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tetrads - BICD100 Genetics W08 Tetrad Analysis Handout...

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Tetrad Analysis In some organisms, mainly fungi, the four haploid products of a meiosis form spores that are held together in a sac (an ascus ). The four products of a single meiosis = a tetrad . Because fungi can grow as haploids we can directly see the genotype--no need for a test cross! The additional information gained from being able to detect all 4 meiotic products allows accurate distances to be calculated without a mapping function Tetrads are of two kinds depending on the arrangement of the spores: Spores jumbled up in tetrad = unordered / nonlinear tetrad Spores in same order as divisions in meiosis = ordered / linear tetrad Ordered tetrads give additional information about the distance of genes from their centromere. A. Unordered ( nonlinear ) tetrads (yeast) A yeast cross is always between haploids of different mating type (a x α ) giving an a/ α diploid that will sporulate 2 a spores and 2 α spores. In a cross AB x ab the diploid is AaBb and the 3 possible tetrads are: Spore genotypes Tetrad type If genes same chromosome If genes different chromosome AB AB ab ab Parental ditype PD NCO or 2-strand DCO NCO between either gene and centromere Ab Ab aB aB Nonparental ditype NPD Four-strand DCO between Aa and Bb NCO between either gene and centromere AB Ab aB ab Tetratype TT (or T) SCO between Aa and Bb OR 3-strand DCO SCO between one gene and its centromere
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