2/4/20141Course Business• Labs this week• Not too much to say1Today’s Objectives• Genetics• Modern Synthesis• DNA2Genetics: Inheritance and VariationHow is genetic variation maintained?How is genetic variation inherited?32/4/20142Theories of inheritance44“Blending” caused problems for Darwin’s advantageous traitsPangenesis• Darwin’s theory of heredity• Body cells produce gemmules that gather in reproductive organs• Accounts for:– Atavisms • Reappearance of traits from earlier generations• Result from awakening of dormant gemmules– Blended inheritance– Lamarckian inheritance– Limb regeneration5Pangenesis is wrong• Darwin’s cousin Galton experimentally tested whether gemmules exist• Transfused blood from different breeds of rabbits• Found that offspring of transfused rabbits did NOT develop traits of the breed whose blood they had gotten6Francis Galton 1822-1911http://darwin-online.org.uk/converted/published/1875_Variation_F880/1875_Variation_F880_fig05.jpghttp://en.wikipedia.org/wiki/File:Francis_Galton_1850s.jpghttp://www.dreamstime.com/royalty-free-stock-photos-gray-rabbit-sit-image63231782/4/20143Gregor Mendel 1822-1884• Austrian monk• Conducted experiments with peas• Discovered genetics• Publication citing his results was largely ignored until 1900 when rediscovered7http://en.wikipedia.org/wiki/File:Gregor_Mendel.pngmendelweb.orgworld-latest-news-headlines.blogspot.com8• Gene: basic unit of inheritance• Allele: variation of a geneY = allele for greeny = allele for yellow92/4/20144A couple definitions…• Genotype: genetic (allelic) make-up of one or more gene loci– For example: SS, Ss, ss; SsTt, ssTt, SStt• Phenotype: expression of a particular allele; physical or behavioral characteristics– For example: tall, brown hair, O blood type• Individuals with the same phenotype may have a different genotype!– Why is this?10What words do you associate with “dominant” or “dominance”?11Are dominant alleles…• …more frequent in a population?• …better adapted to the environment?• …more important for individual survival?• …normal?122/4/20145Some more definitions…• Homozygous (-ote): having the same alleles at the same genetic locus; AA or aa• Heterozygous (-ote): having different alleles at the same genetic locus: Aa• Dominant: trait expressed in a heterozygote (usually written in capital letters); only need one copy to be expressed; AA or Aa• Recessive: trait not expressed in heterozygote; only expressed in homozygote (usually written in lower case letters); need 2 copies to be expressed: aa• Codominance: expression of both traits in a heterozygote; AB13Early Resistance• Paleontologists, geneticists, and other specialists didn’t understand each others’work• Biology was divided into different disciplines -zoology, botany, physiology, etc. with poor communication and professional jealousy• Mendelian genetics seemed impossible to reconcile with slow, gradual, Darwinian change• Hard to see how small, gradual changes could lead to major evolutionary changes14Modern Synthesis – connecting Darwin and Mendel152/4/20146Do natural selection and Mendel’s conclusions conflict? • If inheritance is particulate, how can evolution take place gradually?• Why do phenotypes still appear to be “blended”?• How is variation maintained?• How is variation produced that doesn’t currently exist?16Modern Synthesis1930s-1940s• Evolutionary theory• Population genetics• Paleontology• Various other branches of biology– Systematics– Physiology– Developmental biology17The Modern Synthesis1930s-1940s• R. A. Fisher: Genetical Theory of Natural Selection (1930)• J. B. S. Haldane: The Causes of Evolution (1930)• Theodosius Dobzhansky: Genetics and the Origin of Species (1937)• Julian Huxley: Evolution: The Modern Synthesis (1942)• Ernst Mayr: Systematics and the Origin of Species (1942)• George Gaylord Simpson: Tempo and Mode in Evolution (1944)• NOTE: this is well before Watson and Crick (and others) published on the actual structure of DNA182/4/20147What did the Modern Synthesis add to Darwinian Evolution?Darwin• Primary concern was how evolution worked at the level of an organism– Speciation– Individuals in competition– AdaptationModern Synthesis• How does evolution work at the genetic level?– What is the mechanism?– Genes, phenotypes and how those relate to populations19Do natural selection and Mendel’s conclusions conflict? • If inheritance is particulate, how can evolution take place gradually?• Why do phenotypes still appear to be “blended”?• How is variation maintained?• How is variation produced that doesn’t currently exist?20Monogenic and Polygenic traits21short tall2/4/20148Monogenic and Polygenic traitsMonogenic• Same as “Mendelian” traits• Influence by one gene• Expression not usually influenced by environment• Distribution of phenotypes into just a few discrete categories (e.g., two phenotypes)Polygenic• Influenced by more than one gene• Expression may be much influenced by environment• Distribution of phenotypes is continuous with no discrete categories (many phenotypes can be distinguished)2223242/4/201492526Modern Synthesis Answers• Why do phenotypes still appear to be “blended”?– ANSWER: Continuous (polygenic) traits are controlled by many genes by way of the proteins for which they code. Each gene has a small effect on the phenotype (gradual changes). Genes are particulate but together produce blended phenotypic traits.272/4/201410Any Questions?28How much of a trait is genetic?How much of a trait is environmental? FALSE dichotomy!!29Norm of Reaction• Effects of environmental variation on a phenotype• Phenotypic range seen in individuals with a particular genotype302/4/201411Human Example: PKU (Phenylketonuria)• Autosomal recessive condition– Unable to metabolize phenylalanine• Phenotypic symptoms require high levels of phenylalanine– DIET31Human Example: PKU (Phenylketonuria)• How much do genes and the environment contribute to this condition?• 100% genetic and 100% environmental• Alleles for PKU are inherited, but the phenotype need not be32What can these examples teach us?• Danger of both genetic and environmental determinism.• Understand the limits of “gene for” something• Genetics do not determine our fate – we can construct environments appropriate
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