Chapter 13, Problem 19P

One issue with interrupted-mating experiments such as that in Problem 19 is that gene order may be ambiguous if the genes are close together. Another shortcoming is that such experiments do not provide accurate map distances. The reason is that researchers select for the first Hfr marker transferred into the recipient, but the recovery of F exconjugants with a later Hfr marker is complex, depending both on transfer of the marker into the cell and on crossovers that transfer the marker into the recipient chromosome.

To make more accurate maps, bacterial geneticists often do Hfr × F^- crosses in a different way: They select for exconjugants that contain a late Hfr marker, and then screen for the presence of the earlier markers. This method ensures that all of the markers have entered the F^- cell, so relative gene distances are now accounted for solely by crossover frequencies. Furthermore, gene order is clarified by considering the crossovers responsible for each class of exconjugants.

As an example, suppose you performed the same cross as in Problem 19, but you selected for Arg⁺ exconjugants, and then screened them for the earlier Hfr markers Mal Xyl and Pyr . You obtained the following data:

Exconjugate type	Number of exconjugates
Arg+ Mal+ Xyl+ Pyr+	80
Arg+ Mal+ Xy+l Pyr-	40
Arg+ Mal+ Xyl- Pyr-	20
Arg+ Mal- Xyl- Pyr-	20
Arg+ Mal- Xyl+ Pyr-	1
Arg+ Mal- Xyl- Pyr+	1

a.	Explain why four of the exconjugant types are much more frequent than the other two.
b.	What can you conclude about the relative distances between the four genes?
c.	The data allow you to estimate one other relevant genetic distance. Explain.