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Hardy-Weinberg Principle

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A system of guidelines for genetic inheritance is known as mendelian inheritance. A monk by the name of Gregor Mendel made the initial discoveries of genetics in the 1850s, and his findings were first published in 1866. People have been aware of how qualities are passed on from parents to their offspring for thousands of years. Mendel conducted research on plants and meticulously planned his trials, which made his work unique.

Mendel conducted experiments to examine how qualities were passed down in pea plants. He began his crosses with true-breeding plants and counted natural traits that were either/or (either tall or short). He produced numerous plants by breeding, and he quantified his findings. Test crossings were done to determine the quantity and presence of recessive traits.

Hardy Weinberg Principle

  1. According to the Hardy-Weinberg equilibrium, when evolutionary processes are not at play, both the genotype and allele frequencies in a sizable random-mating population remain constant from generation to generation.
  2. In other words, according to the Hardy-Weinberg Law, if the frequency of the unique alleles in a population is known, it is possible to compute the anticipated frequencies of genotypes in a population under a specific set of circumstances.
  3. The German physician Wilhelm Weinberg and the English mathematician G.H. Hardy made the discovery in 1908.
  4. Numerous factors, like mutations, non-random mating, genetic drift, natural selection, and gene flow, can upset the Hardy-Weinberg equilibrium.
  5. For instance, by introducing new alleles into a population, mutations upset the balance of allele frequencies.
  6. Similar to how nonrandom mating and natural selection cause changes in gene frequencies, they also upset the Hardy-Weinberg equilibrium.
  7. This occurs because specific alleles can either increase or decrease an organism’s probability of surviving.
  8. Another factor that could upset this equilibrium is genetic drift, which takes occurred when allele frequencies randomly increase or decrease in small populations.
  9. Gene flow, which happens when two species mate and introduce new alleles into a population, can also affect the Hardy-Weinberg equilibrium.
  10. Since all of these negative forces frequently occur in nature, the Hardy-Weinberg equilibrium hardly ever occurs.
  11. Genetic differences in nature can therefore be viewed as deviations from the Hardy-Weinberg equilibrium, which describes an idealized state.
Hardy Weinberg Principle


Hardy-Weinberg Equation and Analysis

  • Hardy-Weinberg formula Mendelian genetics theorems are discussed in the context of populations of diploid, sexually reproducing individuals.
  • This theorem states that, under a particular set of circumstances,
    • Allele frequencies in a population don’t change from generation to generation.
    • The expected genotype frequencies are p2, 2pq, and q2 if p and q are the allele frequencies in a population with two alleles, B and b, at the locus.
  • This frequency distribution won’t change from generation to generation after a population reaches a Hardy-Weinberg equilibrium.
  • The frequency of genotype BB is equal to p2, the frequency of genotype Bb is equal to 2pq, and the frequency of genotype bb is equal to q2. As an example, if allele B is more common in the population than allele b, the frequency of genotype B is equal to p, and allele b is more common than p.
  • Mathematically speaking, p+q must equal 1 if a locus has just two alleles.
  • The Hardy-Weinberg genotype, p2 + 2pq + q2, has frequencies that are one and mirror the binomial expansion of (p + q)2.
  • If B allele frequency is 0.7, then we know that the b allele frequency is 0.3.
  • If there are three alleles, B1, B2, and B3, with frequencies p, q, and r, the equilibrium frequencies for the six genotypes will be as follows:
  • (p+q+r)2= p2(B1B1) +q2 (B2B2) +r2(B3B3) + 2pq(B1B2) + 2pr(B1B3)+ 2qr(B2B3)
  • Hardy-Weinberg formula Theorem can be used for loci with more than two alleles, in which case the expected genotype frequencies are given by the multinomial expansion for all k segregating alleles: (p1 + p2 + p3 +…+ pn)2.
  • The Hardy-Weinberg Theorem’s conclusions only hold true when the population matches these presumptions:
  • The relevant locus is not subject to natural selection (i.e. there are no consistent differences in the likelihoods of survival or reproduction between genotypes).
  • Both migration and mutation, which are the sources of new alleles, do not contribute new alleles into the population (the transport of individuals and their genes into or out of the population).
  • Assuming that genetic drift does not result in random fluctuations in allele frequencies from one generation to the next due to sampling error, population size is unlimited. The impacts of drift are likely to be more noticeable in small populations than in large ones because all natural populations are finite and as a result prone to drift.
  • People in the population mate randomly in the locus under consideration. Non-random mating can cause variations from expected genotype frequencies and pave the way for natural selection to bring about evolutionary change, even if it does not change allele frequencies from one generation to the next while the other assumptions are true.
  • Given that the aforementioned assumptions hold that male and female allele frequencies are equal (or else that individuals are hermaphrodites) and that the locus is autosomal, it only takes one generation of random mating to bring genotype frequencies in a population back to Hardy-Weinberg proportions.
  • If allele frequencies differ across sexes, it takes two generations of random mating to reach the Hardy-Weinberg equilibrium.
  • Sex-linked loci take many generations to establish equilibrium since one sex has two copies of the gene and the other only one.
  • The expected Hardy-Weinberg genotype frequencies can be easily determined from these situations if we think of random mating in terms of the likelihood of producing each genotype by the random union of gametes into zygotes.
  • If gametes join randomly to generate zygotes and each allele occurs in sperm and eggs at the same frequency, the probability that any two alleles will combine to make a certain genotype is equal to the product of the allele frequencies.
  • We add the probability of the two ways that the heterozygous genotype (B egg and B sperm, or B egg and B sperm) can be created in order to achieve the expected Hardy-Weinberg frequency of the heterozygous genotype (2pq).
  • It is crucial to understand that the Hardy-Weinberg equilibrium is a neutral equilibrium. This means that, if the other conditions of the theorem are met, a population that has been disturbed by its Hardy-Weinberg genotype frequencies will still achieve equilibrium, but if allele frequencies have changed, it will be a new equilibrium after one generation of random mating.
  • A stable equilibrium, in which a disrupted system returns to the same equilibrium state, differs from a neutral equilibrium by virtue of this feature.
  • The Hardy-Weinberg equilibrium’s instability makes sense given that variations from the equilibrium’s genotype frequencies are typically associated with variations in allele frequencies (p and q), which in turn result in new values for p2, 2pq, and q2.
  • After that, unless the new equilibrium is disturbed again, a population that conforms to the Hardy-Weinberg assumptions will remain there.

Assumptions for the Hardy-Weinberg Principle

  1. There can only be sexual reproduction.
  2. Mating occurs at random.
  3. There are a lot of people living there.
  4. Everything is a diploid
  5. There is no generational overlap
  6. Equality of sex-based allele frequencies
  7. There are no signs of mixture, migration, mutation, or gene flow.

Any deviation from the anticipated outcome could result from any breakdown of the aforementioned assumptions. The resultant subtraction is entirely responsible for the digression. A population must reach Hardy Weinberg proportions after one generation of random mating, according to the law. This population will not have Hardy Weinberg proportions if the assumption of random mating is broken. Inbreeding is the most typical cause of non-random mating. It causes all genes to become more homozygous.

Infringement of the Hardy-Weinberg Equilibrium

  1. Mutation – On the allele frequencies, it barely makes a difference. The range of the mutation rate is 10-4 to 10-8. Most changes to the allele frequencies fall within this category. Recurrent mutations will keep the allele from being lost even if there is a persistent strong selection working against it in the population.
  2. Selection – The allele frequencies generally fluctuate quickly as a result of this. Balancing selection is one of the few types of selection that can achieve equilibrium without any allele loss, whereas directional selection and other types of selection can cause allele loss over time.
  3. Size of the population – Due to the sampling phenomenon known as genetic drift, being small can result in a random change in the allele frequencies. Sampling effects are noticeable when alleles are detected in low copy numbers.
  4. Migration – Genetically speaking, migration can link two or more groups together. The allele frequencies in these groups have a propensity to increase with homozygosity. A few migration models are essentially the Wahlund effect (non-random mating). For these models, Hardy-Weinberg proportions are often useless.

Applications of the Hardy-Weinberg Principle

Genetic variation in natural populations is frequently seen changing as a result of mutation, genetic drift, migration, sexual selection, and natural selection. The Hardy-Weinberg equilibrium offers a formula that distinguishes between populations that aren’t evolving and populations that are. The processes that underlie population evolution can be hypothesized over time if the allele frequencies are noted and estimated for the predicted frequencies based on the values of the Hardy-Weinberg principle.

In order to examine the population genetics of diploid creatures, which satisfy the fundamental assumptions of random mating, huge population, no mutation, migration, or selection, the law provides a prototype, which is often utilized as a point of origination.

However, haploid pathogens do not fit the Hardy-Weinberg equilibrium paradigm. One of the presumptions in this law is broken if a population is not determined to be in Hardy-Weinberg equilibrium. This implies that selection, non-random mating, or migration have had an impact on the population. In this situation, tests are conducted and hypotheses are put out in order to determine the causes of the population’s lack of equilibrium.

Complete Dominance

When total dominance exists and Hardy-Weinberg equilibrium is in place, making it impossible to distinguish between two genotypes, allele frequencies can be found. The allele frequencies of the individuals displaying the recessive phenotype aa can be determined from the frequencies of the two genotypes AA and Aa having the same phenotype as a result of total dominance of A over a. In this case, the square of the frequency of the recessive gene should be equal to the frequency of the aa individual.

Multiple Alleles

The Hardy-Weinberg principle permits the calculation of genotypic frequencies at loci with more than two alleles known as multiple alleles, such as the ABO blood types. In IA, IB, and IC, three alleles are present with p, q, and r frequencies, respectively. p + q + r = 1. The genotype of a population with random mating is given by (p + q + r)2.

Linkage Disequilibrium

Consider two distinct loci with two or more alleles each or two or more alleles on the same chromosome. The frequency of allelic combinations reaches equilibrium as a result of genetic exchange through recombination occurring at two syntenic loci at regular intervals.

Alleles are known to be in a linkage disequilibrium when they are unable to reach equilibrium, which is caused by two or more linked alleles inheriting together more frequently than expected. Supergenes are another name for such gene groupings.

Frequencies of Harmful Recessive Alleles

The law can also be used to calculate the prevalence of dangerous recessive genes carried by heterozygous carriers. If two alleles, A and a, are present at an autosomal locus in a population with p and q frequencies, respectively, and p + q = 1, then the frequencies of the genotypes AA, Aa, and aa will be p2 + q2 + 2pq. If the aa genotype frequently manifests a disease-causing phenotype, such as cystic fibrosis, then the population’s proportion of affected people must be q2, and the heterozygous carrier’s recessive allele frequency must be 2pq.

Hardy-Weinberg Theorem’s Evolutionary Implications

  1. The Hardy-Weinberg Theorem demonstrates that Mendelian loci segregating for multiple alleles in diploid populations may maintain predictable levels of genetic diversity in the absence of influences that alter allele frequencies.
  2. These predictions are frequently visualized by plotting p2, 2pq, and q2 as a function of allele frequencies.
  3. This graphical depiction highlights two key consequences of the Hardy-Weinberg theory
  4. When p = q = 0.5, the population’s heterozygosity (frequency of heterozygotes) is highest.
  5. Given that when q is close to zero, q2 is considerably less than 2pq, and when p is close to zero, p2 is much smaller than 2pq, rare alleles are predominantly found in heterozygotes, as they must be.
  6. The second assertion acquires special significance when we take into account how natural selection can influence the frequencies of novel mutations.
  7. All individuals in a population must be homozygous for a beneficial allele for that allele in order for it to exist if the other Hardy-Weinberg assumptions are true.
  8. The initial increase in the frequency of a common, favorable, dominant allele is greater than that of a rare, advantageous, recessive gene.
  9. Because uncommon alleles are frequently found in heterozygotes, as we have seen, the natural selection won’t detect a novel recessive mutation until it occurs in homozygotes with a frequency high enough (perhaps through drift in a genuine, finite population).
  10. However, because a new dominant mutation affects fitness in heterozygotes, natural selection can detect it right away.
  11. Even though Hardy(1908) demonstrated that dominance alone does not change allele frequencies at a site, dominance interactions between alleles can have a significant impact on evolutionary trajectories.
  12. Selection, mutation, migration, and genetic drift are the processes that alter allele frequencies; when one or more of these processes is active, the population deviates from Hardy-presumptions, and Weinberg’s, and evolution takes place.

FAQs on Hardy-Weinberg Principle

Question 1: How does mutation affect Hardy-Weinberg?


Due to the following, allele frequencies are stable over time: No discernible new mutation rate exists. There is no selection against any specific genotype; all individuals, regardless of genotype, are equally capable of mating and passing on their genes.

Question 2: What is the application of Hardy-Weinberg’s principle to evolution?


In the absence of factors that alter allele frequencies, the Hardy-Weinberg Theorem shows that Mendelian loci segregating for multiple alleles in diploid populations will maintain predictable levels of genetic diversity.

Question 3: What factors affect allele frequency?


The methods that alter allele frequencies throughout time include natural selection, genetic drift, and gene flow.

Question 4: How does mutation affect Hardy Weinberg?


Mutations are permanent adjustments made to the DNA sequence encoding genes. These alterations affect the genes and alleles that make up the genetic diversity of a population. While mutations alter the genotype of a population, they may or may not result in observable or phenotypic alterations. Mutations may have an impact on particular genes or entire chromosomes.

One of the requirements for a population to be in a Hardy-Weinberg equilibrium is that there be no mutations. These alterations affect the genes and alleles that make up the genetic diversity of a population.

Question 5: Which condition is essential for natural selection?


Natural selection requires the occurrence of four factors: reproduction, inheritance, variation in organism fitness, and variation in individual traits among population members. Natural selection happens spontaneously if they are satisfied.

Question 6: What type of cell passes on mutations?


An acquired mutation that manifests in an egg or sperm cell can be passed on to the person’s progeny. An acquired mutation becomes hereditary once it is transmitted from parent to child. If acquired mutations take place in somatic cells, which are body cells other than sperm and egg cells, they are not passed on to subsequent generations.

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Last Updated : 15 Oct, 2022
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