Genetics 3000 Week 5 Notes
Genetics 3000 Week 5 Notes 85033 - GEN 3000 - 002
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85033 - GEN 3000 - 002
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This 4 page Class Notes was uploaded by Lisa Blackburn on Sunday February 14, 2016. The Class Notes belongs to 85033 - GEN 3000 - 002 at Clemson University taught by Kate Leanne Willingha Tsai in Fall 2015. Since its upload, it has received 18 views. For similar materials see Fundamental Genetics in Biomedical Sciences at Clemson University.
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Date Created: 02/14/16
Terms Imporatnat People Key Topics Genetics Week 5 Notes: Chapter 7: Chromosome Mapping Review of Important Terms to remember: o Principle of Segregation: an individual diploid organism will possess two alleles for a single characteristic. These two alleles will separate into gamete randomly. o Principle of Independent Assortment: alleles that are at different loci separate independently from each other. o Chromosome Theory of Heredity: Sutton’s theory. Genes are found on chromosomes. Terms: o Linked Genes: genes on a chromosome that are close together and stay together during meiosis, meaning that do sort independently. Complete linkage: the genes are on the same chromosome and are close together. No crossing over takes place, the gametes will result in parental gametes. Incomplete linkage: the genes are on the same chromosome but there is some amount of distance between them. This will allow for crossing over to possibly take place and will have some recombinant gametes. o Recombination: new combination of alleles for a gamete Interchromosomal Recombination: occurs between genes that are on different chromosomes. This occurs through independent assortment in Anaphase I. Intrachromosomal Recombination: occurs between genes on the same chromosome. This occurs through crossing over taking place in prophase I. o Nonrecombination: the allele combination is the same as the parental gametes. o Recombinant Frequency: the frequency for the recombinant genes to be witnessed. Is found by: (# of recombinant progeny/total # of progeny)*100% o Coupling: (cis) when the wild type alleles are on one chromosome and the mutant alleles are on the other chromosome. o Repulsion: (trans) when each chromosome has one wild type allele and one mutant allele. o Genetic Maps: uses recombination frequencies to make a map of the chromosomes. Is an approximate distance. 1% recombination frequency = 1 centimorgans (cM) o Physical Maps: a map of chromosomes that uses the physical distances. An absolute map. o Double Crossing Over: this will yield in a middle gene being alerted or flipped from what it should be. o Interference: if the number of predicted double crossing over is different from what is observed, then this is caused by interference. Crossovers may influence where another crossing over occurs. Important People: o Morgan: genes on same chromosome segregated together, those closely linked together were not usually subject to recombination o Alfred Henry Sturtevant: a student of Morgan. Generated the first map of chromosomes by using recombinant frequencies. o Harriet Creighton and Barbara McClintock: determined how crossing over worked, followed the process by identifying characteristics that separated. This was the first evidence of breakage of chromosomes to switch information. Key Topics: o Linked Genes: genes that are located on the same chromosome and are close together will sort independently. This will result in an overrepresentation of the parental in the F generation. 2 When independent assortment occurs, the ratio of the F2generation will be 9:3:3:1. This is not seen for linked genes. o How did Mendel see the 9:3:3:1 ratio? He got lucky and picked characteristics that were on the same chromosome but far enough apart to have crossing over taking place. Crossing over looked like independent assortment to Mendel. o Test Crosses: Completely Linkage: results in parental progeny. Half will be parent 1 and the other half will be parent 2. Independent Assortment: will result in a 1:1:1:1 progeny. ¼ will be parent 1. ¼ will be parent 2. ¼ will be recombinant in one way. ¼ will be recombinant in another way. In other words, half will be parental and the other half will be recombinant. No Crossing Over: if no crossing over takes place within the genes of interest, then all the progeny will be parental. ½ will be parent 1 and ½ will be parent 2. With Crossing Over: if crossing over takes place within the genes of interest, then there will be a 1:1 ratio. ½ will be parental and ½ will be recombinant. This is for a single crossing over. o Linkage Mapping: the use of recombinant frequencies to determine a relative distance between genes. High recombination: crossing over has more room to take place, therefore, the genes are farther apart Low recombination: crossing over has less room to take place, therefore, the genes are closer together. This means that it is less likely to be able to detect a crossing over taking place Recombinant Frequency: a relative distance between genes Double Crossing Over: distances can be missed due to a double crossing over taking place, cannot be detected based on phenotype. o Genes may be: Independent Assortment: genes are on different chromosomes and can combine randomly. Results in an equal ratio of possible gametes. Complete Linkage: genes are on the same chromosome but are close together so crossing over is not likely to take place. Results in all parental gametes. Incomplete Linkage: genes are on the same chromosome but are far enough apart to all for crossing over. The gametes will result in some parental gametes and recombinant gametes, but there will be an overrepresentation of parental gametes. o Two-Point Testcrosses: a testcross between two genes Example: looking at genes a,b,c,d Cross gene a and b: get a 50% recombination frequency. Must assume that the genes are located on a different chromosome. Cross gene a and c: get a 50% recombination frequency. Must assume that the genes are located on a different chromosome. Cross gene a and d: get a 50% recombination frequency. Must assume that the genes are located on a different chromosome. Cross genes b and c: get a 20% recombination frequency. Cross genes b and d: get a 10% recombination frequency. Cross genes c and d: get a 28% recombination frequency. Since gene crosses of b and c, b and d, and c and d are all under 50%, this means that they are located on the same chromosome. But in what order? o Would have to be in order dbc or cbd. Gene d is 10 units away from b and b is 20 units from c. When put in this order, gene d is 30 units from gene c (10+20=30). We calculated that gene d is 28 units from c, but we missed double crossing over events which caused us to get 28 instead of 30. Linkage Groups: the more gene/markers the more information that can be gathered. If we calculate that gene A and F are over 50% recombination frequency, we would assume that the genes are on different chromosomes. However, if we know genes are in between them we can find out that A and F are on the same chromosome. Intermediate markers: allow for a linkage group that is at a span greater that 50cM to be connected together. o Three-Point Testcrosses: are more efficient than two-point testcrosses. The order of genes can be established in a single cross Double crossovers can be detected which will provide more information Double crossovers: yields a recombinant chromosome that has an altered middle gene. If a change in a single gene while the other two remain the same, then the changed gene is what is in the middle. Steps of a 3 point testcross: Identify parent groups (which group has the most? Overrepresentation) Identify the double crossing over (which group has the least?) What is the difference? Compare the parental to the DCO. Which gene is different or “switched?” This gene will be the gene in the middle. o Recombinant Frequencies and Predictions of Progeny: Recombinant frequencies can be used to predict the outcome of recombinant progeny of a hybrid crossed with a homozygous recessive. Subtract the frequency from 100, this will give you the frequency of the parental. Divide this number by 2 and you will get the percentage of what each parental will be seen. Take the recombinant frequency and divide by 2. This will give you the percentage of what each type of recombinant will be seen. o Gene mapping with Recombinant Frequencies: 1% recombinant frequency=1 cM. This can be used as a distance of the genes, but it is approximate. A to B: 5 cM B to C: 10 cM A to C: 15 cM What order do these have to be in for this to be true? ABC or CBA. As long as the distances are kept the same, there may be more than one order. Important things to note: Recombinant frequency cannot be greater than 50% for two genes o If it is a 50% recombinant frequency, then 50% of the progeny will be parental, meaning that independent assortment could have took place. o Therefore, when the recombinant frequency is greater than 50% we do not know if the genes are linked or on different chromosomes Genes that are far enough apart can undergo double crossovers and will look like crossing over never took place. o Molecular Markers: Restriction fragment length polymorphism (RFLP): changes in DNA sequence will modify restriction enzyme recognition sites Variable number of tandem repeats (VNTR): differences in copy number Microsatellite markers: number of repeats is different in every one, producing different phenotypes Single nucleotide polymorphism (SNP): a single base change
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