Biology 311 Human Genetics Lecture 6 Hardy Weinberg Equilibrium Fall 2006 Reading Chap 4 pp 117 119 Lecture Outline 1 Allele frequencies in populations 2 Hardy Weinberg equilibrium 3 Disease prediction Genetic counseling Lecture 1 Allele frequencies Population Group of interbreeding individuals Gene Pool All the alleles for a particular gene in a population Gene frequency allele frequency more precise phrase proportion of all alleles for gene A of one type such as A1 A1A1 A2A2 A1A2 A1A2 A2A2 A Pick an allele at random from the gene pool Chance p that allele is A1 q that allele is A2 Pick a second allele at random Chance p that allele is A1 q that allele is A2 Chance that both alleles were A1 p x p p2 Chance that both alleles were A2 q x q q2 Chance that first allele was A1 second was A2 pq Chance that first allele was A2 second was A1 qp Overall chance of one A1 and one A2 allele is 2pq 1 2 Hardy Weinberg equilibrium Gene frequencies and genotypes of a population can be predicted using probabilities Autosomal locus Genotype A1A1 A1A2 A2A2 Frequency p2 2pq q2 X linked locus Females A1A1 A1A2 A2A2 Frequency p2 2pq q2 Males A1 A2 Frequency p q Assumptions Random mating o Inbreeding non random mating decreases heterozygosity and increases homozygosity 3 Disease prediction Genetic counseling See Box 4 6 a Calculate carrier frequency An autosomal recessive condition affects 1 newborn in 10 000 What is the expected frequency of carriers phenotypes frequencies unaffected AA p2 unaffected Aa 2pq Affected individuals q2 1 10 000 q2 q 1 100 1 in 100 alleles at the A locus are a 99 in 100 are A The carrier frequency 2pq 2 99 100 1 100 0 02 2 affected aa q2 If a parent of an affected child remarries what is the risk of producing an affected child in the new marriage Apply the product rule Probability that parent that had affected child is a carrier 1 x Risk of new spouse being carrier probability of affected child 0 02 x 005 X linked recessive red green color blindness affects 1 in 12 British males What proportion of females will be carriers What proportion will be affected Males A normal a colorblind Females AA or Aa normal aa colorblind Females AA Aa aa Frequency p2 2pq q2 Males A a From males q 1 12 therefore p 11 12 Female carriers 2pq 2 x 11 12 x 1 12 22 144 Females affected q2 1 12 x 1 12 1 144 Other forces that affect allele frequencies a mutation A a mutation rate gene generation back mutation rate gene generation b Natural selection Can act on dominant or recessive trait Can have heterozygote superiority Aa AA or aa 3 Frequency p q 1 12 Can have heterozygote inferiority AA Aa and aa Aa s coefficient of selection s 0 no selection s 1 lethal gene Estimate mutation rate Assume mutation and selection are in equilibrium for autosomal recessive trait sq2 mutation rate selection coefficient x allele frequency of recessive for rare autosomal dominant trait sp mutation rate selection coefficient x allele frequency of dominant allele for X linked recessive trait u sq 3 mutation rate selection coefficient x allele frequency of recessive trait only 1 3 of the sex chromosomes those in males are affected For most genetic diseases the mutation rates are actually rather low Natural selection would actually drive one allele to extinction other to fixation Most likely the trait remains in the population due to natural selection acting on both dominant and recessive phenotypes heterozygote superiority 4