Chapter 8.5 – Allelic Frequencies and population genetics
Background:
Deals with the genes and alleles in a whole population
Gene pool – the alleles of all the genes in a population
Allelic frequency – The number of times a specific allele appears in a gene pool
We can look at this more closely by examining a gene that has two alleles, one that is dominant and
one that is recessive. An example of this is the gene responsible for causing cystic fibrosis. This gene
has a dominant allele (F) that leads to normal mucus production and a recessive allele (f) that leads
to thicker mucus being produced. Any human has two of these alleles in each of their cells, one on
each of the pair of homologous chromosomes on which the gene is found. As these alleles are the
same in every cell in the body, we only count one pair of alleles per individual when studying a gene
pool. So, if there are 10,000 people in a population, there will be twice as many (20,000) alleles in
the gene pool for this gene.
The pair of alleles of the CF gene have three different possible combinations, homozygous dominant
(FF), homozygous recessive (ff) and heterozygous (Ff). However, when we look at allele frequencies
it is important to note that the heterozygous combination can exist in two forms (Ff and fF).
In any population, the number of alleles is given as 1.0. In a population of 10,000 people, if everyone
had the genotype FF, the frequency of the dominant allele would be 1.0 and the frequency of the
recessive allele would be 0. If everyone was heterozygous, then the frequency of the dominant allele
(F) would be 0. The frequency of the recessive allele would also be 0.5. Of course, in the real world,
the population is not made up of only one genotype, but a mixture of all three (the proportions of
these vary in each population). How can we work out the allelic frequencies for these mixed
populations?
The Hardy-Weinberg Principle
This provides a mathematical equation that can be used to calculate the frequencies of the alleles in
a particular gene in a population. This principle predicts that the proportion of dominant and
recessive alleles of any gene in a population remains the same from one generation to the next,
provided that 5 conditions are met:
No mutations arise
The population is isolated, i.e. there is no flow of alleles to or from the population
There is no selection (all alleles are equally likely to be passed on to the next generation)
The population is large
Mating within the population is random
Although these 5 conditions are probably never entirely met in a natural population, the Hardy-
Weinberg principle is still useful when studying gene frequencies.
To help us understand this principle let us consider a gene that has two alleles a dominant allele (A)
and a recessive allele (a).
Let the frequency of allele A = p
And the frequency of allele a = q
The first equation we can write is:
Background:
Deals with the genes and alleles in a whole population
Gene pool – the alleles of all the genes in a population
Allelic frequency – The number of times a specific allele appears in a gene pool
We can look at this more closely by examining a gene that has two alleles, one that is dominant and
one that is recessive. An example of this is the gene responsible for causing cystic fibrosis. This gene
has a dominant allele (F) that leads to normal mucus production and a recessive allele (f) that leads
to thicker mucus being produced. Any human has two of these alleles in each of their cells, one on
each of the pair of homologous chromosomes on which the gene is found. As these alleles are the
same in every cell in the body, we only count one pair of alleles per individual when studying a gene
pool. So, if there are 10,000 people in a population, there will be twice as many (20,000) alleles in
the gene pool for this gene.
The pair of alleles of the CF gene have three different possible combinations, homozygous dominant
(FF), homozygous recessive (ff) and heterozygous (Ff). However, when we look at allele frequencies
it is important to note that the heterozygous combination can exist in two forms (Ff and fF).
In any population, the number of alleles is given as 1.0. In a population of 10,000 people, if everyone
had the genotype FF, the frequency of the dominant allele would be 1.0 and the frequency of the
recessive allele would be 0. If everyone was heterozygous, then the frequency of the dominant allele
(F) would be 0. The frequency of the recessive allele would also be 0.5. Of course, in the real world,
the population is not made up of only one genotype, but a mixture of all three (the proportions of
these vary in each population). How can we work out the allelic frequencies for these mixed
populations?
The Hardy-Weinberg Principle
This provides a mathematical equation that can be used to calculate the frequencies of the alleles in
a particular gene in a population. This principle predicts that the proportion of dominant and
recessive alleles of any gene in a population remains the same from one generation to the next,
provided that 5 conditions are met:
No mutations arise
The population is isolated, i.e. there is no flow of alleles to or from the population
There is no selection (all alleles are equally likely to be passed on to the next generation)
The population is large
Mating within the population is random
Although these 5 conditions are probably never entirely met in a natural population, the Hardy-
Weinberg principle is still useful when studying gene frequencies.
To help us understand this principle let us consider a gene that has two alleles a dominant allele (A)
and a recessive allele (a).
Let the frequency of allele A = p
And the frequency of allele a = q
The first equation we can write is:
