IB DP Biology: HL復習筆記10.3.5 Skills: Comparing Allele Frequencies Between Populations
Populations & their Allele Frequencies
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 are 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?(eg. 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.
Distribution of lizards on the island before and after the geological event.
A geological event caused the island to become divided in two; this is shown by the red arrows in the diagram above. One island sank below the ocean, killing all its inhabitants, whilst the other survived. Only two white lizards made it to the surviving half of the island. Fortunately, these were male and female and they were able to begin the recolonisation of the small island.
Calculate the allele frequencies of?Y?and?y?before and after the geological event.
Therefore,?q?is the square root of the white phenotype frequency
Step 3: Subtract this from 1 to find the allele frequency of the dominant allele, Y
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