Genetic DriftGenetic DriftRandom change in allele frequencyRandom change in allele frequency– Just by chance or chance events (migrations, Just by chance or chance events (migrations,

natural disasters, etc)natural disasters, etc)

Most effect on smaller populations (≤100)Most effect on smaller populations (≤100)

Founder effect- allele frequencies change Founder effect- allele frequencies change as a result of migration of a small as a result of migration of a small subgroup of a pop.subgroup of a pop.– Chance events can reduce pop. size or create Chance events can reduce pop. size or create

new, smaller pop.s from an original larger new, smaller pop.s from an original larger pop.pop.

Genetic equilibriumGenetic equilibriumHardy-Weinberg principle- allele frequencies Hardy-Weinberg principle- allele frequencies in a population will remain constant unless 1 in a population will remain constant unless 1 or or factors causes those alleles to change factors causes those alleles to changeGenetic Equilibrium- allele frequencies in the Genetic Equilibrium- allele frequencies in the pop. remain the same, nothing is changingpop. remain the same, nothing is changing– Conditions under which evolution will Conditions under which evolution will notnot occur occur

5 Conditions5 Conditions– Random MatingRandom Mating– No Immigration or Emigration (Migration)No Immigration or Emigration (Migration)– No MutationsNo Mutations– No Natural SelectionNo Natural Selection– Large PopulationsLarge Populations

Hardy-Weinberg EquationHardy-Weinberg Equationpp22 + 2pq + q + 2pq + q22 = 1 = 1p + q = 1p + q = 1– p=frequency of dominant allele, q= frequency of p=frequency of dominant allele, q= frequency of

recessive allelerecessive allele– pp22 = frequency of individuals who are homozygous = frequency of individuals who are homozygous

dominant, qdominant, q22 = frequency of individuals who are = frequency of individuals who are homozygous recessive, 2pq= frequency of homozygous recessive, 2pq= frequency of individuals who are heterozygousindividuals who are heterozygous

If the frequency of people in the United States If the frequency of people in the United States that have a certain recessive trait is 2% (or that have a certain recessive trait is 2% (or 0.02), what is the frequency of the allele for 0.02), what is the frequency of the allele for that trait?that trait?– qq22 = frequency of individuals who are homozygous = frequency of individuals who are homozygous

recessive = 0.02recessive = 0.02– q= frequency of recessive allele= √0.02 = 0.14 or q= frequency of recessive allele= √0.02 = 0.14 or

14%14%

Sample ProblemSample ProblemA certain recessive disease has a frequency of 1 A certain recessive disease has a frequency of 1 in 2500 people. What is the frequency of each in 2500 people. What is the frequency of each allele and the frequency of carriers? (Assuming allele and the frequency of carriers? (Assuming the population is at Hardy-Weinberg the population is at Hardy-Weinberg equilibrium)equilibrium)– What is the genotype of a recessive disease?What is the genotype of a recessive disease?– So qSo q22= 1/2500= 0.0004, and q= √0.0004 = 0.02= 1/2500= 0.0004, and q= √0.0004 = 0.02– If q is present 0.02 or 2% of the time, and p is the If q is present 0.02 or 2% of the time, and p is the

only other allele, how often is p present?only other allele, how often is p present?– So q= 0.02, p= 0.98, those are the frequencies of So q= 0.02, p= 0.98, those are the frequencies of

the alleles!the alleles!– What is the genotype of a carrier of a recessive What is the genotype of a carrier of a recessive

disease?disease?pp22 + 2pq + q + 2pq + q22 = 1 = 1– So 2pq= 2 x 0.02 x 0.98= 0.04So 2pq= 2 x 0.02 x 0.98= 0.04– Frequency of homozygous dominant= 0.98Frequency of homozygous dominant= 0.9822= 0.96= 0.96– Frequency of homozygous recessive= 0.02Frequency of homozygous recessive= 0.0222= 0.0004= 0.0004– 0.96 + 0.04 + 0.0004= 10.96 + 0.04 + 0.0004= 1