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Notes February 29th to March 4

by: Joseph Merritt Ramsey

Notes February 29th to March 4 CELL 2050

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Joseph Merritt Ramsey

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Genetic Linkage Included in Depth With Expanded Diagrams to Understand Gamete Separation in Relation to Linkage
Dr. Meenakshi Vijayaraghavan
Class Notes
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This 12 page Class Notes was uploaded by Joseph Merritt Ramsey on Monday March 7, 2016. The Class Notes belongs to CELL 2050 at Tulane University taught by Dr. Meenakshi Vijayaraghavan in Winter 2016. Since its upload, it has received 50 views. For similar materials see Genetics in Science at Tulane University.


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Date Created: 03/07/16
March 2, 2016 Chapter 6: Genetic Linkage  Genetic Mapping Overview o Displays  1. What Genes are Present  2. What Physical Order They’re In o Vocab to Know  Bateson and Punnet’s Work o Two Factor Heterogeneous Cross o So they question is – why was it so far off?  Crossing Over o Pachytene Phase o Crossing Over increased the variability o So Watson decided to look at three traits o How Could They Prove Crossing Over is Occurring?  Took a look at corn  Corn has 10 Chromosome, and various Chromosome 9’s  have some abnormalities  I. Translocated Piece of 8  II. Knob on the End  Both ends were also known to have specific gene loci  So they decided to run a cross (Test Cross): Heterozygote  with Recessive  Proved?  I. Were able to view the Crossed Over portions under  the microscope  II. The crossing Over created new phenotypic  combinations  Mitotic Recombination o This occurrence is much less likely  This process is always Nuclear, so not like variegated leaves o Two Periods of Occurrence:  I. It Can Happen Very Early on in Development – different  body tissues  II. Or It Can Happen After Development – mosaicism, patches o Ex) Drosophila Body Types  Body Color and Bristle Type is found on the X Chromosome  So females have 2 X­Chromosomes in their cells  When they replicate, the homologues on the two can match  up and cross over  So think about it: couldn’t occur in males in this  scenario  Also consider what would happen at either of the locations of development:  I. Early Stages – divided body tissues  II. Somatic Cells – can form a path of phenotypically  altered appearance  Dr. V’s Questions for Clarity o Linkage Groups vs. Linked  Linkage Groups –   Linked –  o Chi­Squared Analysis  The possibilities in this case are “Recombinant” and  “Nonrecombinant,” so the DF = 1 o Crossing Over Occurs in Pachytene Phase of Prophase I between  Non­Homologues o Genetic Mapping  It is done with a Test Cross  So there needs to be a recessive individual  To Lay it Out: o Keep in mind that whole segments of Chromosomes are being  moved, not simply alleles  To prove this, a Heteromorphic Allele was utilized  (Chromosome 9 in corn) o Mitotic Recombination  This is incredibly rare  But it should not be confused with the effects of Maternal  Inheritance in the form of Variegation  This effect is Mosaicism, a Nuclear Phenomenon  March 4, 2016 Chapter 6: Genetic Linkage – Mapping  Genetic Mapping o Overview  Also known as Chromosome Mapping  Definition – the arrangement of genes along the Chromosome  Distance is being measured and is the key consideration  Loci are being defined o Uses:  Eugenics and Disease Analyses  Evolutionary Comparisons o The Process  Recombination Frequency is measured  This is the capacity to Cross Over  They are measured in Map Units (mu) or CentiMorgan (CM)  Comes out to be that 10% frequency related to 10mu o Test Crossing   Always need a Heterodominant paired with a Recessive  So with Sex Chromosomes an extra consideration  must be made: Hemizygous Recessive Can Occur in  Males  Heterozygote + Homozygote  See how, when following gamete formation, the pieces start  to really fit together  Some Profiles of Examples of Genetic Mapping o I. Multiple Genes – Drosophila  Have about 10 Identified Genes on the X­Chromosome  To analyze, they need to be broken into pairs  What becomes evident is that going with frequency higher  than 30% and up to 50% leads to Underestimation  Multiple Crossovers can occur at that point  So the preferred method is start close and move  outward  The smaller numbers are more closely linked and are  therefore more accurate overall  So these are the genes we’re looking at (Five Total Loci)  So to determine the significance of crossing over, each needs  to be compared in pairs o II. Trihybrid Cross – Drosophila   The same general idea will apply  Three Groups of Offspring Now Become Evident  1. Parental Combinations  2. Intermediate (Single Crosses)  3. Double Crosses  These groups will be important in the ultimate analysis of  gene mapping o III. General Goal  Compare the frequencies between two alleles to begin  mapping the Chromosome  Calculating the Map Distance o Formula Crossover Frequency BetweenGene A∧B ¿x100 ( TotalOffspring This is equivalent to… RecombinantOffspring¿x100 ( TotalOffspring o You always have to compare two genes, but you can’t use a  Dihybrid cross – there’s nothing else with which to compare them o If the genes are far apart, there’s a higher likelihood of  recombination occurring o It’s also always best to start “close up,” so to say, and “zoom out”  So calculate the smaller numbers (more accurate) and add  them  Instead of taking the big numbers first o Example I: Simple, The Bristle Types (Dihybrid)  Recall, the colored picture in the test cross, the numbers  were about 450 and 75 for Parental and Recombinant,  respectively.  So look at how to plug those numbers in: o Example II: More Complex, The Above Trihybrid Cross  But when more genes are involved, more options become  apparent  The next example will outline two manners in which you can determine the mapping units in a Trihybrid Cross  As a side note, Polyhybrid Crosses become too complicated  to be useful and efficient   Interference o One crossing over prevents another from occurring o Calculating Interference P (¿¿GENE1/2CROSSOVER)x(P GENE 2/3CROSSOVER Expected DoubleCrosses=P TWOCROSSOVER observed doublecrossovers c=confidencecoefficient= expected doublecrossovers Interference=1−c o When Prevention Occurs, it is called Positive Interference o Negative Interference Never Occurs Chapter 9: Molecular Structure of Genetic Material  What is Genetic Material? o 1) Stores Info o 2) Replicates o 3) Transmission o 4) Allows for Variance  How Did We Get to Where We Are? o 1) Experiment I – Griffith’s Bacteria  General Overview  Griffith experimented with Bacteria  In general, Smooth Bacteria are virulent o They have a smooth outside protein capsule o This capsule is made of Complex Carbohydrate Trees  Rough Bacteria are nonvirulent o They lack the coating  Overall, there are Two Types of the Bacteria  Bacterial Mutations  Smooth mutations led to the rough capsule,  nonvirulent form  So while the capsule type can switch back and forth,  the bacteria type cannot  Experimental Results Living Type 3S Type 2R, Dead 3S  Aside: How do Vaccines Work?  Our immune cells begin to synthesize antibodies to tag infectious diseases  So our cells search for Cell Surface Antigens (define  as what antibodies bind to) on Bacteria o Smooth is more complicated to synthesize


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