Question

using the results of the transduction experiment below, determine the order of the genes

phenotype	number of transductants
F+ M- R-	116
F+ M+ R-	447
F+ M- R+	7
F- M+ R+	490

total number: 1060

Is M closer to F or R?

the answer is that M is closer to F not R, but if cotransduction frequency for FM is 42 (447/1060) and frequency for MR is 46 (490/1060) how can this be? is there something I'm missing?

clear explanation please!

Answer 1

To check the transduction frequency we need to assume the donor is with a phenotype F+M+R+ and the recipient is with a phenotype F-M-R-

Now among the recombinants F+M+ has a cotransduction frequency (F+M+R-)= 447/1060= 0.421

While the cotransduction frequency of F+R+ in the recombinant ( F+M-R+)= 7/1060= 0.0066 which is clearly less than F+M+.

So F needs to be closer to M than R.

So the gene arrangement will be either FMR or MFR

Now the least number of recombinants F+M-R+ is produced by highest number of recombinant events. In FMR two recombinant events take place , once at F and once at R .But in case of MFR only one recombinant event takes place containing both FR together. So FMR should be the correct gene order.

Now you are right in saying M will be closer to R than F because transduction frequency of FM is less than MR.