What type of allelic interaction is this

A lilly from a true-breeding line with white flowers and short corollas (line A) is crossed to a lilly from a true-breeding line with black flowers and long corollas (line B). The resulting offspring, the F1, are all red-flowered, with corollas of medium length. Suppose an individual from the F1 is crossed to an individual from line A. The resulting offspring are as follows: 50% white flowers and short corollas, 50% red flowers and medium corollas. What type of allelic interaction is this? How many loci are involved? Diagram the cross.