As an illustration of this progression, consider the concept ‘gene’. The idea that inherited traits might be passed from parent to offspring through discrete units first led to abstract notions of some sort of ‘unit of heredity’. Later, tests based on breeding and recombination allowed such a unit to be operationally defined in a rigorous (if sometimes laborious) way. Eventually, alternative definitions based on the physical characteristics of genes began to supplant such tests, so that most modern biologists now see genes as stretches of DNA with particular sequence characteristics, which happen to control inheritance (as opposed to units of heredity that happen to be instantiated in DNA). Evidence that this more physical definition of the gene has, for all practical purposes, replaced the one based on the operation of breeding can be seen in how readily we accept gene annotations for organisms, such as mammoths [1], which we could never actually breed.