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15 - GENE MAPPING IN EUKARYOTES CHAPTER OUTLINE EARLY STUDIES OF GENETIC LINKAGE: MORGAN'S EXPERIMENTS WITH DROSOPHILA GENE RECOMBINATION AND THE ROLE OF CHROMOSOMAL EXCHANGE Corn Experiments D rosophiln Experiments CONSTRUCTING GENETIC MAPS Detecting Linkage Through Testcrosses Gene Mapping with TWo-Point Testcrosses Generating a Genetic Map Gene Mapping with Three-Point Testcrosses Calculating Accurate Map Distances THINKING ANALYTICALLY When working on a mapping problem, start by systematically organizing the data. This will allow you to extract critical information by inspection. Arrange the data by using symbols to represent the traits, writing out the crosses and their progeny, and tabulating the number of off- spring in each progeny class. After organizing the data, inspect two genes at a time. This often allows you to determine if the two genes are linked, and, if linked, whether they are sex linked. Any trait appearing in progeny in just one sex type should be checked for sex linkage. Mnppittg genes using meiotic recombination and testcrosses.It is essential to remember that two genes that show 50 percent recombination in a testcross may either be on different chromosomes or be on the same chromosome but very far apart. The map distances between two genes can exceed 50 map units, but their recombination frequency cannot exceed 50 percent. If two genes showing 50 percent recombination are each linked to a third, intermediate gene (i.e., they show less than 50 percent recombination with the third gene), they will be on the same chromosome but more than 50 map units apart. In a three-point testcross, a systematic, methodical approach is essential. 1. Assign gene symbols to the phenotypes that are seen and rewrite the crosses. 2. Reorganize the progeny genotypes according to the frequency of each class. 337
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338 CHAPTER 15 REVIEW OF KEY TERMS, SYMBOLS, AND CONCEPTS In your own words, write a brief, precise definition of each term in the groups below. Check your definitions using the text. Then develop a concept map using the terms in each list. linkage linked genes linkage group chromosome parental genotypes, classes recombinant genotypes, classes genetic recombination independent assortment crossing-over chiasma, chiasmata cytological markers genetic, gene markers translocation reciprocal exchange genetic map meiotic recombination centimorgan (cM), map unit recombination frequency coupling vs. repulsion two-point testcross three-point testcross two-, three-, four-strand, double crossovers interference coefficient of coincidence mapping function DNA recombination Holliday model Holliday intermediate branch migration patched duplex spliced duplex heteroduplex 3. Assign parental, single-crossovet and double-crossover classes-parentals will be most frequent, double crossovers will be least frequent. 4. Infer the gene order by comparing the parental and double-crossover classes. 5. Rewrite the testcross; infer which single crossover gives rise to which class. 6. Calculate the recombination frequency and map distance for each gene interval.
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This note was uploaded on 04/15/2011 for the course BIOL 3900 taught by Professor Oommen during the Fall '09 term at Georgia State University, Atlanta.

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