Lab 2 Protocol - Genetics 310 Crossing Over and Gene...

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Genetics 310 Crossing Over and Gene Distance in Sordaria I. Introduction Many fungi are composed of a mass of threads known as mycelium. One of the threads in a mycelium is called a hypha (plural hyphae). Sordaria fimicola is a sac fungus, phylum Ascomycota. All of the nuclei in the growing Sordaria mycelium are haploid. In this fungus, sexual spores form in sac-shaped hyphae called asci as a result of the union of two mycelia of opposite mating types (see figure 1). When the mycelium of two different Sordaria mating types (think of this as male and female) come together, the male nucleus of one hyphal cell enters a hyphal cell of the female sex organ. A series of mitotic divisions of this dikaryotic cell occurs before two haploid nuclei of different mating types enter a forming ascus and combine to form a diploid zygote. Immediately after fertilization, meiosis occurs, forming haploid nuclei in each ascus. Each ascus then contains a meiotic tetrad of nuclei. In Sordaria , these haploid nuclei each divide again to form a total of eight spores in a mature ascus (see Figure 1). The products of one meiotic event (but in duplicate) line up in a linear order within each ascus, thus, preserving a meiotic event that can be observed. Many of these mature asci form inside of a larger structure called a perithecium (the asci look like pea pods inside of a large sac). This linear arrangement of meiotic products in Sordaria provides a means of reconstructing crossing-over events during meiosis and of measuring gene distances from crossover data. The wild type ascospores of Sordaria are dark brown. In this laboratory, two different spore color mutants, t (for tan) and g (for gray) are being studied. The corresponding wild type strain for these mutants would have the genes t+ and g+, as the mutations occur at two different loci that affect spore color.
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