Mutations in the rII region of a bacteriophage T4 prevent its growth on E. coli strain K. You obtain numerous mutations in this region and want to determine how many genes are located there. You perform double infections of E. coli K with the different pairwise combinations of mutations both in cis and in trans-configuration. (Recall that cis-configuration means that two mutations are located in the same phage genome, and trans means that they are located in different genomes).

Cis-test Trans-test
rIIA rIIB / + + ==> growth rIIA + / + rIIB ==> growth
rIIA rIIC / + + ==> growth rIIA + / + rIIC ==> growth
rIIB rIIC / + + ==> growth rIIB + / + rIIC ==> no growth
rIIA rIID / + + ==> no growth rIIA + / + rIID ==> no growth

What can be concluded from this experiment?