The relative frequency of an allele in a population is calculated by dividing the number of times the allele appears in the population by the total number of copies of the gene in the population.
Here's a more detailed breakdown:
Understanding Allele Frequency
Allele frequency, a fundamental concept in population genetics, describes how common a particular allele is within a population. It's a crucial measure for tracking evolutionary changes.
Steps to Calculate Relative Allele Frequency
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Determine the number of individuals with each genotype: Identify how many individuals in your population are homozygous dominant (e.g., AA), heterozygous (e.g., Aa), and homozygous recessive (e.g., aa) for the gene in question.
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Calculate the total number of alleles: Since each individual carries two copies of each gene (for diploid organisms), multiply the number of individuals in the population by 2. This gives you the total number of alleles for that gene in the population.
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Calculate the number of copies of the allele of interest:
- For a dominant allele (A): Count the number of A alleles. This will be 2 times the number of homozygous dominant individuals (AA) plus 1 times the number of heterozygous individuals (Aa). So, 2*(number of AA) + (number of Aa).
- For a recessive allele (a): Count the number of a alleles. This will be 2 times the number of homozygous recessive individuals (aa) plus 1 times the number of heterozygous individuals (Aa). So, 2*(number of aa) + (number of Aa).
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Calculate the relative frequency:
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Divide the number of copies of the allele of interest (calculated in step 3) by the total number of alleles (calculated in step 2).
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Formula:
- Frequency of allele A (p) = [2(number of AA) + (number of Aa)] / [2 (total population size)]
- Frequency of allele a (q) = [2(number of aa) + (number of Aa)] / [2 (total population size)]
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Express as a decimal or percentage: The result is the relative frequency of the allele, typically expressed as a decimal (between 0 and 1) or as a percentage.
Example
Imagine a population of 500 pea plants. The gene for flower color has two alleles: purple (P) and white (p). We observe:
- 320 plants with purple flowers (PP)
- 160 plants with purple flowers (Pp)
- 20 plants with white flowers (pp)
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Total number of alleles: 500 plants * 2 alleles/plant = 1000 alleles
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Number of P alleles: (2 * 320) + 160 = 640 + 160 = 800
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Number of p alleles: (2 * 20) + 160 = 40 + 160 = 200
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Relative frequency:
- Frequency of P (p) = 800 / 1000 = 0.8 or 80%
- Frequency of p (q) = 200 / 1000 = 0.2 or 20%
Therefore, in this population, the relative frequency of the purple allele (P) is 0.8 (or 80%), and the relative frequency of the white allele (p) is 0.2 (or 20%). Note that p + q = 1.
Importance of Allele Frequency
Allele frequencies are important for understanding the genetic makeup of populations and how they change over time due to factors such as natural selection, genetic drift, mutation, and gene flow. This information is essential for fields like conservation biology, medicine, and agriculture.
