GENETIC POLYMORPHISM

GENETIC POLYMORPHISM

Introduction

Poly : By “poly”, we mean “multi” or “more than one”. Hence, genes having multiple traits, i.e. two or more than two traits, result in polymorphism.

Morph : This is a term that refers to various forms or stages in the life span of an organism. The different forms or stages can also be stated as “morphs.”

Polymorphic : The combined term refers to the existence of more than one form of traits in a species.

When we combine the two terms as polymorphism, it indicates the presence of a gene with multiple variations in polymorphic traits. If we take the above-given example of jaguars, if they have one polymorphic trait for that gene, it is called “monomorphic” because “mono” means “one”. It means if jaguars have only one skin color, then they are monomorphic

One of the most characteristic features of any natural population is its diversity. This diversity is obvious when we consider the human species, for we are attuned to sensing differences in human appearance, personality, sexuality, and so on. In the populations of flies or dandelions such well marked diversity does not occur but it exists nevertheless. In genetic terminology natural populations are said to be polymorphic.

genetic polymorphism

Genetic Polymorphism

Polymorphism is most apparent when it affects a visible or behavioral phenotype, but is not at all restricted to such traits. R. Lewontin and J. Hubby, in 1966, undertook the first extensive analysis of protein polymorphisms in natural population of Drosophila pseudoobscura by subjecting extracts of individual flies to get electrophoresis and observing the rates of migration of various proteins, which represented 18 gene loci. They found, quite unexpectedly, that many of the proteins existed in the population in the form of isoelectric variants, meaning that for a given type of protein some individuals possessed a fast-migrating species and others a slow-migrating species.

Numerous subsequent studies of such diverse species as barley, wild oat, horse-shoe crab, mouse and man have all produced the same result : an abundance of protein polymorphism is found wherever it is sought. Protein polymorphism signals the existence of allelism, and it has been estimated that 20 to 50 per cent of all structural gene loci in a given species exist in two or more allelic forms in any given population

The polymorphism may arise in a population by the following three basic avenues—transient polymorphism, balanced polymorphism and random fixations of natural mutations.

Transient polymorphism

Transient polymorphism is a by-product of directional natural selection. If we imagine that allele a1 has a selective advantage over a2 , then with time a1 should proceed toward fixation at p = 1, and a2 should proceed toward elimination at q = 0. While this process is occurring, both a1 and a2 will be present in the gene pool and a1 /a2 heterozygotes will be present in the population. As the name implies, transient polymorphism represents a temporary situation. For example, during the course of industrial melanization, both dark and light moths would be expected to cohabit the Manchester trees for the interim, but the proportion of light moths would be seen to diminish with time as dark moths gradually predominated.

Balanced polymorphism

Balanced polymorphism is also relatively permanent kind of equilib[1]rium in which alleles a1 and a2 are present in the population at some steady-state frequencies. Balanced polymorphism is originated by disruptive or diversifying selection and heterosis.

Random fixation of natural fixation

The random fixation of natural fixation method of origin of polymorphism is also called Neutral Mutation–Random Genetic Drift hypothesis or “Non-Darwinian Evolution” and this idea has been developed by S. Wright and Kimura. This hypothesis is based on the following two assumptions :

  1. The first assumption states that selectively neutral mutations can occur in genes that code for proteins. This will clearly be true in the case of “synonymous” mutations in which one codon is replaced by another codon dictating the same amino acids, but it is also proposed to be true in the case of mutations that lead to amino acid substitutions. The idea is that an acidic amino acid might occur in an “unimportant” region of the protein, with the result that the emergent mutant protein is identical to the original in all functional aspects.
  2. The second assumption states that neutral alleles, being neither selectively advantageous nor disadvantageous, simply drift in the gene pool. Thus, if a neutral mutation arises in woman’s germ cell and this germ cell gives rise to a female child, the probability is about 0.5 that the mutant allele will be transmitted to a grandchild and 0.5 that it will not. If it is not, then q becomes equal to zero and the allele is lost. Polymorphism of cytochrome c of all eukaryotes and haemoglobin protein of all vertebrates strongly support the Wright-Kimura hypothesis

Conclusion

There is an involvement of one of two or more variants of a particular DNA sequence in polymorphism. It is observed that variation at a single base pair is the most common type of polymorphism. Long stretches of DNA can also be involved in polymorphism. This is called SNP.

  • In biology, it is the genetic variation that is not in a continued state, or we can say that the discontinuous state concluding in the happening of a few various types of solo organisms among the members of its species.
  • It is the occurrence of two or more forms in the population of a species.
  • A disconnected genetic dissimilarity can be divided among the individuals of a population into two or more than two sharply clear-cut forms.
  • If we glance over another example, then the first instance is the human’s blood group and its types.
  • In comparison, the individuals do not fall into sharp classes or groups but are almost imperceptibly ranked among the wide extremes.

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