BIOL 2107 Lecture 5: Linkage and Chromosome Mapping II

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Two model organisms for understanding mechanisms of recombination include saccharomyces cerevisiae (bakers yeast) and neurospora crassa (bread mold). All four haploid products of each meiosis are contained within an ascus (sac). Ascospores (spore contained in an ascus) can germinate and survive as viable haploids that divide by mitosis. Haploid strains of opposite mating type (a and a) can be mated and the resulting diploid induced to undergo meiosis. Phenotype of haploid fungi is a direct representation of their genotype. Generation of diploid yeast cells that are heterozygous for two unlinked genes his4 trp1 (a) x his4 trp1 (a) his4/his4; trp1/trp1 (a/a) When genes are unlinked, the number of pd = number of npd. When genes are linked, number of pd >> number of npd. How crossovers between linked genes generate different tetrads. Rf = (npd + 1/2t)/total tetrads * 100. Evidence that recombination takes place at the four-strand stage. Evidence for exception to the rule that recombination is reciprocal.

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