Genetic polymorphism is defined as the occurrence together in the same habitat at the same time of two or more distinct forms of a species in such proportions that the rarest of them cannot be maintained merely by recurrent mutation. Genetic polymorphism can also be defined as the occurrence in the samepopulation of two or more alleles at one locus, each with appreciable frequency.
If the mutation frequency is more than 2 per cent in a population, it is called as polymorphism. DNA polymorphism as DNA having more than one form, with a frequency of above 2 percent in a population.
- Single Nucleotide Polymorphisms (SNPs),
- Restriction site Polymorphisms (RSPs) or Restricted
- Fragment Length Polymorphisms (RFLP) and
- Variable Number of Tandem Repeats (VNTRs).
I. Single Nucleotide Polymorphisms (SNPs): A single nucleotide is substituted by a different nucleotide.
There are two types of nucleotide substitutions resulting SNPs.
1) Transition: A substitution occurs between Purines (A,G) or between Pyrimidines (C,T). .
2) Transvertion: A substitution occurs between a Purine and a Pyrimidine.
Insertions: A new nucleotide will be inserted in the sequence.
Deletions: An existing nucleotide will be deleted in the sequence.
II. Restriction site Polymorphisms (RSPs) or Restricted Fragment Length Polymorphisms (RFLP): A sub set of SNPs cause a loss or gain of a restriction site (restriction site is the location where a particular enzyme cuts the DNA sequence at particular sequence location into pieces of DNA). Due to change in nucleotide at particular site location, enables the enzyme to cut DNA into pieces. This leads to create different length of DNA piece in an individual and another length of DNA piece in another individual. This is called Restriction site polymorphism. RSPs are described as restricted fragment length polymorphisms (RFLP).
III.Variable Number of Tandem Repeats (VNTRs): It is divided into two types:
- Micro-satellite polymorphism is also called as Short Tandem Repeats (STRs). A small array of tandem repeats of a simple sequence (usually less than 10 basepairs).
Ex: dinuleotide repeat, trinuleotide repeat
2. Mini-satellite polymorphism: A collection of moderately sized arrays of tandemly repeated DNA sequence which are dispersed over considerable portions
of the nuclear genome.
Genetic polymorphisms in human populations
Genetic polymorphisms are very common phenomena in all human populations. Most of the polymorphisms encountered in human populations so far fall into two main categories: blood-cell antigens (blood groups) and blood proteins (serum proteins).
- sickle-cell anaemia- controlled by a gene which produces the disease when homozygous and is responsible merely for the sickling trait when heterozygous. This gene affects the formation of haemoglobin, but not for all, it only affects the structure of erythrocytes that assumes a sickle-like shape, that leads to haemolysis severe enough to cause an extreme and often fatal anaemia. the homozygotes suffer from this heavily lethal disease which usually eliminates them, the heterozygotes are quite common in certain regions. Allison (1954) discovered that the sickle-cell trait confers marked immunity against malaria especially, due to Plasmodium falciparum. The polymorphism is established only in those places where malaria is common.
- Thalassaemia- Many homozygotes, and a few of the heterozygotes, die. The gene is present in 10 per cent of the population in some of the districts of Greece and Italy where malaria is endemic.
- The ability to taste phenyl-thio- carbamide (PTC), or phenyl-thio-urea (PTU). Non-tasters are homozygous for the recessive allele t. When both parents are non-tasters, all their children are non-tasters.
Genetic polymorphisms may be ‘transient ’ or ‘balanced’.
Genetic polymorphisms are called balanced, if selection favors the heterozygotes. When selection favors the heterozygotes, a stable equilibrium may be achieved and substantial frequencies of both alleles may be maintained in one environment. The balanced or stable polymorphism is the result of natural selection operating as a stabilizing agent.
It is difficult to establish whether a polymorphism is stable or transient. Direct evidence for at least one balanced polymorphism is available; the polymorphism for the group of haemoglobins, including haemoglobin S, in the presence of malaria.
Genetic polymorphism and selection
It helps in natural selection and thus survival in adverse conditions.
Eg. sickle-cell anaemia in maleria prevalent territory