Experiment 4 Crossing Over In this lab pictures of a prepared slide of the fungus Sordaria fimicola will be used to examine the effects of crossing over The life cycle of this fungus begins in the haploid state However after the combination of two different types of strains they develop a diploid nucleus As the life cycle continues the diploid nucleus undergoes meiosis along with mitosis and produces eight haploid ascospores which are stored Figure 6 in a sac called an ascus Figure 6 Once maturation has been reached the sac will burst allowing the ascospores to be released These spores are haploid and thus begin the life cycle again Sordaria fimicola is often used to observe crossing over because the wild type strain has black ascospores and the mutant type has tan ascospores When the two strains go through meiosis the location of the ascospores will directly show if crossing over has occurred If crossover does not occur the ascospores will be arranged like Figure 7 If crossover does occur the ascospores will be arranged like Figure 8 Figure 8 Figure 7 The measurement to describe the difference between genes is referred to as map unit As the distance increases between genes the likelihood of crossing over becomes greater therefore demonstrating that the proportion of crossovers corresponds with the 2013 eScience Labs LLC All Rights Reserved distance between genes As a rule the percentage of recombinations is equal to the number of map units between two genes or a gene and the centromere Materials Sordaria fimicola Digital Slide Images Procedure 1 Examine the three fields of view of the Sordaria fimicola 2 There are at least 10 different hybrids crossovers in the following images Count the total number of clearly visible crossovers as well as the number of clearly visible non crossovers Record your data for each image in Table 4 Sordaria fimicola 400X Image 1 2013 eScience Labs LLC All Rights Reserved Sordaria fimicola 400X Image 2 Sordaria fimicola 400X Image 3 2013 eScience Labs LLC All Rights Reserved Table 4 Sordaria fimicola Crossover Data Image Number of Crossovers Number of Non Crossovers Image 1 6 4 Image 2 0 10 Image 3 6 4 Post Lab Questions 1 Determine the percentage of crossovers To do this divide the number of crossovers by the total number and multiply it by 100 Image 1 60 Image 2 0 Image 3 60 2 Determine the map distance To accomplish this divide the percentage of crossovers by two Note the number is divided by two because crossover occurs once between two chromosomes Image 1 30 Image 2 0 Image 3 30 2013 eScience Labs LLC All Rights Reserved
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