Exam how do we know if a trait is caused by one gene or by two genes that interact? example: dihybrid cross of pure-breeding parents produces three phenotypes in f2 progeny if single gene with incomplete dominance, then f2 progeny should be in 1: 2: 1 ratio if two independently assorting genes and recessive epistasis, then f2 progeny should be in 9: 3: 4 ratio these ratios can appear very similar. further breeding studies can reveal which hypothesis is correct.