Basics of Genetics: Allele Frequency

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Allele frequency is the term used to describe how often a gene variant occurs in a population. Alleles are best described as variant forms of a gene located at the same genetic locus, or same position, on a chromosome. For example, if the allele frequency is 20 percent in a specific population, then among those within in this specific population, the allele will be carried by one in five chromosomes. Other gene variants will occupy the other four out of five.


This is calculated by dividing how many times the specific allele is observed within a specific population by the total amount of copies of all of the alleles located on that specific genetic locus in the population. These frequencies can be represented as a percentage, a decimal, or a fraction. These are a reflection of genetic diversity in a population. When these frequencies change over time, it can mean that genetic drift is going on or that new mutations have been presented into a population.


Evolution occurs when variation within a population is converted into variation between populations in time and space. Natural selection is generally thought to be the primary factor for figuring out the relative allele frequency in a population. It differentiates between a population’s phenotypes ability to reproduce. One phenotype may be more successful at surviving endemic onslaughts of predators or parasites than another phenotype, who may be more effective at penetrating new habitats. Directional selection is a principal pattern than selection in natural populations follow. Mutation is an evolutionary agent and it is a property that is required of the genetic material for evolution to occur. Mutations result in new alleles being introduced into the gene pool.

A higher fitness level results from advantageous genes and, therefore, the bearer has more offspring. However, beneficial mutations are rare, making them very hard to study. When organisms divide rapidly, such as bacteria and yeast, provide the advantage that mutations that are beneficial could possibly be monitored in a laboratory. Within a population that is growing, several new mutations are introduced, with a large amount of deleterious mutations. Depending on how large the population is, the majority of the deleterious mutations will be purged from a population. However, a population’s overall fitness can be spread and increased with advantageous mutations. An informative genetic marker must be available that is capable of discriminating between clonal lineages to identify those who are carrying favorable mutations. Until recently, there was not a marker system in existence for yeast or Escherichia coli. New technologies and allele frequencies are being discovered.


Abedon, S.T. (2005). Population Genetics. Retrieved on September 5, 2010 from Ohio State University:

Nature Education. (2010). Allele Frequency. Retrieved on September 5, 2010 from Nature Education: