A Study of the Genetics of Sordaria fimicola I

A Study of the Genetics of Sordaria fimicola I - BioG104 A...

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BioG104: Chapter 11 January 25, 2008 A study of the genetics of Sordaria fimicola (201-217) It is difficult to infer genotypes from phenotypes in diploid organisms because of dominant-recessive relationships o Easier to do in haploids – have only 1 allele for each gene genotype = phenotype Crossing over: exchange of genetic material between homologous chromosomes during prophase I when double-chromatid, homologous chromosomes are in synapsis o Chiasmata: breakage refusion points during crossing over; exist between synapsed chromosomes Results from 2 homologues being switched in an equal and reciprocal fashion o Used in determining location of gene on chromosome If a chiasmata can form at any point between 2 homologous chromosomes, then the frequency of crossing over in the region between 2 different genes on a chromosome should vary directly with the physical distance between the genes Cross: mating; made between parents whose genotypes are partially known o Each meiotic division produces 4 haploid nuclei collectively called a tetrad o In most organisms, it’s impossible to examine the assortment of alleles in an individual meiotic division because meiotic products are made randomly Neurospora crassa (bread mold) and Sordaria fimicola – meiosis occurs within ascus (isolates each tetrad) and the 4 products of meiosis occur within the ascus in the order in which they arose during meiosis o Tetrad analysis: makes it possible to determine if crossing over has occurred and determine if genes are linked, while mapping locations of genes on chromosomes Genetic make up of each cell can be studied with respect to particular trait and determine the meiotic division that produced the tetrad Sordaria Life Cycle: o Fertilization consists of 2 separate, sequential events: Plasmogamy: fusion of the contents of 2 cells to form 1 cell with 2 haploid nuclei
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Karyogamy: fusion of the haploid nuclei to form a zygote with a diploid nucleus
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A Study of the Genetics of Sordaria fimicola I - BioG104 A...

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