When the allele frequencies in a population stop changing, the population is at genetic equilibrium. However, how can a population be examined for equilibrium? What is classified as unchanging? In order to solve this problem, Godfrey Hardy and Wilhelm Weinberg developed the Hardy-Weinberg principle. The formula used is p^2 + 2pq + q^2 = 1.
p^2 signifies the frequency of AA, or the dominant alleles. 2pq signifies the frequency of Aa, or the heterzygote. q^2 signifies the frequency of aa, or the recessive alleles. 1 is the sum of all of the individuals in the specific population. Through this equation, a population can be examined as being at genetic equilibrium or not. Genetic equilibrium signifies a stop in the evolutionary process, or stasis. However, the principle only holds true when the species reproduces through sexual reproduction.
Genetic equilibrium is always true if five specific conditions are met. First off the population size must be considerably large. There mustn?t be gene flow (migration) in the population. No mutations can be occurring in the population. Environmental factors mustn?t be causing natural selection to occur. Lastly, and most importantly, random mating must occur. Random mating is where each organism in the population has an equal chance of mating with another.
Let us use the Hardy-Weinberg theory in a practical situation. Imagine that we have 64 homozygous recessive individuals in our population of 10000. q^2 is therefor .0064. q is therefor .08. p=1-q=1-.08=.92. According to this, AA is p^2, which is .8464. Heterzygotic frequency is 2pq, which is .1472.
The result of this theorem, is that in the large populations (which is a prerequisite for this formula), inheritance can?t by itself change the frequencies of the different alleles. The theorem also shows that dominant alleles aren?t always more prevalent than recessive alleles. It is important to remember that the Hardy-Weinberg theory only works in the ideal world. Scientists don?t expect to go take genetic samples of a population and have genetic equilibrium turn out. However, it is useful to have a standard to compare real populations to. The Hardy-Weinberg theory is an extension of Mendelian genetics for populations.
In Mendelian genetics the dominant allele appear to prevail over the recessive allele. However, the theory shows that although they replace the recessive alleles, they aren?t inherently more frequent. This is demonstrated in the detailed formula.
To truly understand the rule you need to understand punnit squares;
An allele is one of the genes which appears on a given locus. There can be two more alleles at a loci. Heterozygous refers to one dominant allele and one recessive allele. Homozygous recessive refers to two recessive alleles. Homozygous dominant refers to two dominant alleles. A population refers to a group of the same species, in the same location, at the same time.
It is interesting to note that the Hardy-Weinberg could not theoretically be applied to humans. By nature humans pick their mates based on phenotype, which is by definition non-random mating. This defies one of the five criteria in order for a population to be at genetic equilibrium
The link below will give a simple tutorial on punnit squeares and Mendelson.
http://www.docstoc.com/docs/679713/Mendelian_Genetics .
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