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Sample Exam question-have fun!!


I will post a few more next week in preparation for the final exam.

4. In corn, synthesis of purple pigment is controlled by two genes acting sequentially through colourless ('white') intermediates

                  gene A                         gene B

white 1-----------------> white 2-----------------> purple

UAG nonsense mutations were obtained in the genes A and B, and we can call them an and bn. These both lacked enzyme activity and were recessive to their wild type alleles A and B. A nonsense mutation is caused by a nucleotide change that results in a stop codon; in this case the same nonsense codon was produced, UAG.

A nonsense suppressor is also available; it is a mutation in a duplicate copy of a tRNA gene. The mutational site is in the anticodon and this causes the tRNA to insert the amino acid tryptophan at the nonsense codon UAG, thereby allowing the protein synthesis machinery to complete translation, and the result is a wild type phenotype. We will represent the nonsense suppressor allele as Ts and its wild type allele T+. Neither Ts nor T+ have any detectable effect on the wild type alleles A and B or on any other aspect of the phenotype.

i). (6 points) Would you expect Ts to be dominant to T+ or not? Explain.




ii). (14 points) A trihybrid Aan Bbn TsT+ is selfed. If all the genes are unlinked, what phenotypic ratio do you expect in the progeny. Explain carefully. (Assume absence of purple results in a light colour which we can call white.)





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