Hardy-Weinburg Equilibrium

The Hardy-Weinberg theorem states that the frequency of alleles and genotypes in a populations gene pool remain constant over the generations unless acted upon by agents other than sexual recombination. For example, take a population of mice that consists of 1,000 members. A specific allele, albino allele, is recessive within this species. 80% of the population expresses the normal phenotype- brown coloring, while the remaining 20% are albino. 640 members of the population have the genotype AA, 320 have Aa, and 40 have aa.

If completely random mating were to occur, there would be an 80% chance that a gamete would bear the normal allele, A, and a 20% chance that the gamete would bear the albino allele, a. The resulting offspring will display the following genotype ratios: AA will have 64%, Aa 32% (the chance of the offspring having the A allele is 96%), and aa 4%. The offspring have the same genotype ratio as their parents. This example was one of Hardy-Weinberg equilibrium. The next generation will express the same genotype ratio as their parents, and so on.

But what exactly is needed to create Hardy-Weinberg equilibrium? (Basically, a population in Hardy-Weinberg equilibrium s not evolving in any way. ) Five specific factors are needed to create Hardy-Weinberg equilibrium within a population- a very large population, isolation from other populations, no net mutations, random mating, and no natural selection. The first element needed to create Hardy-Weinberg equilibrium is a very large population size. The larger the population, the less likely it is for genetic drift to occur.

Genetic drift is a chance fluctuation in the gene pool that may change the frequencies of alleles. A large population can better represent the gene pool of the previous generation than a small one. In order to completely eliminate all chances of genetic drift, a population would have to be infinitely large. Thus, we can see here that perfect Hardy-Weinberg equilibrium, which has no changes in the frequency of alleles, would require no genetic drift at all, and genetic drift itself is only possible in a population of infinite size.

There are two types of genetic drift- the bottleneck effect and the founder effect. Both severely decrease the variability within a population, altering the frequencies of alleles and thus making Hardy-Weinberg equilibrium impossible. If a disaster occurs in a population, killing off many members, the surviving members will not be representative of the original population. In the mouse example, a fire could have killed half of the population. Certain alleles would be overexpressed, certain underexpressed, and others not expressed at all.

Which alleles are expressed occurs completely by chance. Genetic drift in a new colony is called the founder effect. The population that results has a gene pool the expresses only those characteristics that the few founders carried with them, thus decreasing variability. If a few of the mice in the example were separated from their original population and colonized a new area, chances are that the founders of the new colony would not express the same genotype ratio in their gene pool as their original population.

It is obvious that these types of genetic drift would become increasingly less meaningful as population size increases. For example, if 100 mice out of a population of a population of 300,000 experience a bottlenecking, genetic drift within the population would be minimal, perhaps even going unnoticed. However, if 100 mice out of a population of 200 experienced a bottlenecking, there would be a massive change in the gene pool. We can see that the larger the population is, the better suited it is to avoid evolution in the form of genetic drift.

This is why having a large number of individuals is one of the necessary elements of a population in Hardy-Weinberg equilibrium. For the allelic frequency to remain constant in a population at equilibrium, no new alleles can come into the population, and no alleles can be lost. Both immigration and emigration can alter allelic frequency. Therefore, a population must be isolated from other populations in order to maintain the frequency of its alleles.

If our mouse population lived in a locked warehouse where no other mice could enter or exit, it would be ideal for Hardy-Weinberg equilibrium because there would be no gene flow. Gene flow is a genetic exchange due to the migration of fertile individuals or gametes between individuals. If one male mouse expressing the dominant allele for purple fur, if there was such a thing, were to enter the warehouse and mate with several of the females, the gene pool would be altered; evolution would occur.

For a population to be at Hardy-Weinberg equilibrium, there can be no change in allelic frequency due to mutation. A mutation is a change in an organisms DNA. Any mutation in a particular gene would change the balance of alleles in the gene pool. Mutations may remain hidden in large populations for a number of generations, but may show more quickly in a small population. If a mouse mutated in such a way that it was black and white spotted and bred into the population, that spotted gene would infiltrate the gene pool, altering the frequencies of alleles.

For a population to be in Hardy-Weinberg equilibrium, mating must be at random. In assortative mating, individuals tend to choose mates similar to themselves; for example, larger mice tend to choose mates of larger size and smaller mice tend to choose smaller mates. Though this does not alter allelic frequencies, it results in fewer heterozygous individuals than you would expect in a population where mating is random. Since a populations genetic structure consists of its frequencies of alleles and genotypes, fewer heterozygous individuals would mean a change in genotype frequency.

A change means that the equilibrium would be offset. In a population at Hardy-Weinberg equilibrium, there is no natural selection. This means that no individual expressing a certain allele is selected over individuals with other alleles. If selection occurs, those alleles that are selected for will become more common. For example, if resistance to a particular disease allows certain mice to live in an environment that has been introduced to that disease, the allele for resistance may become more frequent in the population.

In order to create a population in Hardy-Weinberg equilibrium, five conditions need to be met- a very large population, isolation from other populations, no net mutations, random mating, and no natural selection. However, because of certain factors, such as genetic drift, gene flow, unavoidable mutations, nonrandom mating, and natural selection, having a population in Hardy-Weinberg equilibrium is impossible. Populations will evolve over time.

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