Bio1B Evolution 3 Last lecture Natural selection principles lines of evidence in the Origin Descent with modification Estimation interpretation of phylogeny Today Phylogenetics molecular data and the molecular clock More history Darwin Mendel the Neodarwinian synthesis Mechanisms of evolution Evolution in populations population genetics Allele genotype and phenotype frequencies Predicting genotype freq s Hardy Castle Weinberg Equilibrium Application Null model for evolution Application Predicting heterozygote frequencies for recessive traits DNA sequence variation Fitting observed patterns of sequence variation at homologous aligned sites to a phylogenetic hypothesis The molecular clock dating divergence events Hemoglobin and divergence of chimpanzee human lineages Fig 16 9 Dating the emergence of HIV Fig 16 10 1 Mendel s principles of inheritance 1865 see Ch 14 Alternative versions of genes alleles account for variation in inherited characters For each character an organism inherits 2 alleles one from each parent If the 2 alleles at a locus differ then the dominant allele determines phenotype The 2 alleles for a heritable character segregate during gamete formation Law of Segregation Each pair of alleles segregates independently of others during gamete formation for unlinked genes Dominance of purple P over white p flower color see also F 8 1 in text P Generation Mendel s garden Brno Co dominance heterozygote is intermediate pink in snapdragons see also Fig 8 10 in text P Generation Purple flowers PP P Appearance Genetic makeup Gametes White flowers pp p Red C RC R White CWCW Gametes CR CW F1 Generation Pink C RC W F1 Generation Appearance Genetic makeup Purple flowers Pp 12 Gametes 12 P p Gametes1 2 CR 1 2 CW F1 sperm P p PP Pp Pp pp F2 Generation 1 P F1 eggs 1 2C R 1 2C W Sperm 1 2 CW R p 3 2C F2 Generation Eggs C RC R C RC W CRCW CWCW 1 Genotype and allele frequencies for a locus with two alleles see also Figs 15 3 15 10 in text Futuyma 2nd Ed 2 Hardy Weinberg Equilibrium general case Gametes for each generation are drawn at random from the gene pool of the previous generation 80 CR p 0 8 20 CW q 0 2 male gametes Expected genotype frequencies qp A2A1 q2 A2A2 Sperm CR 80 pq 64 C RC R 16 C RC W qp A2A2 q2 4 CWCW 16 C RC W A1A1 p2 A1A2 2pq CW 20 p2 CR 80 pq A1A2 Eggs p2 A1A1 CW 20 f A2 q f A1 p female gametes f A1 p f A2 q q2 See also Fig 15 11 in text for dominant case Hardy Weinberg Equilibrium Predicts genotype phenotype frequencies from allele frequencies Genotype frequencies in expected proportions in a single generation Allele genotype frequencies constant across generations inheritance alone does not cause evolution Assumptions Random mating for this gene trait No mutation selection migration Large population no drift Applications of HWE A null model for evolution Deviations from expected proportions indicate something interesting but what Cystic fibrosis Mapped to chloride transport gene on chromosome 7 Common mutation F508 is recessive and at p 0 02 in caucasian population F het 2pq 0 04 carriers F hom p2 0 0004 affected Predicting frequency of heterozygotes for recessive alleles e g cystic fibrosis 3 Hardy Weinberg genotype frequencies as a function of allele frequencies at a locus with two alleles Futuyma 2nd Ed 4
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