Name the law that states that the sum of | Class 12 Biology Chapter Evolution, Evolution NCERT Solutions

G.H. Hardy and W. Weinberg independently discovered a rule that related allelic and genotypic frequency in the Mendelian population (shares common gene pool). There is an aspect of this law which explains that the allelic frequencies at an autosomal locus in the population will not change from one generation to the next (allelic- frequency equilibrium), i.e. will remain constant. There are five factors that are responsible for the failure of Hardy- Weinberg equilibrium: gene flow, genetic drift, non-random mating or genetic recombination, natural selection and mutation.

Equation of Hardy and Weinberg;    p² + 2pq + q² = 1

Where; p is the frequency of dominant allele, q is the frequency of recessive allele and pq defines the heterozygous population.

There are some assumption that are necessary for this law, population should be panmictic and there would be no natural selection, no mutation, no migration occurs.    

1. Genetic drift: Random genetic drift occurs as the result of non- directional fluctuations in the transfer of alleles from one generation to the next in the small population purely by the chance irrespective of selective advantage or disadvantage of the gene. Genetic drift results in the increased variation within a population and an increase in the divergence between populations.  
2. Gene migration or gene flow: It is the movement of alleles from one gene pool to another. There are some different models for the gene flow like Continent-island model (it is one way migration to a separated population), Stepping-stone model (it is a migration where migrants come only from neighbouring populations).
3. Mutation: Mutation is the source of all genetic variations and is considered as random phenomenon; and occurs in all directions.  
4. Natural selection: Darwin described the process of natural selection in which the forms of organisms in a population that are best adapted to the environment increases in frequency relative to less adapted forms over a number of generations.
5. Genetic recombination: The exchange of genetic material between different organisms that leads to the production of offspring with the combination of traits is genetic recombination.