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The Hardy-Weinberg Equation

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Presentation on theme: "The Hardy-Weinberg Equation"— Presentation transcript:

1 The Hardy-Weinberg Equation
Population genetics at its driest

2 Useful definitions Ones you should remember:
Allele, genotype, phenotype, frequency

3 Useful definitions Ones that may be new:
Gene pool - all the alleles of all the genes in a population; a pool of genetic resources Population - a group of organisms of the same species that live in a specific location and breed with one another more often than they breed with members of other populations

4 To study population genetics…
…it would be useful if we could calculate the presence of certain alleles and genotypes (and therefore phenotypes) within a population

5 The Hardy-Weinberg Law
Allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences In other words, the H-W law allows us to predict changes in allelic frequency in a non-evolving population

6 The Hardy-Weinberg Law
In order for a population to be in allelic/genotypic equilibrium, the following conditions must be met… Large population A population must be large enough that chance occurrences cannot significantly change allelic frequencies significantly eg. Coin flip No mutation There must be no change in the number of copies of an allele due to mutation No immigration or emigration Individuals must not move in and out of that population

7 The Hardy-Weinberg Law
In order for a population to be in allelic/genotypic equilibrium, the following conditions must be met… Random mating In order for all alleles to have an equal chance of being passed down to the next generation, mating within the population must be random Random reproductive success Just as mating must be random, the survival of offspring to reproductive age, or reproductive success, must also be random

8 The Hardy-Weinberg Law
Obviously this perfect equilibrium cannot exist in real life Some or all of these types of forces all act on living populations at various times and evolution at some level occurs in all living organisms The H-W formulas allow us to detect changes in allelic frequencies from generation to generation, thus allowing us to determine when/how evolution is occurring The Hardy-Weinberg equilibrium equation: p2 + 2pq + q2 = 1

9 The Hardy-Weinberg Law
p2 + 2pq + q2 = 1 Where… p = frequency (%) of the dominant allele q = frequency (%) of the recessive allele 100% of all alleles are either dominant or recessive therefore another useful equation is p + q = 1 Also… p2 = frequency (%) of homozygous dominant genotype 2pq = frequency (%) of the heterozygous genotype q2 = frequency (%) of the homozygous recessive genotype

10 Example H-W problem p2 + 2pq + q2 = 1
Albinism s a rare genetically inherited trait that is only expressed in the phenotype of homozygous recessive individuals (aa).  The most characteristic symptom is a marked deficiency in the skin and hair pigment melanin.  This condition can occur among any human group as well as among other animal species.  The average human frequency of albinism in North America is only about 1 in 20,00

11 Try this… You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: The frequency of the "aa" genotype (36%) The frequency of the "a" allele (60%) The frequency of the "A" allele (40%) The frequencies of the genotypes "AA" and "Aa“ (16%, 48%) The frequencies of the two possible phenotypes if "A" is completely dominant over "a“ (64% dominant, 36% recessive)

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