In-depth Notes on Population Genetics

Introduction to Population Genetics
  • Definition: Population genetics studies the distribution and change in frequency of alleles within populations.
    • Focuses on groups of individuals of the same species that can interbreed.
Learning Outcomes
  • Ability to calculate genotypes and allele frequencies.
Key Concepts in Population Genetics
  • Populations: A group of individuals that can interbreed.
  • Genetic Diversity: Refers to the variety of alleles and genotypes present in a population.
  • Population Structure: Patterns of genetic variation and organization of genetic variation in populations.
  • Changes in Genetic Structure: How genetic variations change over time within a population.
Genotype and Allele Frequencies

  • Genotype: A set of alleles at a locus in an individual.

    • Homozygous: Two identical alleles (e.g., RR or rr).
    • Heterozygous: Two different alleles (e.g., Rr).

  • Calculating Allele Frequencies:

    • Formula: [ \text{Allele frequency} = \frac{\text{Number of copies of the allele}}{\text{Total number of alleles}} ]
    • Example with butterflies:
    • Total butterflies: 1000
    • Genotypes: 200 rr, 500 Rr, 300 RR
      • Genotype frequency calculations: 200/1000 (0.2 for rr), 500/1000 (0.5 for Rr), 300/1000 (0.3 for RR).

  • Calculation Example:

    • 200 rr = 0.2
    • 500 Rr = 0.5
    • 300 RR = 0.3
    • Ensure genotype frequencies sum to 1: (0.2 + 0.5 + 0.3 = 1)
Detailed Calculation of Allele Frequencies
  • Alleles Breakdown:
    • For 2000 total alleles (1000 individuals each contributing 2 alleles):
    • 900 copies of r, frequency = ( \frac{900}{2000} = 0.45 )
    • 1100 copies of R, frequency = ( \frac{1100}{2000} = 0.55 )
    • Total allele frequencies should also sum to 1.
Problem Example
  • Calculate frequencies of genotypes based on the proboscis color of insects:
    • Given data:
    • 66 homozygous blue (AA)
    • 31 heterozygous blue (Aa)
    • 4 homozygous green (aa)
    • Genotype frequencies = ( \frac{66}{101} = 0.66 ) for AA, ( \frac{31}{101} = 0.30 ) for Aa, and ( \frac{4}{101} = 0.04 )
    • Total frequencies = 1.00
    • Alleles breakdown:
    • Total A = (2 x 66) + 31 = 163
    • Total a = 31 + (2 x 4) = 39
    • Calculating allele frequencies:
    • A frequency = ( \frac{163}{206} = 0.807 )
    • a frequency = ( \frac{39}{206} = 0.193 )
Summary of Key Definitions
  • Locus: Specific location on a chromosome or genome for a particular gene or DNA sequence.
  • Loci: Plural of locus, referring to multiple locations across chromosomes.
  • DNA Variant: Any change from the typical DNA sequence, can include:
    • Single nucleotide variant
    • Deletion variant
    • Insertion variant
    • Repeat expansion variant
  • Allele: A version or variant of a gene.
  • Genotype: The genetic constitution of an individual at a particular locus, inherited from both parents.