Lecture: Natural Selection and Genetic Drift and Genetic Equilibrium

Genetic Drift Random change in allele frequency Just by chance or chance events Most effect on smaller populations (100 individuals or less) Founder effect- allele frequencies change as a result of the migration of a small subgroup of a population Chance events can like migrations or natural disasters can reduce population size or create new, smaller populations from an original larger population

Genetic equilibrium Hardy-Weinberg principle- allele frequencies in a population will remain constant unless one or more factors causes those alleles to change Genetic Equilibrium- allele frequencies in the population remain the same, nothing is changing Conditions under which evolution will not occur 5 Conditions Random Mating No Immigration or Emigration (Migration) No Mutations No Natural Selection Large Populations

Hardy-Weinberg Equation p2 + 2pq + q2 = 1 p + q = 1 p=frequency of dominant allele, q= frequency of recessive allele p2 = frequency of individuals who are homozygous dominant, q2 = frequency of individuals who are homozygous recessive, 2pq= frequency of individuals who are heterozygous If the frequency of people in the United States that have a certain recessive trait is 2% (or 0.02), what is the frequency of the allele for that trait? q2 = frequency of individuals who are homozygous recessive = 0.02 q= frequency of recessive allele= √0.02 = 0.14 or 14%

Sample Problem A certain recessive disease has a frequency of 1 in 2500 people. What is the frequency of each allele and the frequency of carriers? (Assuming the population is at Hardy-Weinberg equilibrium) What is the genotype of a recessive disease? So q2= 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 only other allele, how often is p present? So q= 0.02, p= 0.98, those are the frequencies of the alleles! What is the genotype of a carrier of a recessive disease? p2 + 2pq + q2 = 1 So 2pq= 2 x 0.02 x 0.98= 0.04 Frequency of homozygous dominant= 0.982= 0.96 Frequency of homozygous recessive= 0.022= 0.0004 0.96 + 0.04 + 0.0004= 1