype:

The best way of detection of genetic polymorphism is the average heterozygosity at various loci. The higher the heterozygosity, the higher the polymorphism will be. The regular occurrence of several phenotypes in a population is an indication of genetic polymorphism. The polymorphism can be detected on the basis of plant characters such as shape, colour, surface, size of various plant characters. The polymorphism is observed for both oligogenic and polygenic traits.

ii. Biochemical Markers:

Sometimes it is difficult or impossible to identify the polymorphic alleles by visual observations. In such situation the best way of detecting the polymorphic alleles is the isozyme studies or gel electrophoretic studies. Sometimes mutations give rise to protein polymorphism and the variant forms of protein differ only at few amino acid sites. This type of polymorphism can be easily detected by gel electrophoretic studies which throw light on amino acid banding pattern.

iii. Molecular Markers:

Sometimes polymorphic differences are at molecular or DNA level. In other words, the differences are in nucleotide sequences in the DNA. These can be observed by restriction fragment length polymorphisms (RFLPs), amplified fragment length polymorphisms (AFLPs), random amplified polymorphic DNA (RAPD), single sequence repeat (SSR) etc. The molecular or DNA markers are very accurate in detecting the level of polymorphism in a population.

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Term Paper # 5. Theories of Genetic Polymorphism:

Two theories have been put forth to explain the wide spread existence of polymorphic variation. These are selectionist theory and neutralist theory.

A brief account of these theories is presented below:

i. Selection Theory:

This theory states that polymorphism is balanced or stable and the stable equilibrium is maintained by selective forces. For example, the balanced polymorphism is the selection in favour of heterozygotes.

ii. Neutral Mutation Theory:

This theory was proposed by Kimura (1983) and further elaborated by Crow (1986). According to this theory some polymorphisms are due to presence of mutant alleles that are nearly neutral with regard to fitness. Such alleles were mutated in distant past and are still present in the population contributing to polymorphism. This theory is widely accepted,

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Term Paper # 6. Advantages of Genetic Polymorphism:

There are several advantages of genetic polymorphism which are briefly presented below:

i. Genetic Diversity:

Polymorphic population has greater genetic diversity than pure lines and inbred lines. The genetic diversity avoids danger of uniformity and provides protection from biotic and abiotic stresses to the population.

ii. Broad Genetic Base:

Polymorphic population has broad genetic base due to presence of several phenotypes. Such population has greater buffering capacity to environmental changes.

iii. Adaptation:

Genetic polymorphism enhances the adaptive value of a population by providing increased diversity of genotypes in a population. It also enhances adaptability of a population, because heterozygotes are more adaptable than homozygotes. Genetic polymorphism gives rise to variation of quantitative characters.

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Term Paper # 7. Disadvantages of Genetic Polymorphism:

There are some demerits or disadvantages of genetic polymorphism which are briefly discussed below:

i. Difficult to get Purelines:

It is difficult to get purelines from a polymorphic population. Inbreeding does not have much effect in polymorphic population. It is difficult to control the number of loci that have to be kept in polymorphic state.

ii. Less Uniform:

The polymorphic populations are less uniform due to presence of genetic diversity. The produce of such population is also less uniform and less attractive.

iii. Low Yield:

The yield of polymorphic population is poorer than the best genotype present in the polymorphic population.