Origins of randomness [in animal patterning]

The model in the main text assumes that randomness enters through initial conditions. If the two parts of a single animal—say opposite wings on a butterfly—form together, then these initial conditions can be expected to be the same. But usually even the two sides of a single animal are never physically together, and they normally end up having quite uncorrelated random features. In cases such as fingerprints and zebra stripes there is some correlation between different sides, suggesting an intrinsic component to the randomness that occurs. (The fingerprints of identical twins are typically similar but not identical; iris patterns are quite different.) Note that at least sometimes random initial patterns are formed by cells that have the same type, but different lineages—as in cells expressing genes from the two different X chromosomes in a female animal such as a typical tortoiseshell cat. (In general, quite a few traits—particularly related to aging—can show significant variation in strains of organisms that are genetically identical.)