In radishes the shape may be long or round or oval. The color may be red or white or purple. A long red variety was crossed with a round white variety. The F1 were all oval purple. In the F2 the following results were obtained: 8 long red, 15 long purple, 17 oval red, 31 oval purple, 7 long white, 15 oval white, 8 round red, 16 round purple, 9 round white. What is the genetic basis for these characters?