Chapter 13 13 1 Mendel s Experimental System What Questions Was Mendel Trying to Answer o What are the basic patterns in the transmission of traits from parents to offspring 2 hypothesis at the time Blending inheritance The traits observed in a mother and father blend together to form the traits in the offspring Offspring s traits are intermediate between mother s and father s Inheritance of acquired characters Traits present in parents are modified through use and passed on to offspring in the modified form Garden Peas First Model Organism in Genetics o Mendel could control matings Pollen grains Self fertilization Cross fertilization o Mendel studied different traits Phenotype Seed shape seed color pod shape pod color flower color flower and pod position and stem length 2 distinct phenotypes existed Began work with pure lines Produced hybrids A mating between parents that each carry two different determinants for the same trait Crossed pure lines with just 1 trait Round seeds with wrinkled seeds 13 2 Mendel s Experiments with a Single Trait Monohybrid Cross o o o o o o Results Traits didn t blend together No intermediate formed The wrinkled seeds seemed to disappear Self pollinated the F1 seeds F2 generation results 5474 round 1850 wrinkled 3 1 Wrinkled seeds had reappeared Recessive Dominant traits Reciprocal cross Mendel thought parent gender had an effect Turned out to be wrong for seed shape Particulate Inheritance o Hereditary determinants maintain their integrity from generation to generation Instead of blending together they act like discrete entities or parts o Principle of Segregation To explain 3 1 Mendel thought the 2 members of each gene pair must segregate into different gamete cells during formation of eggs and sperm Each gamete contains one allele of each gene o Heterozygous Homozygous o Genetic model Set of hypotheses that explains how a particular trait is inherited Dihybrid Cross o o o o 9 3 3 1 Testcross A mating between two individuals both heterozygous for two traits Independent assortment Alleles of different gametes are transmitted independently of one another A mating between one parent with an unknown genotype with an homozygous recessive other parent 13 4 The Chromosome Theory of Inheritance Chromosomes are composed of Mendel s hereditary genes Anaphase of meiosis 1 principle of segregation Principle of independent assortment Chromosome theory chromosomes at meiosis I Sex linkage o X linkage o Y linkage A gene that resides on the Y chromosome Autosomal inheritance X linked inheritance o Genes that are on non sex chromosomes o Mendel s rules can be explained by the independent alignment and separation of homologous A gene that resides on the X chromosome A female has 2 copies of the gene more chances to be dominant whereas the male only has 1 copy since his other chromosome is a Y o Convinced most biologists that the chromosome theory of inheritance was correct 13 5 Extending Mendel s Rules Linkage o o Crossing Over The physical association between genes on the same chromosome Exception to independent assortment Involves a physical exchange of segments from homologous chromosomes o Occurs during prophase I o o Occurs at least once in every homologous pair usually several times o Linked genes are inherited together unless crossing over occurs Incomplete dominance o Heterozygotes have an intermediate phenotype o Ex red white pink flowers Codominance o Heterozygotes show both phenotypes o Ex blood type Multiple Allelism o More than two alleles of the same gene o Polymorphic Pleiotropic o o A gene that influences many traits Ex Marfan Syndrome gene A change in a single allele affects more than one trait Other things that determine phenotypes More than two distinct phenotypes are present in a population due to multiple allelism o o Physical environment Interactions between other genes 14 1 What Are Genes Made Of Hershey Chase Experiment o Found that DNA is the hereditary material not protein o DNA contained all the information for life s complexity Secondary Structure of DNA o Watson Crick Model DNA is a long linear polymer made up of many monomers called deoxyribonucleotides Consist of a deoxyribose molecule a phosphate group and a nitrogenous base Phosphodiester bonds forms between the hydroxyl group of a 3 carbon of one and the phosphate group on the 5 carbon of another Primary structure A strand of DNA has a polarity A sugar phosphate backbone A series of nitrogenous bases o One end has exposed hydroxyl group on the 3 carbon The other has an exposed phosphate on the 5 carbon 3 end a 5 end o Double Helix o Antiparallel o Complementary base pairing o The existing strands of DNA serve as a template for the production of new strands Bases are added to the new strands according to complementary base pairing 14 2 The Meselson Stahl Experiment 3 alternative hypothesis for how the new and old strands of DNA interact during replication 1 The old strands separate and each are used as a template Semiconservative replication Each new daughter strand consists of one old strand and one new one 2 The bases temporarily turn outward so that complementary strands no longer face each other They could serve as a template for the synthesis of an entirely new double helix all at once Conservative replication 3 The parent helix is cut in short sections before being unwound copied and put back together Results in an intact parental molecule entirely new daughter molecule New and old segments would alternate Stretches of old DNA would be interspersed with new DNA down the length of the daughters Their Experiment Dispersive replication 1 Wanted to tag mark parental and daughter strands to make them distinguishable from each other 2 Needed to choose an organism to study E coli Because it is small and grows quickly and readily 3 Grew the cells in presence of different isotopes of nitrogen If different N isotopes were available in the growth medium when different generations were produced then the parental and daughter strands will have different densities Density gradient centrifugation Seperates molecules based on their density o Conclusion 1 Data from generation 1 conflict with conservative replication 2 Data from generation 2 conflict with dispersive replication 3 Replication is semi conservative Each new DNA molecule consists of one old strand and one new strand 14 3 A Comprehensive Model for DNA Synthesis DNA Polymerase o o o Polymerizes deoxyribonucleotides to DNA Found to