Peanuts have two types of growth behavior: runner, which grows low to the ground, and bunch, which grows upright and branched. A true-breeding line of runner peanuts is crossed to a true-breeding line of bunch peanuts. All of the F1 progeny grow as runners, so runner appears to be dominant. These F1 runner peanuts are then crossed to true-breeding bunch peanuts (NOT a self-cross) and the progeny show both phenotypes in a ratio of 3 bunch : 1 runner. Clearly one gene cannot explain this. How many genes are involved in the growth behavior phenotype, and what must the genotypes in the cross be? Assume recessive loss-of function mutations are involved.
