Hardy Weinberg

Populations and Gene Pools

Genetic variation is required for a population to evolve, but does not guarantee that it will. One or more factors that cause evolution must be at work for a population to evolve.

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring.

• A gene pool refers to all the alleles, for every gene, present in individuals of a population.

  • A gene pool represents all the possible 

alleles that could be inherited by the 

next generation. 

Allele Frequencies

• The frequency of an allele in a population can be calculated. 

  • For diploid organisms, the total number of alleles at a locus (gene location on a chromosome) is the total number of individuals times 2 

  • The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles.

• By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies.

  • p  usually represents the frequency of dominant alleles, while q represents the frequency of recessive alleles. 

• The frequency of all alleles in a population will add up to 1

  • p + q = 1

The Hardy-Weinberg Equation

• If p and q represent the frequencies of the only two possible alleles in a population at a particular locus, then p² + 2pq +q² = 1

  • p = frequency of dominant allele (usually)

  • q = frequency of recessive allele

  • p² = frequency of homozygous dominant genotype

  • q² = frequency of homozygous recessive genotype

  • 2pq = frequency of heterozygous genotype

• This is called the Hardy-Weinberg equation. It describes the genetic makeup expected for a population that is not evolving.

  • If the population is not evolving we say that the population is in Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equation describes a hypothetical population that is not evolving.

  • Allele and genotype frequencies remain constant from generation to generation for a population in Hardy-Weinberg equilibrium.

• If the data observed for the population differ from the expected values, then the population may be evolving.

  • In real populations, allele and genotype frequencies usually change over time.

• There are five conditions needed for a population to be in Hardy-Weinberg equilibrium, and they are rarely met in nature:

1. No mutations 

2. Random mating (no sexual selection)

3. No natural selection

4. Extremely large population size

5. No gene flow (no emigration out of the population or immigration into the population)

If any of the conditions above are not met, the population is NOT in HW Equilibrium and is experiencing evolution.

• EX: The occurrence of PKU (a recessive genetic disorder) is said to be in Hardy-Weinberg Equilibrium for the following reasons:

1. The PKU gene mutation rate is very low

2. Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions.

4. The population is large.

5. Migration has no effect, as many other populations have similar allele frequencies.