Chapter 13 Mendel s Experiments with a Single Trait Monohybrid Cross mating between parents that each carry two different genetic determinants for the same trait Fig 13 2 Through monohybrid crosses Mendel disproved theory of blending inheritance claimed that traits observed in a mother and father blend together to form the traits observed in their offspring As a result an offspring s traits are intermediate between the mother s and father s traits Recessive and dominant traits Recessive and dominant identify which phenotype is observed in individuals carrying two different genetic determinants for a given trait Recessive temporarily latent or hidden Dominant trait shown Reciprocal cross a set of matings where the mothers phenotype in the initial cross is the father s phenotype in a subsequent cross Mendel proposed particulate inheritance hereditary determinants for traits do not blend together or acquire new or modified characteristics through use hereditary determinants maintain their integrity from generation to generation Instead of blending together they act like discrete entities or particles Genes alleles genotypes Gene indicates the trait Alleles different versions of the same gene Genotype alleles that are found in a particular individual Physical traits of phenotype The Principle of Segregation Two members of each gene pair separate into different gamete cells during the formation of egg and sperm in parents Homozygous containing two copies of the same allele Heterozygous containing two different alleles for the same gene Fig 13 4 13 7 Principle of Segregation takes place in the separation of homologous chromosomes in meiosis I AI II Chromosomes are composed of hereditary determinants genes o Each has a locus physical location of a gene Dihybrid Cross a mating between two individuals heterozygous for two traits o Fig 13 5 The principle of Independent Assortment alleles of different genes are transmitted independently of one another fig 13 8 if alleles for different genes are located on different nonhomologous chromosomes they assort independently of each other at meiosis I Sutton and Boveri Formalized Mendel s rules with chromosome theory of inheritance o States that Mendel s rules can be explained by the independent alignment and separation of homologous chromosomes at meiosis I Thomas Hunt Morgan experiments Drosophila melanogaster fruit fly Characterized common phenotype as wild type Discrete easy to recognize phenotype different from normal phenotype mutant having a mutation a change in a gene Nettie Stevens discovery of sex chromosomes though X and Y chromosomes contain different genes they have regions that are similar enough to lead to proper paring during prophase of meiosis I The X and Y chromosomes are sex chromosomes o female XX male XY sex linkage or sex linked inheritance general term for inheritance of genes on either sex chromosome according to the hypothesis of X linkage a female fruit fly has two copies of the gene that specifies eye color because she has two X chromosomes A male in contrast has only one copy of the eye color gene because he has only one X chromosome inherited from his mother Linkage physical association among genes on the same chromosome Linkage and sex linkage are not the same o linked can be two or more genes on the same chromosome o sex linked means gene located on the sex chromosome o linked genes violate the principle of independent assortment p 243 closeness of genes makes it highly unlikely that they will be split crossing over recombinant individuals have a combination of alleles on their X chromosome different from the combinations of alleles present in the parental generation box 13 1 linked genes are inherited together unless crossing over occurs between them When crossing over takes place genetic recombination occurs Dominance is not necessarily an all or none phenomenon o Dominant and recessive describe which phenotype is observed when two different alleles of a gene occur in same individual o There can be combinations in which alleles do not produce completely dominant or recessive phenotype o Incomplete dominance Heterozygotes have intermediate phenotype 4 o clock flowers fig 13 15 o codominance heterozygotes have the phenotype associated with each individual allele blood ABO blood type Pg 247 Environment and phenotype individuals phenotype is often as much a product of the physical environment as it is a product of the genotype quantitative versus qualitative traits Mendel worked with discrete traits characteristics that are qualitatively different from each other o Quantitative traits differ by degree also influenced by environment o Quantitative traits forms a bell curve normal distribution Herman Nilsson Ehle if many genes each contribute a small amount to the value of a quantitative trait then a continuous bell shaped normal distribution results for the population as a whole Fig 13 21 Quantitative traits are produced by the independent actions of many genes although it is now clear that some genes have much greater effects on the trait in question than other genes do Transmission of quantitative traits is said to result from polygenic inheritance each gene adds a small amount to the value of the phenotype Summary table 13 4 pg 250 Chapter 14 14 1 What are genes made of The Hershey Chase Experiment o How T2 virus infects E Coli o Experiment based on the fact that proteins contain sulfur but not phosphorus and DNA contains phosphorus but not sulfur o Using radioactive isotopes of sulfur and phosphorus Hershey and Chase and found that the radioactive protein stayed outside the cell while DNA was found inside the host cell The Secondary Structure of DNA o Primary structure has 3 components backbone of sugar and phosphate groups nitrogen containing bases rungs of ladder directionality polarity 3 exposed hydroxyl 5 exposed phosphate o Secondary structure o Antiparallel twist around each other into helix because of snug fit of nitrogenous bases o Stabilized by hydrogen bonds between A T and C G complementary base pairing stabilized by hydrophobic interactions inside helix o Existing strands of DNA serve as template for production of new strands with bases being added to the new strands according to complementary base pairing 14 2 Testing Early Hypothesis about DNA Synthesis The Meselson Stahl Experiment semiconservative conservative and dispersive theories of DNA replication genome entire complement of DNA difference in N
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