Imagine you do a test cross between a purple-flowered pea plant having serrated leaves (a dominant trait) and a white-flowered pea plant having smooth edges. If the purple-flowered plant is heterozygous for both traits, the expected ratio in the offspring is 1 purple-serrated: 1 purple-smooth:1 white-serrated:1 white-smooth. Instead, you see 4 purple-serrated:1 purple-smooth:1 white-serrated:4 white-smooth. What is the explanation of this ratio?