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POPULATION GENETICS

POPULATION GENETICS. Dr. Attya Bhatti General Genetics. Population genetics. The branch of genetics that studies the genetic makeup of groups of individuals and how a group’s genetic composition changes with time.

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POPULATION GENETICS

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  1. POPULATION GENETICS Dr. AttyaBhatti General Genetics

  2. Population genetics • The branch of genetics that studies the genetic makeup of groups of individuals and how a group’s genetic composition changes with time. • Population geneticists usually focus their attention on a Mendelian population, which is a group of interbreeding, sexually reproducing individuals that have a common set of genes, the gene pool. • A population evolves through changes in its gene pool; so population genetics is therefore also the study of evolution.

  3. Population genetics • Study the variation in alleles within and between groups and the evolutionary forces responsible for shaping the patterns of genetic variation found in nature

  4. Population Genetics The genetical study of the process of evolution • Change of allele frequencies • Genotype frequencies • Phenotype frequencies

  5. Terminology used in population Genetics • A population is a community of sexually inbreeding individuals. Mating b/w individuals can be random or assortative. • Random mating is the condition in which an individual in a population has equal chance of mating with other individual in that population. • Assortative mating is non random.

  6. Terminology used in population Genetics • The Gene pool of a population is the total of all the genes in the reproductive gametes of the population, Each gene in the gene pool have different alleles. • The allele frequency is the frequency of a specific allele of a gene in a population. • The genotypic frequency is the frequency of individuals with specific genotype in the populations.

  7. Population genetics • The phenotypic frequency is the frequency of individuals with specific phenotype in the populations.

  8. Factors causing genotype frequency changes • Selection • Mutation • Random Drift • Migration • Recombination • Non-random Mating

  9. The primary goal of population genetics is to understand the processes that shape a population’s gene pool. • First, must ask • what effects reproduction and Mendelianprinciples have on the genotypic and allelic frequencies: • How do the segregation of alleles in gamete formation and the combining of alleles in fertilization influence the gene pool?

  10. Assumptions • For an autosomal locus with two alleles, the Hardy- Weinberg law can be stated as follows: • Assumptions—If a population is large, randomly mating, and not affected by mutation, migration, or natural selection, then: • Prediction 1—the allelic frequencies of a population do not change; and • Prediction 2—the genotypic frequencies stabilize (will not change) after one generation in the proportions p2 (the frequency of AA), 2pq (the frequency of Aa), and q2 (the frequency of aa), where p equals the frequency of allele A and q equals the frequency of allele a.

  11. Hardy-Weinberg EquilibriumGodfrey H. Hardy and Wilhelm Weinberg in 1908. States that “ Under certain conditions , if the population is large and randomly mating, the genotypic frequencies of the population will remain stable from generation to generation.” Hardy-Weinberg conditions: • No mutations • No selection against one of the genotypes. • No migration or immigration.

  12. No consanguineous mating • Equations:Allele freq. at locus can be expressed as • p + q = 1 • Genotypic frequencies are expressed as • p2 + 2pq + q2 = 1 • p2-= freq. of indivi. with genotype AA • q2= freq. of indivi. with genotype aa • 2pq = freq. of indivi. with genotype Aa

  13. Genotype Frequency The proportion of individuals in a group with a particular genotype. (Genotype can refer to one locus, two loci, or the whole genome) 40 AA, 47 Aa, 13 aa = 100 Total individuals pAA = 40/100 = 0.4 pAa = 47/100 = 0.47 paa = 13/100 = 0.13

  14. Calculation of Genotypic Frequencies • A frequency is simply a proportion or a percentage, usually expressed as a decimal fraction. • For example, if 20% of the alleles at a particular locus in a population are A, we would say that the frequency of the A allele in the population is 20. • For large populations, where it is not practical to determine the genes of all individuals, a sample of individuals from the population is usually taken and the genotypic and allelic frequencies are calculated for this sample 22 for a discussion of samples). • The genotypic and allelic frequencies of the sample are then used to represent the gene pool of the population.

  15. Calculation of Genotypic Frequencies • To calculate a genotypic frequency, we simply add up the number of individuals possessing the genotype and divide by the total number of individuals in the sample (N). • For a locus with three genotypes AA, Aa, and aa, the frequency (f ) of each genotype is: The sum of all the genotypic frequencies always equals 1.

  16. Hardy-Weinberg Equilibrium How to predict genotype frequencies from allele frequencies?

  17. Assumptions (1) Organism is diploid (2) Reproduction is sexual (3) Generations are non-overlapping (4) Mating occurs at random (5) Population size is very large (6) Migration is zero (7) Mutation is zero (8) Natural selection does not affect the gene in question 1-locus, 2-alleles Assume that all of the H-W conditions are met. pAA + pAa + paa = 1 Frequency of A allele?

  18. These are the Hardy-Weinberg frequencies.

  19. Review of Hardy-Weinberg Allele Frequency The proportion of all alleles in all individuals in the group in question which are of a particular type. (often referred to as "gene frequency") e.g. 40 individuals which are AA 47 individuals which are Aa 13 individuals which are aa Genotype

  20. Allele frequency of A = 127/200 = 0.635 pA=0.635 pa = 73/200 = 0.365 = 1- pA

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