Summary OCR A-Level Biology 6.2.12 Hardy Weinberg Principle

Hardy-Weinberg Principle
Factors Affecting Allele Frequency
 Mutation Rate
 Migration and Isolation
 Population Size
 Natural Selection
 Environmental Change
 Genetic Drift
 Gene Flow
 Non-random Mating

Hardy-Weinberg Principle
 Assumptions:
o Population is large enough to make sampling error negligible
o Mating in the population is random
o No selective pressures for or against any genotype so no selection
o No mutation, migration or genetic drift
 Frequency of dominant allele = p
 Frequency of recessive allele = q
 Only two alleles in population so p + q = 100% or 1.0
 Possible allele combinations:
o Homozygous Dominant: pp = p²
o Homozygous Recessive: qq = q²
o Heterozygous x2: pq x2 = 2pq
 p² + 2pq + q² = 100% or 1.0

Examples:
 25% population recessive trait, 75%  36% population recessive trait, 64%
dominant trait dominant trait
 q² = 0.25 so q=0.5  q² = 0.36 so q=0.6
 1.0 – 0.5 = p = 0.5  1.0 – 0.6 = p = 0.4
 2pq = 2x0.5x0.5 = 0.5  2pq = 2x0.4x0.6 = 0.48
 So 50% of the population are  So 48% of the population are
heterozygous heterozygous