If you start out with two pure breeing parents and cross them to get an F1 generation then self-cross the F1 generation with itself ( self-cross ) then cross the F2 with themselves ( self the F2 gen ) you will 3:1 ratio of the dominant phenotype in the F3 generation. Can anyone show me how to do the punnett square for the selfing of the F2 generation ? The thing I dont understand is how to do a punnett square when there are three different gamete combinations being crossed. This seems impossible to me. Thank you for your help.