Mixed  infections  of  two rIIA mutants on E. coli B produce wild-type
   recombinants that can infect the K12 strain (left). The plate at right
   is a B strain control that reflect total phage progeny.

   To calculate the frequency of recombination:
   2 (recombinant phages) / total phages):
   2 (4 * 10^3 / 8 * 10^9) = 2 (0.5 * 10^-6) = 10^-6
   We  need  to  multiply  by  2  since  only  one  of the two reciprocal
   recombinants is detected.