Labeling convention for a lineage tree.

(a) Each lineal position is labeled with a binary number. The founder of the tree is located at generation g = 1. (b) Each subtree is labeled with two indices (ℓ, τ) representing the longitudinal (ℓ) and transverse (τ) coordinates of its root. Because, as we discuss later, roots of subtrees are associated with sources of variation we need to create a ‘subtree’ located outside the lineage, called (0, 0) (in red), to represent variation among pedigrees. Note that, in an unordered tree, τ values are unidentifiable and will often be ignored.