Genetic Equillibrium - Hardy and Weinberg

 

Population genetics is the study of gene frequency in the population and  their change under varied conditions. Population or Mendelian  population is a group of individuals present in a geographical area which has a common gene pool. 

Gene frequency of the percentage of an allele in relation to the total allele  present in a population. All the genes and their alleles in the population create a gene  pool. A change in relative frequency of allele  brings the evolution. This occurs due to some selective pressure but normally the allele tries to maintain an equilibrium with reference to one another over the generation irrespective of the phenotypic expression. This is known as genetic equilibrium.

The  genetic equilibrium will operate under certain conditions :  the population is stable throughout  the year. There is no immigration or emigration. The size of the population is large. Mutation is  negligible. The sex ratio is unity and mating is random. 

GH Hardy and Withelm Weinberg gave an equation independently in 1908 about the structure of population. It is known as Hardy Weinberg equilibrium or genetic equilibrium. The allelic frequencies in a population are stable and remain constant from generation to generation. The sum of  two alleles (P and Q) to form 100% or 1.

They  segregate during gamete  formation and then combine randomly at the time of the fertilization with equal chances of each of them combining with its own type or the other. This can be understood as follows 

p+q=1

(p+q) 2 =1 

p2+2pq+q2 =1

The percentage value of homozygous  and heterozygous individuals remains constant over the generations. Recessive traits appear only when both the recessive alleles come together. The information is used to know the frequency of other combinations. Suppose a recessive trait is present in 16% of the sample population than 

q2 =16 =0.16

q = 0.4

As p+q =1 then value of p would be 1.0 - 0.4 = 0.6

Frequency of homozygous allele is  p x p = 0.6 x 0.6 = 0.36 = 36%

Frequency of heterozygous allele is  2 x p  =2 x 0.6 x 0.4 = 0.48 = 48%

There are five basic principles that can change the Hardy Weinberg equilibrium and bring about variation at the genetic level are - Mutations, Recombination, Gene migration, Genetic drift and Natural selection. 

Mutation are  sudden unpredictable stable and inheritable changes which occur in organisms due to permanent change in their chromosome and gene. The frequency of mutation varies from gene to gene. The Rate  of mutation is increased by the presence of radiation and some Chemicals called Mutagen. Mutated genes add new alleles to the gene pool. The gene pool  is the aggregate of all the genes and their presence in a population gene pool. It is not static; it shows the change in frequency of different alleles of the gene. Evolution occurs due to change in the gene pool of a species. mutatioñ  are the source of all the pre adaptation replacements of alleles that are removed by other evolutionary agents. 

Gene recombination is a new combination of alleles of different genes which come together in an individual due to sexual reproduction. All phenotypic and genotypic differences found amongst the individual species are due to gene recombination because no two individuals of species are similar except  for monozygotic twins. Gene recombinations occur due to the crossing over, random fusion of allele, independent assortment of chromosomes. 

Gene migration is addition or loss of allele from a population due to entry or exit  of an organism. Gene migration leads to gene flow. Gene flow is the spreading of alleles in a population and between populations due to breeding and subsequent intercrossing. The gene migration changes the gene  frequency and phenotypes of  individuals.

Genetic drift is an evolutionary change in allele frequencies  in  small populations,. if a wrong allele is transferred from the gene pool of the current generation to the next generation then genetic drift is caused.It does not occur due to any environmental influences.

In large populations, the allele frequency of the genes remains relatively stable because the genes are not affecting fitness and do not have a natural selection pressure against the alleles. 

The genetic drift can be observed In the bottleneck effect in which the population size is  decreases due to competition, predators, or diseases.The frequency of certain alleles in a population changes due to elimination of organisms. This is observed during natural disasters like volcanic eruptions, earthquakes, etc., leading to the death of most of the population.

Founder Effect is another one which shows the founder effect.

In the founder effect, a new population is founded in a new location due to physical or geographical barriers.The new population formed does not interact and mate with the original population.As a result, the allelic frequencies of the new population will be different from the original population. There are many species that are found only on a particular island. This is due to the founder effect. E.g., two birds of the same species reach an island. Their alleles will be responsible for the diversity on that island.

Natural selection or survival of the fattest is a major factor that adds variation in the population and changes the gene frequency in the gene pool leading to the evolution or formation of a new pool. It operates through differential or non random reproduction. It is because all the offspring do not have a similar type of gene combination. Some have better genotypes which promote adaptation, others have poor adaptability. Most of the individuals with inferior genotypes are eliminated before reaching maturity. Even those which reach maturity do not have equally good genotype. the sum of them do not reproduce. The most successful also produce the maximum offspring. Moderate individuals produce fewer offspring. The phenomena of finding mates and producing offspring with success is called differential reproduction.