Talk about simple interaction between genes.

The term gene interaction is often used to express the idea that many genes may interact to affect a single phenotype. When these F1 walnuts are mated in cross 4, all four comb shapes are produced in a ratio that approximates 9:3:3:1. This observation immediately suggests a cross involving two gene pairs, because the resulting data display the same ratio as in Mendel’s dihybrid crosses. Since only one character is involved (comb shape), we propose that the two gene pairs “interact” to produce each phenotype