Explain the test - cross. I have to explain the usefullness of the test-cross. I had my paper written then was told I need to incorporate the Punnett square, monohybrid cross, dihybrid cross etc. into the paper. I am now confused - maybe I have just researched so much I cannot make sense of any of it. How is the Punnett Square, test cross, monohybrid cross, dihybrid cross all tied in together?