Question

In a transduction experiment where the recipient is Met-, Ala-, Gal- you observe a recombinant phenotype Met+, Gal+ in some strains and Gal+ Ala+ in some strains. But in none of the cases can you find Met+ Ala+. What is the sequence of the gene?

(a) Met, Ala, Gal

(b) Met, Gal, Ala

(c) Gal, Met, Ala

(d) Gal, Ala, Met

This question was posed to me in semester exam.

My question comes from Gene Mapping in Bacteria by Transduction topic in division Genomics : The Mapping and Sequencing of Genomes of Cytogenetics

Answer 1

Right choice is (b) Met, Gal, Ala

To explain I would say: The recombinant genotype Met+ Ala+ will only result in the case when there is a double cross over such that the middle portion is exchanged twice. This is of least occurrence and is deterministic for the sequence.