Two yeast strains were mated and sporulated (allowed to carry out meiosis). One strain was a haploid with normal chromosomes and the linked genetic markers ura3 (requires uracil for growth) and arg9 (requires arginine for growth) surrounding their centromeres. The other strain was wild type for the two markers $(U R A 3 \text { and } A R G 9)$ but had an inversion in this region of the chromosome as shown here in pink: During meiosis, several different kinds of crossover events could occur. For each of the following events, give the genotype and phenotype of the resulting four haploid spores. Assume that any chromosomal deficiencies are lethal in haploid yeast. Do not consider crossovers between sister chromatids. a. $A$ single crossover outside the inverted region b. A single crossover between $U R A 3$ and the centromere c. A double crossover involving the same two chromatids each time, where one crossover occurs between $U R A 3$ and the centromere and the other occurs between $A R G 9$ and the centromere
Added by Manuel W.
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A single crossover outside the inverted region: In this case, the crossover occurs outside the inverted region, so the resulting genotypes and phenotypes of the four haploid spores will be: Show more…
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Suppose a haploid yeast strain carrying two recessive linked markers his4 and leu2 was crossed with a strain that was wild type for $H I S 4$ and $L E U 2$ but had an inversion of this region of the chromosome as shown here in blue. Several different kinds of crossover events could occur during meiosis in the resulting diploid. For each of the following events, state the genotype and phenotype of the resulting four haploid spores. Do not consider crossover events between chromatids attached to the same centromere. a. A single crossover between the markers HIS4 and $L E U 2$ b. A double crossover involving the same chromatids each time, where both crossovers occur between the markers $H I S 4$ and $L E U 2$ c. A single crossover between the centromere and the beginning of the inverted region
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A yeast geneticist irradiates haploid cells of a strain that is an adenine-requiring auxotrophic mutant, caused by mutation of the gene ade1. Millions of the irradiated cells are plated on minimal medium, and a small number of cells divide and produce prototrophic colonies. These colonies are crossed individually with a wildtype strain. Two types of results are obtained: (1) prototroph $\times$ wild type : progeny all prototrophic (2) prototroph $\times$ wild type : progeny $75 \%$ prototrophic, $25 \%$ adenine-requiring auxotrophs a. Explain the difference between these two types of results. b. Write the genotypes of the prototrophs in each case. c. What progeny phenotypes and ratios do you predict from crossing a prototroph of type 2 by the original ade1 auxotroph?
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