Why is H the answer Consider the cross Hfr strS eryS arg met

Why is H the answer?

Consider the cross: Hfr: str^S ery^S arg+ met- gal-lac-lys- x F^-: str^R ery^R arg- met-gal+lac+ lys+ where the genetic map is shown below (the

Solution

arg              gal

                              Hfr1                       Hfr3

                                                          Lac

                               Str                         Hfr2

                                                         ery

                                     H

                                   lys                 met

During conugation technique, the part of DNA is inserted into the F^- cells from Hfr cells then giving the characters of Hfr to F^- cells. According to genetic map of DNA both Hfr and F^- cells, the streptomycin resistance or sensitive part is located on the other part from other genes i.e arg, met, gal, lac, and lys which are negative for F^- cells while positive for Hfr cells. After conjugation, the genetical part for streptomycin resistance will be present while other genes will be replaced by Hfr genes.

Streptomycin - sensitivity allele ( str^s ) and streptomycin- resistance allele ( str^r ) in the recipient are used to kill the Hfr cells after conjugation.

          

             

                                                  Chromosome integrated with F factor

            F factor                           Hfr cell

(Chromosome) F⁺ cells                     F⁺ cell             

                                                  

                                

                                               Hfr cell

                                             Chromosome transfer across the F pilus

                                        

                              

DNA complement is synthesized