A cross is made between yeast cells with different alleles for a set of linked genes: pr+q × p+rq+. The resulting tetrads show a 3:1 ratio for r+ to r instead of the expected 2:2 ratio. Can you explain how this could have occurred?
What will be an ideal response?
Gene conversion by a mismatch repair mechanism could have caused this deviation from expected ratios. During pairing of homologous chromosomes in meiosis, recombination between the two inner chromatids occurred, resulting in heteroduplex (mismatched) DNA strands. Both mismatches were repaired by excision and DNA synthesis to match the parent DNA with the r+ allele.
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