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Allele Frequencies: Skills (DP IB Biology)

Revision Note

Lára Marie McIvor

Last updated

Allele Frequencies in Isolated Populations

Comparison of allele frequencies of geographically isolated populations

  • Allele frequency is a term that assigns a relative frequency of an allele at a particular gene locus

    • Alleles can vary from each other by as little as one nucleotide

  • When a degree of geographic separation exists between two populations, this can cause differences in the frequencies of alleles to emerge

  • Human allele frequencies vary by geography and ethnicity

    • Examples of clear-cut allele frequency differences are rare in human populations because of the ease of travel and interbreeding

    • This leads to a scarcity of truly isolated populations

  • If there is more than one allele in existence for a particular gene, the respective allele frequencies must add up to 1

  • Online databases list the frequencies of human alleles

  • Alleles are sometimes referred to as polymorphisms which just means many (poly-) different forms (-morphisms) of a gene

    • The most common type is called a single nucleotide polymorphism (SNP)

  • Mathematical formulae such as the Hardy-Weinberg formula can be used to calculate phenotype frequencies from allele frequencies and vice versa

  • Comparing allele frequencies can provide information for

    • Identifying genetic associations with particular diseases

    • Estimating the number of individuals with disease susceptibility within a population

    • Estimating the level of drug resistance in a population

    • Performing evolutionary and anthropological studies (e.g. tracing the history of humans through time)

Basis of allele frequency analysis

  • For any polymorphism, each individual carries two alleles per locus

    • One is inherited from the mother, the other from the father

    • Exception - this does not apply to alleles present on the X or Y chromosome

  • Within a population, there are twice as many total alleles as there are individuals

  • Homozygous individuals each contribute two of that allele to the total number of that particular allele

  • Heterozygous individuals each contribute one of a particular allele to the total number of that allele

    • For example, if there are eight individuals with the ZZ genotype, they contribute 16 Z alleles

    • Thirty-four Zz heterozygous individuals contribute a total of 34 Z alleles and 34 z alleles to the total

Worked Example

Consider the following isolated island populated by a certain species of lizard. On this island, the ratio of white lizards to yellow lizards is 4:9. The yellow pigmentation is caused by a dominant allele, Y. Lizards possessing the homozygous recessive genotype (yy) are white in colour.

The founder effect in lizards


Distribution of lizards on the island before and after the geological event

Two of these lizards found themselves on a floating log that was carried in an ocean current to another island. Their migration is shown by the red arrows in the image. Fortunately, these were male and female and they were able to begin the colonisation of the new island.

Calculate the allele frequencies of Y and y on the old island and the new island.

Answer:

Mathematical formulae such as the Hardy-Weinberg formula can be used to calculate allele frequencies from phenotype frequencies (and vice versa)

p + q = 1

Where: 

  • The letter p represents the frequency of the dominant allele (Y)

  • The letter q represents the frequency of the recessive allele (y)

Step 1: Calculate the phenotype frequencies on the old island

If 4/13 are the white lizards, the phenotype frequency of yy is 4 ÷ 13 = 0.3077


Step 2: Calculate the allele frequency of y (from the phenotype frequency in Step 1)


The probability of two y alleles coming together through fertilisation is given by (q x q) or q2
Therefore, q is the square root of the white phenotype frequency

q equals square root of 4 over 13 end root equals 0.5547


Step 3: Subtract this from 1 to find the allele frequency of the dominant allele, Y


1 - 0.5547 = 0.4453

Frequency of Y allele on the old island = 0.4453 or 44.5%

Frequency of the y allele on the old island = 0.5547 or 55.5%

Frequency of Y allele on the new island = 0 or 0%

Frequency of the y allele on the new island = 1.0 or 100%

Examiner Tips and Tricks

Mathematical derivations of allele frequencies are not required for your exams, although it helps to appreciate that the sum total of all the allele frequencies must add up to 1, in order to appreciate the variation within a species.

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    Lára Marie McIvor

    Author: Lára Marie McIvor

    Expertise: Biology Lead

    Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.