Instructions

A cross was performed between two haploid strains of yeast one with the genotype B e h and the other of genotype b E H. The B, E and H loci are all tightly linked, with the E locus placed between the B and H loci. Transient diploid cells formed, and then underwent meiosis to produce spores contained in tetrads. Upon examination of many tetrads produced from this cross, one tetrad was identified containing spores of the following genotypes: Beh, BEh, bEH bEH The molecular events of recombination are thought to follow the double-strand break model. For this question, you do not need to explain how the heteroduplexes arose, but do need to discuss later relevant events. a) Using a line to represent a single strand of DNA, draw a diagram of all chromatids of a homologous pair of chromosomes during the Holliday intermediate stage of the double-strand break model of recombination. Label Holliday junctions, heteroduplexes, sister chromatids and non-sister chromatids 4 marks] b) With reference to your diagram from (a), explain the events that occurred to produce the number of E and e alleles observed in the tetrad above. What other numbers of E and e is expected with the same low frequency? 4 marks] c) With reference to your diagram from (a), explain the events that occurred to produce the observed arrangement of alleles at the B and H loci in the tetrad above. Assuming no change to the outcome at the E locus, what other tetrad is expected with equal frequency? (i.e. consider the B and H loci, and give genotypes for all four product of the meiosis) 4 marks]

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