Presentation on theme: "Genetic Drift Random change in allele frequency –Just by chance or chance events (migrations, natural disasters, etc) Most effect on smaller populations."— Presentation transcript:
Genetic Drift Random change in allele frequency –Just by chance or chance events (migrations, natural disasters, etc) Most effect on smaller populations (≤100) Founder effect- allele frequencies change as a result of migration of a small subgroup of a pop. –Chance events can reduce pop. size or create new, smaller pop.s from an original larger pop.
Genetic equilibrium Hardy-Weinberg principle- allele frequencies in a population will remain constant unless 1 or factors causes those alleles to change Genetic Equilibrium- allele frequencies in the pop. 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 p 2 + 2pq + q 2 = 1 p + q = 1 –p=frequency of dominant allele, q= frequency of recessive allele –p 2 = frequency of individuals who are homozygous dominant, q 2 = 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? –q 2 = 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 q 2 = 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? p 2 + 2pq + q 2 = 1 –So 2pq= 2 x 0.02 x 0.98= 0.04 –Frequency of homozygous dominant= 0.98 2 = 0.96 –Frequency of homozygous recessive= 0.02 2 = 0.0004 –0.96 + 0.04 + 0.0004= 1