The Hardy-Weiberg Principle

Members of a population share a gene pool A species is defined as a group of similar organisms that can reproduce to give fertile offspring. A population is a group of organisms of the same species living in a particular area at a particular time - they have the potential to interbreed. Species can exist as one or more populations, e.g. there are populations of the American Black Bear in parts of America and in parts of Canada. The gene pool is the complete range of alleles present in a population. How often an allele occurs in a population is called the allele frequency. It's usually given as a percentage or a number

The Hardy-Weinberg Principle predicts allele frequencies won't change The Hardy-Weinberg principle is a mathematical model. It predicts that the frequencies of alleles in a population won't change from one generation to the next. But this prediction is only true under certain conditions - it has to be a large population where there's no immigration, emigration, mutations or natural selection. There also needs to be random mating - all possible genotype can breed with all others The Hardy-Weinberg equations can be used to calculate the frequency of particular alleles, genotypes and phenotypes. The equations can also be used to test whether or not the Hardy-Weinberg principle applies to particular alleles in particular populations, i.e. to test whether selection or any other factors are influencing allele frequencies - if frequencies do change between generations in a large population then there is an influence of some kind.

The Hardy-Weinberg equations can be used to predict allele frequencyWhen a gene has two alleles, you can figure out the frequency of one of the alleles of the genes if you know the frequency of the other allele, using this equation:p + q = 1Where p = the frequency of one allele, usually the dominant one q = the frequency of the other allele, usually the recessive oneThe total frequency of all possible alleles for a characteristic in a certain population is 1.0. So the frequencies of the individual alleles must add up to 1.0.

The Hardy-Weinberg equations can be used to predict genotype and phenotype frequencyYou can figure out the frequency of one genotype if you know the frequencies of the others using this equation:p^2 + 2pq + q^2 = 1Where: p^2 = the frequency of the homozygous dominant genotype 2pq = the frequency of the heterozygous genotype q^2 = the frequency of the homozygous recessive genotype EXAMPLEIf there are two alleles for flower colour, there are three possible genotypes - RR, Rr and rr. If the frequency of genotype RR (p^2) is 0.34 and the frequency of genotype Rr (2pq) is 0.27, the frequency of genotype rr (q^2) must be: 1 - 0.34 - 0.27 = 0.39

The Hardy-Weinberg equations can be used to predict the percentage of a population that has a certain genotypeEXAMPLEThe frequency of cystic fibrosis (genotype ff) in the UK is currently 1 birth in every 2500. From this information you can estimate the percentage of people in the UK that are cystic fibrosis carriers (Ff). To do this you need to find the frequency of heterozygous genotype Ff, i.e. 2pq, using both equations. p + q = 1 and p^2 = 2pq + q^2 = 1First calculate q:Frequency of cystic fibrosis (ff) is 1 in 2500ff = q^2 = 1 / 2500 = 0.0004so q = square root of 0.0004 = 0.02Next calculate p:p + q = 1p = 1 - 0.02 = 0.98Then calculate 2pq:2pq = 2 x 0.98 x 0.02 = 0.039so the percentage is 3.9%