The following points highlight the top five criteria for genetic polymorphism. They are: 1. Selection 2. Mutation and Mutation Rates 3. Genetic Drift 4. Migration Pressure or Gene Flow 5. Meiotic Drive.
Criteria # 1. Selection:
Selection is the non-random or selective reproduction of genotypes. The elucidation of the selective forces which may have given rise to any particular polymorphism is turning out to be among the most difficult and intractable problems in human genetics.
Criteria # 2. Mutation and Mutation Rates:
Mutations are thought to occur more or less at random but if it occur in “one-way” i.e. change from one allele to another then it produces alterations in the frequency of a particular allele in a population. This is known as mutation pressure or genetic load.
Criteria # 3. Genetic Drift:
If the allele frequencies fluctuate randomly due to the inbreeding in a small population, then all loci will be represented by two or more alleles and the direction and magnitude of the effect will be dependent on a chance factor.
In a small breeding population, the new allele will on average be transmitted to only half the children of the individual who first receives it, and a distinct chance that it will not be transmitted to the next generation; the odds in favour of its being lost by chance are compounded in successive generations.
Criteria # 4. Migration Pressure or Gene Flow:
It will not be surprising that allele frequencies may be altered in local populations of a species by an exchange of alleles with other local populations. Migration of species is the major source of the gene flow.
Criteria # 5. Meiotic Drive:
Any irregularities during the time of meiosis may also alter the allele frequencies in a population, but it is most important point which is to be remembered that most of these irregularities have been sporadic and non-hereditary.