Calculation of Allele and Genotype Frequencies & HardyWeinberg Equilibrium Theory
INTRODUCTION
Population geneticists study frequencies of genotypes and alleles within populations rather than
the ratios of phenotypes that Mendelian geneticists use. By comparing these frequencies with
those predicted by null models that assume no evolutionary mechanisms are acting within
populations, they draw conclusions regarding the evolutionary forces in operation.
In a constant
environment, genes will continue to sort similarly for generations upon generations.
The
observation of this constancy led two researchers, G. Hardy and W. Weinberg, to express an
important relationship in evolution.
The law that describes this relationship bears their names.
The HardyWeinberg Equilibrium Theory serves as the basic null model for population genetics.
Every individual has alleles that were passed on from their parents.
If we take all of the alleles
of a group of individuals of the same species (that is, a population) we have what is called the
gene pool.
The frequency, or proportion, of individuals in that population that possess a certain
allele is called the allele frequency.
Populations can have allele frequencies, but individuals
cannot.
This obviously makes populations the best hierarchical unit, or level, to study evolution,
as evolution is basically the study of the change in allele frequencies over time.
Allele Frequencies Consider an individual locus and a population of diploid individuals where
two different alleles, A and a, can be found at that locus. If your population consists of 100
individuals, then that group possesses 200 alleles for this locus (100 individuals x 2 alleles at that
locus per individual). The number of A alleles present in that population expressed as a fraction
of all the alleles (A or a) at that locus represents the frequency of the A allele in the population.
1. To calculate allele frequencies for populations of diploid organisms, first multiply the number
of individuals in the population by 2 to obtain the total number of alleles at that locus.
2. Select
one of the alleles for your first set of calculations. Let’s first choose the A allele from the
example provided above.
a. Individuals homozygous for the A allele will each possess 2 A
alleles. Multiply the number of AA homozygotes by 2 to calculate the number of A alleles.
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 Spring '08
 Chew
 Genetics, Evolution, Population Genetics, Hardy Weinberg Assumptions

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