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The Code of Life Gregor Mendel 1822-1884; an austrian monk who performed the first comprehensive and systematic genetic experiments. Certain strains of peas bred true – tall plants give tall plants Short peas bred gave short plants A Mendelian Genetic Primer Genes come in pairs that separate in the formation of gametes. The members of the pair may be identical (homozygous) or non-identical (heterozygous). Each form of a particular gene is an allele. Genes, Alleles, and Chromosomes Genes, Alleles, and Chromosomes Mendel’s hypothesis Each adult possessed two sets of genes, one contributed from each parent. Mendel characterized genes as dominant or recessive Mendels genetic model served provide an understanding of years of data collected on pea plant breeding Mendel’s Monohybrid Cross – P to F1 Simple cross • Each parent is homozygous • All the offspring are heterozygous – and all look alike A Punnett square, something we’ll cover in a moment. Second generation cross • • • • • • Parents are heterozygous The offspring includes: One homozygous dominant One homozygous recessive Two heterozygous The phenotype ratio is 3 dominant to one recessive expression Phenotype vs Genotype Years of experimentation on pea plants lead to an understanding of Genetic consistency Characters investigated by Mendel Dihybrid Cross Principles of Segregations and Independent Assortment. Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another? The alternative and incorrect hypothesis: dependent inheritance. Because it’s the only way to explain the pattern of inheritance. Principle of Segregation Demystified Principle of Segregation Demystified Segregation The principle of segregation is explained by the behavior of homologous chromosomes at meiosis. Genetics works for people too • In the cross Aa x Aa, where A is a dominant allele for (standard) pigmentation • a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino. Multiple Alleles • Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. • The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene. A Molecular Perspective of Genes The answer to the question “What’s a gene?” depends on we’re interested in. At the molecular level: a gene is a sequence of DNA capable of producing some element of biological function. • Biological function: • It may be an observable trait, (like skin color), • A cellular property, ( cell cycle), • A molecular property, like the three dimensional shape of a protein. Alleles at the Molecular Level Each form of a gene is an allele. The standard (wild type) and altered (mutant) forms of the gene associated with hemoglobin and sickle cell anemia provide an example. The DNA sequences of both alleles of the “hemoglobin gene” are 99.9% identical – a single nucleotide difference makes for a single amino acid difference, which makes for a difference in protein shape, function and, ultimately, phenotype. normal red blood cell sickled red blood cell What’s So Special About DNA? DNA is one of the most boring macromolecules imaginable its made of only four building blocks and has a perfectly monotonous structure. Worse yet, DNA just sits there - it doesn’t catalyze reactions or build the cell or organism. So, what’s so good about DNA? The answer lies in DNA’s ability to store and copy information. Building DNA Building Blocks DNA is Made of Two Long Chains of Nucleotides Joined by DNA is Made of Two Long Chains of Nucleotides Joined by Hydrogen Bonds Hydrogen Bonds A Nucleotide G and C are complementary as are A and T Two Views ofTwo theViews Double Helix of the Double Helix Simple As It IsBase in Principle, Complementary Pairing Allows Each of DNARequires to Serve as a DNAStrand Replication Template for DNA That Replication Many Enzymes Work Coordinately DNA is a perfect illustration of function following form (structure dictates function). DNA polymerases are the first and foremost of the replication enzymes. DNA Replication – Something Old and Something New In Each Daughter Molecule Accidents Happen With Some “Accidents” (Base Mismatches) Leading to Mutation A mutation is a heritable change in DNA sequence. Mutations due to replication errors only happen once in every billion replicated nucleotides. DNA Damage is Often the Root Cause of Mutation DNA Damage is Often the Root Cause of Mutation DNA is chemically altered (i.e. damaged) spontaneously and by DNA is chemically altered (i.e. damaged) spontaneously and by chemicals and chemicals and radiation. radiation. Mutation asas Villain Mutation Villain Cancerous growths that Cancerous growths that result from loss of a protein result from loss of a that polices forDNA errors. protein thatDNA polices for errors. Cancer Incidence Increases Sharply with Age Cancer Incidence Increases Sharply with Age The increase is due at least in part to the age-related accumulation of The increase is due at least in part to the age-related accumulation of multiple multiple mutations in single cells. mutations in single cells. Genetics and Evolution • While mutations can be seen as Villains – they are also the “hero’s” of evolution • It is only by mutation that genetic variations are added to a population- leading to long term change over time • The effects of evolution are felt by individuals, but it is the population as a whole that actually evolves. Evolution is simply a change in frequencies of alleles in the gene pool of a population. Hardy Weinberg Equilibrium: Evolution will not take place if these conditions are met: • • • • • • 1. mutation is not occurring 2. natural selection is not occurring 3. the population is infinitely large 4. all members of the population breed 5. all mating is totally random 6. everyone produces the same number of offspring • 7. there is no migration in or out of the population