Question

What is Genetic ppolymorphism? Explain breifly the different types of polymorphism.

Answer 1

Genetic Polymorphism:

• One of the most characteristic feature 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 nonetheless.
• In genetic terminology, natural populations are said to be polymorphic.
• Polymorphism is most apparent when it affects a visible or behavioral phenotype, but it 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 populations of Drosophila pseudoobscura by subjecting extracts of individual flies to get electrophoresis and observing the rates of migration of various pro teins, 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 existense 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.

1. Transient polymorphism:

• Transient polymorphism is a by product of directional natural selection.
• If we imagine that allele a has a selective advantage over a,, then with time a, should pro ceed toward fixation at p=1, and a, should proceed toward elimination at q=0.
• While this process is occurring, both a, and a, will be present in the gene pool and a/a, 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 a dark moths gradually predominated.

2. Balanced polymorphism:

• Balanced polymorphism is also relatively permanent kind of equilibrium in which alleles a, and are present in the population at some steady-state frequencies.

Balanced polymorphism is orignated by disruptive or diversifying selection and heterosis.

3. 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 following two assumptions
  • 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 pro posed 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.
  • (ii) 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 probabi lity 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.
  
  
  

Genetic Polymorphism:

 The simultaneous occurrence of two or more discontinuous genotypes or alleles in a population is known as Genetic Polymorphism.