9/28/201711Today’s Objectives(What Darwin did not know!)• DNA: What it is and what it does• Mendelian genetics:– How is genetic variation achieved?– How is it passed to future generations?29/28/20172Theories of inheritance3The homunculus (little man)Gregor Mendel (1822-1884)• Augustinian monk living in Brno (modern Czech Republic)• Did experiments with heredity in pea plants, trying to breed for certain discrete traits: color, stem length, size, shape49/28/20173Experiment 1• Crossed “true breeding” plants for long and short stem (P generation)• F1 generation were all long stem• Crossed this generation with itself• F2 generation had 25% short stem, 75% long stemxF1:F2:P:xlonglong longlonglonglonglonglong shortshort5xF1:F2:P:xlonglong longlonglonglonglonglong shortWhat does Mendel conclude from this experiment?short6- information for the short stem plant — not evident in the F1 generation — is still transmitted to the F2 generation9/28/20174Mendel’s First 3 Postulates (there is a 4th. . . wait for it!)• 1. Particles of Inheritance (genes) occur in pairs: 1 from each parent. During fertilization, alleles from both parents are joined to form a pair.• 2. Genetic information is inherited from both parents, but only one trait (called dominant) is expressed in the offspring.• 3. During gamete production (meiosis), homologous alleles segregate randomly into different gametes. (Mendell’s Law of Segregation)7What word do you associate with “dominance”?89/28/20175Are dominant alleles…• …more frequent in a population?• …better adapted to the environment?• …more important for individual survival?• …normal?9None of the aboveSome more definitions…• Heterozygous (-ote): having different alleles at the same genetic locus; AO blood type• Homozygous (-ote): having the same alleles at the same genetic locus; AA blood type• Dominant: trait expressed in a heterozygote (usually written in capital letters, i.e. AA = A blood type); only need one copy to be expressed• Recessive: trait not expressed in heterozygote; only expressed in homozygote (usually written in lower case letters, i.e. oo = O blood type); need 2 copies to be expressed• Codominance: expression of both traits in a heterozygote (i.e. AB = AB blood type)109/28/20176Two more…• 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 type11Know contrasting pairs• Dominant Recessive (also co-dominant)• Homozygous Heterozygous• Genotype Phenotype (and norm of reaction)129/28/20177Nature versus Nurture FallacyEvery trait results from the interaction of some genetic program with the environment. This can be conceptualized with the idea of Canalization.Every gene is canalized to varying extents (i.e., has a norm of reaction) in its expression from genotype to phenotype.13xF1:P:SS x sshomozygotesSs SsSs SsSSs sRevisiting Mendel’s 1stexperimentheterozygoteslonglong longlonglongshort149/28/20178xF1:F2:P:xSS x sshomozygotesSs x SsheterozygotesSS SsSs ssSsS slonglong longlonglonglonglonglong shortshort15What if you have more than one trait?169/28/20179xF1:F2:P:xSSCC x sscc homozygotesSC x sc =heterozygoteslonglong longlonglonglonglonglong shortshort17Tall & purpleShort & whiteSsCc SsCcSSCc SSccSsCC SsCcSCScSC ScSsCc SsccssCC ssCcSSCC SSCc SsCC SsCcSsCc Sscc ssCc ssccsC scsCscMendel’s 4thPostulate: The Law of Independent Assortment• This principle was based on Mendel’s experiments breeding for two different traits at once• Discovered that the presence or absence of one characteristic does not effect the presence or absence of another characteristic• Independent Assortment: the distribution of one pair of alleles does not influence the distribution or expression of another pair189/28/201710xF1:F2:P:xSSCC x sscc homozygotesSC x sc =heterozygoteslonglong longlonglonglonglonglong shortshort19Tall & purpleShort & whiteSsCc SsCcSSCc SSccSsCC SsCcSCScSC ScSsCc SsccssCC ssCcSSCC SSCc SsCC SsCcSsCc Sscc ssCc ssccsC scsCscALA#21) In what year did Charles Darwin publish “On the Origin of Species?”2) What is sexual dimorphism? Provide an example (i.e. a sexually dimorphic organism and the dimorphic feature).3) What are the 2 functions of DNA?9/28/201711Do 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 when natural selection culls away particular variants? • How is variation produced that doesn’t currently exist?21Modern Synthesis• Combination of Darwin & Mendel’s conclusions to explain how variation is maintained229/28/201712The Modern Synthesis Answers Darwin’s Critics1a. Before Mendel, Darwin was considered wrong because blending inheritance would remove all variation.23The Modern Synthesis Answers Darwin’s Critics1b. After Mendel, inheritance was understood as particulate, so how can evolution result from gradual changes? 249/28/201713Monogenic and Polygenic Traits25short tallThe Modern Synthesis Answers Darwin’s Critics1. Answer: Continuous (polygenic) traits are controlled by many genes by way of the proteins for which they code. Genes are particulate but together produce blended phenotypic traits. 269/28/201714Figure 03.0527Figure 03.06289/28/201715Figure 03.0729BMP4BMP2BMP7Polygenic traits appear continuous but are controlled by individual genes309/28/201716Any Questions?31Norm of Reaction• Phenotypic range seen in individuals with a particular genotype.• Norm of Reaction = Phenotypic Plasticity329/28/201717How much of a trait is genetic vs. environmental?FALSE dichotomy 33Phenylketonuria (PKU)• Autosomal recessive condition– Unable to metabolize phenylalanine due to deficiency in the enzyme (phenylalanine hydroxylase) which breaks phenylalanine down into tyrosine.• Phenotypic symptoms: Protein-rich foods and sweeteners can act as poisons for people with phenylketonuria.– Diet349/28/201718PKU• How much do genes and the environment contribute to this condition?• 100% genetic & 100% environmental– Influences aren’t a simple weighted sum• Allele for PKU is inherited, but the phenotype need not be.• Genetics do not determine our fate – we can construct
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