# HMB265H1 Lecture Notes - Lecture 11: Allele Frequency, Population Genetics, Genotype Frequency

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HMB264 Lecture 10 Population genetics
Population genetics
- Population genetics is the study of the genetics of a population and how the alleles vary
with time
- Population is defined as the group of interbreeding individuals of the same species that
inhabit the same space at the same time
- Gene pool is the sum of all the alleles carried by all members of the population
- Gene flow describes how alleles can move between populations when migrate and mate
- Microevolution is defined as the changes in allele frequencies within a population
Terms used to describe populations
- Phenotype frequency is defined as the proportion of individuals in a population that have
a certain phenotype
- Genotype frequency is defined as the proportion of individuals in a population that carry
a particular genotype
- E.g. consider we have a gene with two alleles (A and B) in a population of 20 individuals
o 12 individuals are AA
o 4 individuals are AB
o 4 individuals are BB
- The genotype frequencies would be:
o AA = 12/20 = 0.6
o AB = 4/20 = 0.2
o BB = 4/20 = 0.2
Calculating allele frequencies
- Allele frequency is defined as the proportion of gene copies in a population that are of a
given allele type
- If we take the example from before and calculate the allele frequency, we first have to
multiply the number of individuals by 2 as each individual carries 2 alleles per locus
o 20 individuals = 40 alleles
o 12 AA individuals 24 A alleles
o 4 AB individuals 4 A alleles and 4 B alleles
o 4 BB individuals 8 B alleles
- Frequency of A alleles = (24 + 4)/40 = 0.7
- Frequency of B alleles = (4 + 8)/40 = 0.3
The Hardy-Weinberg Law correlates allele and genotype frequencies
- Five simplifying assumptions must be met for a population to be at “Hardy-Weinberg
equilibrium”
o Infinitely large population
o Individuals mate at random
o No new mutations appear in gene pool
o No migration into or out of the population
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