Population genetics

Population genetics

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  HARDY WEINBERG LAW– FACTORS AFFECTING EQUILIBRIUM FREQUENCIES IN RANDOM MATING POPULATIONS PREPARED BY.... KIRAN DASANAL [UG13AGR1879] The Hardy-Weinberg Equilibrium Allele Frequencies in a Population G.H. Hardy English Mathematician Dr. Wilhelm Weinberg German Physician
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  INTRODUCTION • Cross-pollinated cropsare highly heterozygous due to the free intermating among their plants. They are often referred to as random mating populations. • Mendelian population • A population, in this case, consists of all such individuals that share the same gene pool
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  Hardy-Weinberg law • TheHardy-Weinberg law is the fundamental law of population genetics and provides the basis for studying Mendelian populations. • This law was independently developed by Hardy (1908) in England and Weinberg (1909) in Germany.
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  Hardy-Weinberg law  The Hardy-Weinberglaw states that the gene and genotype frequencies in a Mendelian population remain constant generation after generation if there is no selection, mutation, migration or random drift.
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  Example of Hardy-Weinberglaw • Let us consider a single gene with two alleles, A and a, in a random mating population. • There would be three genotypes, AA, Aa and aa, for this gene in the population. • Suppose the population has N individuals of which D individuals are AA, H individuals are Aa and R individuals are aa. • so that D +H + R = N.
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  Cont………… • p =(2D + H) / 2N or = (D + ½ H) / N and • q = (2R + H) / 2N or = (R + ½ H) / N • Therefore, p + q = 1 • Such a population would be at equilibrium since the genotypic frequencies would be stable, that is, would not change, from one generation to the next. This equilibrium is known as Hardy- Weinberg equilibrium.
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  Factors affecting equilibrium frequencies
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  Migration • Migration isthe movement of individuals into a population from a different population. • Migration may introduce new alleles into the population or may change the frequencies of existing alleles. • In plant breeding programmes, migration is represented by intervarietal crosses, polycrosses, etc., wherein the breeder brings together into a single population two or more separate populations.
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  Mutation • Mutation isa sudden and heritable change in an organism and is generally due to structural change in a gene. • It is the ultimate source of all the variation present in biological materials. • Mutation may produce a new allele not present in the population or may change the frequencies of existing alleles.
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  Random drift • Randomdrift or genetic drift is a random change in gene frequency due to sampling error. • Random drift occurs in small populations because sampling error is greater in a smaller population than in a larger one. • The breeder cannot do anything to prevent this genetic drift, except to use very large populations.
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  Inbreeding • Mating betweenindividuals sharing a common parent in their ancestry is known as inbreeding. • Inbreeding reduces the proportion of heterozygotes or heterozygosity and increases the frequency of homozygotes or Homozygosity.
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  Selection • Differential reproductionrates of various genetypes is known as selection. • The breeder is able to improve the various characteristics by selecting for the desirable types. In a random mating population. • selection is expected to change gene frequencies rather than to eliminate one or the other allele.
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