Hardy Weinberg principal

           Hardy Weinberg principal

➢ It was proposed by G.H. Hardy, an English mathematician and W. Weinberg, 

a German physician independently in 1908. 

➢ It describes a theoretical situation in which a population is undergoing no 

evolutionary change. 

➢ In fact, it defines the genetic structure of a non-evolving population. 

➢ Mutations introduce new genes into a species resulting a change in gene 

frequencies.

➢ Gene frequency is the frequency with which a particular allele occurs in a 

population.

➢ If certain conditions existed, gene frequencies are supposed to remain fixed 

and even remain the same through generations.

➢ This is called genetic equilibrium.

Essential condition HWP

No mutation:

➢ Sudden appearance of variations are called mutations.

➢ There should not be either gene or chromosomal mutation. 

No gene flow:

➢ Gene flow refers to the movement of alleles from one population to another 

as a result of interbreeding between members of the two populations. 

➢ The removal of alleles from one population or addition of alleles into another 

population is called gene flow or gene migration. 

➢ There must not be gene flow between the population.

No genetic drift :

➢ Genetic drift is also known as “Sewall Wright Effect”.

➢ It occurs only by chance. 

➢ It is non directional. 

➢ Genetic drift can cause elimination of certain alleles or fixation of the other 

alleles in the population.

No genetic recombination:

➢ The alleles of parental linkage groups separate and new associations of 

alleles are formed in the gamete cells.

➢ This process is known as genetic recombination.

➢ Crossing over during meiosis is a major source of genetic variation within 

population.

No Natural selection:

➢ There must be no natural selection pressure with respect to the alleles in 

question.

Mathematical expiration

➢ Individual frequencies of alleles may be named p, q etc. 

➢ In a diploid p and q represent the frequency of allele A and allele a.

➢ The frequency of AA individuals in a population is p

2

. 

➢ The probability that an allele A with a frequency of p appears on both the 

chromosomes of a diploid individual is the product of the probabilities i.e., p2

. 

➢ Similarly of aa is q2

, of Aa is 2 pq.

➢ Thus p

2 + 2 pq +q2 =1. 

➢ This is a binomial expansion of (p + q)2

. 

➢ It is possible to calculate all allele and genotype frequencies using the 

expressions

○ Allele frequency p + q = 1.

○ Genotype frequency p2 + 2pq +q2 =1.

➢ Constant gene frequencies over several generations indicate that evolution is not 

taking place. 

➢ Changing gene frequencies would indicate that evolution is in progress. 

➢ In other words, evolution occurs when the genetic equilibrium is upset.

➢ Evolution is a departure from Hardy-Weinberg Principle.