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2/4 notes + chapter 4 textbook notes for EXAM 1

by: Elizabeth Notetaker

2/4 notes + chapter 4 textbook notes for EXAM 1 Bio 230

Marketplace > West Chester University of Pennsylvania > Biology > Bio 230 > 2 4 notes chapter 4 textbook notes for EXAM 1
Elizabeth Notetaker
GPA 3.9

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the chapter 4 notes only include what is on exam 1 for chapter 4
Dr. Donze- Reiner
Class Notes
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This 8 page Class Notes was uploaded by Elizabeth Notetaker on Sunday February 7, 2016. The Class Notes belongs to Bio 230 at West Chester University of Pennsylvania taught by Dr. Donze- Reiner in Spring 2016. Since its upload, it has received 11 views. For similar materials see Genetics in Biology at West Chester University of Pennsylvania.


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Date Created: 02/07/16
1. You usually have to use several different kinds of maps 2. Distorted maps are still useful maps 3. Sometimes a site on a chromosome map is only charted because it helps to find other more useful genes. Why is gene mapping important? 1. Complex genotypes require knowing the position of genes for experiments 2. Figuring out a gene's structure and function (wild-type and mutant) 3. Evolutionary genetic mechanisms can be figured out by seeing how genomes diverged between species Chromosome map- unidimensional arrangement of genes Loci- gene positions Recombination-based maps --> map loci of genes shown by mutant phenotypes showing single gene inheritance Physical maps--> show genes as segments arranged along the DNA molecule that makes a chromosome 4.1 Diagnostics of Linkages Recombination maps- use linkage analysis to determine how far away gene loci are on a chromosome Linked- term used for when the loci of 2 genes are on the same chromosome (don't show mendelian inheritance pattern) Recombination frequency to recognize linkage A scientist doing crosses expecting 9:3:3:1 ratios realized that they were not at all these rations, but rather 2 combinations were the large majority as if they were associated. The idea of linkage was proposed The linked genes are the ones the parents have Linked genes produce a recombinant frequency less than 50% How crossovers produce recombinants for linked genes Crossing overs- when homologous chromosomes break and exchange DNA with each other Crossover products- the results of crossing over (new combinations) Chiasma forms ( x shape structure between chromosomes that are crossing over) Visible manifestation of crossovers Linkage Symbolism & Terminology Cis conformation- 2 dominant alleles on same homolog (AB/ab) Trans conformation- 2 dominant alleles are on different homologs (Ab/aB) 1. No punctuation (AB) means they are on the same homolog 2. Slash (A/B) means 2 different homologs 3. Alleles are always written in the same order 4. Semicolon (A;B) means they are known to be on separate chromosomes 5. Dot (A•B) means unknown linkage How do we know that crossing over is a breakage and rejoining process? Scientists figured out that crossing over occurred by a physical exchange of chromosome segments by looking at corn chromosomes that have visible characteristics of shapes. The recombinants had a new combination of the shapes, showing that the chromosomes physically switched spots. 4 chromatid phase Crossovers occur between nonsister chromatids If crossing over occurs at the 2 chromosome phase between sister chromatids (which are identical) no change would happen in the genotype. Multiple crossovers can include more than 2 chromatids Any single crossover is between 2 chromatids, but double crossovers can happen. 4.2 Mapping by Recombinant Frequency The farther apart genes are, the more likely they will cross over # of recombinants is a clue to how far apart genes are Sometimes a gene is so far apart on a chromosome that it pretty much acts independent. Map units Linkage map- Uses recombinant frequencies % as an index of linear distance between genes Genetic map unit (m.u.)- the distance between genes for 1/100 recombinants Recombination frequency of 10.7= 10.7 map units Centimorgan (cM) - same as map unit This method is pretty accurate except: 1. Recombination hot spots - where crossing over happens a lot 2. Recombination blocks- where not much crossing over happens at all Three Point Test Cross Three point test cross/ three factor cross- trihybrid cross with a triply recessive tester 1. Analyze 2 loci at a time 2. determine parentals and recombinants 3. Calculate recombination frequency for each 4. Determine distance This test can determine gene order, while 2 point can not *sometimes the numbers don't add up perfectly- this is because of double crossovers (you should technically count them twice), but for what we are doing it works well enough


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