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Population Genetics

Population Genetics. Measuring Evolutionary Change Over Time. I. Evolution as Genetic Change. Trait : An inherited physical characteristic Gene : An inherited segment of DNA on a chromosome that codes for a trait Allele : One of a number of different forms of a gene for a particular trait

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Population Genetics

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  1. Population Genetics Measuring Evolutionary Change Over Time

  2. I. Evolution as Genetic Change Trait: An inherited physical characteristic Gene: An inherited segment of DNA on a chromosome that codes for a trait Allele: One of a number of different forms of a gene for a particular trait Homozygous: Having 2 identical alleles for a trait (AA or aa) Heterozygous: Having 2 different alleles for a trait (Aa) Genotype: The genetic make-up of an organism Phenotype: The physical traits of an organism Dominant: An allele that is always expressed in the phenotype (A) Recessive: An allele that is only expressed in the phenotype when homozygous (a)

  3. Chromosome: A structure in the nucleus of a cell that contains genetic information (DNA organized around proteins), that is passed from one generation to the next DNA: A double stranded macromolecule that stores genetic information that codes for proteins

  4. Gamete: Specialized reproductive cells that have half the number of chromosomes Meiosis: A process that produces haploid gametes from diploid cells Haploid: A cell containing a single set of chromosomes (1n) Diploid: A cell that contains two sets of chromosomes (2n) Review of Simple Punnett Squares Possible alleles from male gametes rr Rr Genotypic Ratios in Offspring Generation: 50% - Rr 50% - rr Possible alleles from female gametes Rr rr

  5. II. Allelic Frequencies and Genetic Equilibrium A.Variation: (raw material for evolution) • The variation of traits in a population is often controlled by genes • Changes in the genes causes changes in the corresponding traits • Gene Pool: The collection of genes (and alleles) for all the traits in a population • C. Allelic Frequency: The percentage (%) of a specific allele of a gene in the gene pool of a population • Genetic Equilibrium: A population in which allele frequencies do not change from generation to generation

  6. III. The Hardy-Weinberg Principle A. In 1908, a British mathematician, Godfrey Hardy, and a German physician, Wilhelm Weinberg, independently outlined the conditions necessary for genetic equilibrium • The Hardy-Weinberg Principle states that a population will remain in genetic equilibrium if all of the following conditions are met: (no evolution!) • No mutations occur • Individuals neither enter nor leave the population through migration • Population is large • Individuals mate randomly • Natural Selection does not occur • If even one of these conditions does not hold true, allele frequencies of the population may change and evolution will occur.

  7. IV. Measuring Evolution using Allelic Frequencies Population of Organisms Unique Gene Pool • Hardy-Weinberg conditions are met: • no mutation • no migration • large population size • random mating • no natural selection One or more Hardy-Weinberg conditions are NOT met: Allelic Frequencies change Allelic Frequencies do NOT change Genetic Equilibrium Genetic Disequilibrium Evolution will NOT occur Evolution will occur

  8. A. The Hardy-Weinberg Mathematical Formulas • p + q = 1 (used to measure allele frequencies) • If ‘A’ and ‘a’ are alleles for a particular gene and each individual has two alleles, then p represents the frequency of the A allele • If ‘A’ and ‘a’ are alleles for a particular gene and each individual has two alleles, then q represents the frequency of the a allele • p2 + 2pq + q2 = 1 (used to measure genotype frequencies) • p2 represents the frequency of the homozygous dominant condition (AA) • 2pq represents the frequency of the heterozygous condition (Aa) • q2represents the frequency of the homozygous recessive condition (aa)

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