Various mutations of common species exhibit columnar form much narrower than the normal form. From observation these skinny forms come in two flavours: A: Branch angle is decreased, making the branch closer to parallel with the source twig. Example: Parkland Pillar Asian Birch. B: Receptor for the growth hormones for apical dominance don't work. Every branch 'thinks' that it is the leader and hence heads straight up. Example Swedish Aspen. As a model of fastigate tree genetics would the following be reasonable: * The defective gene for either A or B is fairly scarce in the general population. * The defective gene is recessive. * Consequently the expression of the gene is fairly rare, as both parents have to be carriers. A colleague of mine proposed an alternative that fastigate trees are polygenetic -- It takes the right combination of multiple genes to exhibit the trait. Is there any way to tell in a particular case which model (or neither) is applicable?