# Population Genetics

Topics: Allele frequency, Evolution, Genetic drift Pages: 5 (2030 words) Published: April 9, 2013
In 1831, Charles Darwin, proposed a theory of evolution occurring by the process of natural selection. This has come to be known as the Theory of Natural Selection. Darwin worked on his theory for 20 years and after learning that Alfred Russel Wallace, another naturalist, had developed similar ideas, the two made a joint announcement of their discovery in 1858. Darwin published 'On the Origin of Species by Means of Natural Selection' in 1859, 28 years after he proposed his theory of natural selection. [1] Darwin’s theory of natural selection states that evolutionary change comes through the production of variation in each generation and differential survival of individuals with different combinations of these variable characters. [2] Evolution is not only the development of new species from older ones, but is the minor changes within a species from generation to generation over long periods of time that can result in the gradual transition to new species. In the early 20th century Godfrey Hardy, an English mathematician, and Wilhelm Weinberg, a German physician developed a theory on evaluation. Their theory defines evolution as being the sum total of the genetically inherited changes in the individuals who are the members of a population's gene pool. They observed that evolution is simply a change in frequencies of alleles in the gene pool of a population. Hardy and Weinberg, and the population geneticists who followed observed that evolution will not occur in a population if seven conditions are met: 1. Mutation is not occurring

2. Natural selection is not occurring
3. The population is infinitely large
4. All members of the population breed
5. All mating is totally random
6. Everyone produces the same number of offspring
7. There is no migration in or out of the population
With their observations they came up with an equation to illustrate their theory, which is known as the Hardy-Weinberg equilibrium equation. It is a simple equation that is used to track genetic changes from one generation to another by finding the probable genotype frequencies in a population. [3] The Hardy-Weinberg equilibrium equation is as follows:

(p² + 2pq + q² = 1)
- p is defined as the frequency of the dominant allele
-q as the frequency of the recessive allele for a trait controlled by a pair of alleles (A and a). P accounts for all of the alleles in individuals that are homozygous dominant (AA) and half of the alleles in people that is heterozygous (Aa) for this trait in a population. (p = AA + ½Aa) Q accounts for all of the alleles in individuals that is homozygous recessive (aa) and the other half of the alleles in people that is heterozygous (Aa). (q = aa + ½Aa) If there is only two alleles as with this case, the frequency of one plus the frequency of the other must equal 100% - p + q = 1 Since this is logically true, then the following must also be correct - p = 1 - q From this knowledge Hardy and Weinberg realized that the chances of all possible combinations of alleles occurring randomly is - (p + q)² = 1 or p² + 2pq + q² = 1 The comparison between genotype frequencies from the next generation and those of the current generation in a population, it can be learnt whether or not evolution has occurred and the direction and rate of the selected trait. However, the Hardy-Weinberg equation cannot determine which of the various possible causes of evolution were responsible for the changes in gene pool frequencies. [3] Huntington's Chorea (Huntington's Disease) in the Afrikaner Population of South Africa On the 2nd of August 1980, M. R. HAYDEN, H. C. HOPKINS, M. MACRAE, P. H. BEIGHTON, published an article in the South African Medical Journal outlining their research in the high prevalence of Huntington’s Chorea among the Afrikaner population in South Africa. [4] According to the article their research found that the origin of the gene for the disorder in the Afrikaner population has been traced back over 14 generations...