Hardy-Weinberg.

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Presentation transcript:

Hardy-Weinberg

Hardy-Weinberg Principle To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place So scientists asked: Are there any conditions under which evolution will NOT occur? Is there any way to recognize when that is the case? The answers to these questions are provided by the Hardy-Weinberg principle The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change

Hardy-Weinberg Principle If allele frequencies do not change, then genetic equilibrium is reached and the population will not evolve Five conditions are required to maintain genetic equilibrium: Random mating Large population No immigration/emigration No mutations No selective pressures

Hardy-Weinberg – Allele Frequency The Hardy-Weinberg equations apply to traits that are controlled by two alleles, one dominant (A) and one recessive (a) The equations use “p” to represent the dominant allele and “q” to represent the recessive allele The sum of the frequencies for these alleles must always equal the entire population (100%) In mathematical form, this can be written as the equation: p + q = 1

Hardy-Weinberg – Allele Frequency

Hardy-Weinberg – Genotype Frequency There is also an equation for the genotype frequencies: p2 + 2pq + q2 = 1 p2 = the frequency of AA 2pq = the frequency of Aa q2 = the frequency of aa 1 = the sum of frequencies of all genotypes (100%)

Hardy-Weinberg – Genotype Frequency In a particular generation, we find that p = 0.8 and q = 0.2 How can you figure out the relative frequencies of AA, Aa, and aa individuals? First, write the following equation: p2 + 2pq + q2 = 1 (this represents A2 + 2Aa + a2 = 1) Fill in the values: (0.8)2 + 2(0.8)(0.2) + (0.2)2 = 1

Hardy-Weinberg – Genotype Frequency Calculate: 0.64 + 0.32 + 0.04 = 1 Convert the fractions into percentages: 0.64 x 100 = 64%  Frequency of AA individuals is 64% 0.32 x 100 = 32%  Frequency of Aa individuals is 32% 0.04 x 100 = 4%  Frequency of aa individuals is 4%

Hardy-Weinberg – Genotype Frequency As long as the Hardy-Weinberg conditions hold, neither the allele frequencies nor genotypes frequencies should change from generation to generation a.ka. No evolution is occurring So how do we know if evolution is occurring? If the allele or genotype frequencies change between generations