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Evolution Hardy Weinberg Principle

Hardy Weinberg principle: An Introduction

Last updated date: 24th Mar 2023
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The principle or the equilibrium of discussion is named after G. H. Hardy and Wilhelm Weinberg. The genotype frequencies calculated following the Hardy–Weinberg rules can be used to test for population stratification and non-random mating. The Hardy-Weinberg principle studies the genotype frequencies in non-evolving populations.

For example, external forces like mutations are reported to introduce new alleles, in turn disrupting the equilibrium of allele frequencies. Natural selection and non-random, on the other hand, are believed to alter the gene frequencies resulting in disruption of the Hardy-Weinberg equilibrium. This disturbance mainly occurs because few alleles are reported to assist or harm the reproductive success of the individuals carrying them.

Genetic drift can also cause disruption in equilibrium, majorly in small populations. Gene flow that occurs when new alleles are introduced into a population due to breeding can also alter the Hardy-Weinberg equilibrium (HWE). These external forces are always present in nature; in that case, the HWE will always be disrupted. Hence, the Hardy-Weinberg equilibrium is hypothetical.

Hardy Weinberg Equilibrium

The main assumptions for the Hardy Weinberg Principle are the following-

  1. In order to maintain equilibrium, only sexual reproduction can occur.

  2. Individuals of the population should randomly mate.

  3. The size of the population should be indefinitely large, and there must be diploid entities.

  4. The generations must not overlap, and the sex ratio should be equal.

  5. Gene flow, selection, mutation, migration and other evolutionary influences must be absent.

The Hardy-Weinberg equation can be explained by considering a simple genetic locus containing two alleles, A and a. The equation can be written as-

p2 + 2pq + q2 = 1

where p is the frequency of the allele "A" and q is the frequency of allele "a". p2 denotes the frequency of the homozygous genotype AA, q2 indicates the frequency of the homozygous genotype aa, and 2pq represents Aa, which is the frequency of the heterozygous genotype. In studies related to population genetics, the Hardy-Weinberg equation is used to compute the difference between the observed genotype frequencies and the calculated frequencies given by the equation.

Application of Hardy Weinberg Law

Applications of the Hardy Weinberg principle are mentioned below-

  1. Population stratification and non-random mating can be studied from the Hardy-Weinberg genotype frequency tests.

  2. As variations occur in genes due to mutation, genetic drift, migration, sexual selection and natural selection, the Hardy Weinberg principle acts as a statistical criterion for differentiating a non-evolving population from evolving populations.

  3. The idea of evolution in a population can be obtained from the allele frequencies that are recorded and calculated following the Hardy-Weinberg principle.

  4. The law can be used as a template to research the population genetics of diploid organisms. However, the law stands invalid for haploid organisms.

Hardy Weinberg Law in Plant Breeding

Plant breeding deals with the alteration of the traits of plants to generate desired characteristics. In population genetics, the most frequently used mathematical model is the Hardy–Weinberg Equilibrium (HWE). The genotype and allele frequencies of future generations can be computed with the help of this principle. The equilibrium of earlier and current populations can also be interpreted from this principle.

Hence, the HWE has ecological significance. The facultative clonal plants are inherently problematic subjects for the application of the Hardy Weinberg principle. The problematic areas or assumptions that are not followed according to HWE are the generations overlapping in the case of clonal plants. Life spans of these plants are extreme i.e; some live for a short span and some for a long span. Hence, the study of generations cannot be done in such cases.

In the case of dioecious plants, the criteria of equal sex ratio are not maintained. In spite of these limitations, HWE can be used to obtain values such as expected heterozygosity or fixation index. In plants where clonality is not maintained, the Hardy-Wienberg principle can be used to calculate the genotype frequencies.

Hardy Weinberg Population Genetics

The relationship among genotype frequencies, allele frequencies, and factors that are reported to alter these frequencies over time are examined with the help of population genetics. The Hardy-Weinberg principle is applicable to individual genes containing two alleles, a dominant and a recessive allele. A population with such a gene can be described in terms of its genotype numbers or genotype frequencies. The allele frequency of each genotype can be calculated by dividing the number of individuals with a particular genotype divided by the total number of genotypes in a population.

The HWE gives us an idea about the genetic composition and the inheritance of genes in living organisms. The idea of genetic variation can also be obtained. The study of HWE in population genetics helps us to better understand the contribution of genes to the incidence of diseases. Hardy-Weinberg law is essential to do a comparative study between the real variations in a population to the calculated one by the Hardy-Weinberg principle.

If there is a difference between the observed and predicted valu, then it indicates that the equilibrium in the population is disturbed. The prediction of the occurrence of a negative recessive gene in the heterozygous carriers can also be made by the study of HWE in population genetics.

Key Features of Hardy Weinberg Principle

  • The frequency of alleles in a population can be designated by p2 + q2 + 2pq = 1, where p2 is the frequency of homozygous dominant genotype, q2 is the frequency of recessive genotype, and 2pq is the frequency of heterozygous genotype.

  • In presence of the disruptive forces like selection, mutation and genetic drift the HWE is not maintained.

  • The Hardy Weinberg equilibrium is maintained if sexual reproduction and random mating should occur and the sex ratio should be equal.

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FAQs on Evolution Hardy Weinberg Principle

1. What is an allele frequency?

Allele frequency is calculated by dividing the number of times the allele of attention has appeared in a population by the total number of alleles at a particular genetic locus in the population. It is represented as a decimal.

2. What is population genetics?

Population genetics studies the overall gene pool and variations in the genetic composition of populations. The variations occur as a result of factors, such as natural selection. The concept is focused on the Hardy-Weinberg law.

3. What is random mating?

Random mating occurs in a panmictic population, i.e. when the probability of mating between individuals is not dependent on their genetic constitution within the population. 

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