Three-factor Crosses

To confirm the order of the metA, aceA, and iclR genes determined from two-factor crosses, three-factor crosses were done. Given the results shown below, what is the order of these three genes?

DonormetA::Tn10 iclR-ace+
Recipientmet+ iclR+ace-

Selected phenotypeRecombinantsNumber obtained
TetRIclR- Ace+300
IclR- Ace-3
IclR+ Ace+30
IclR+ Ace-60

ANSWER: The rare class of recombinants would require a double cross-over as shown in the diagram below: