Progressive Greying is the name of the heritable mutation that defines the London Fancy. It is the genetic mechanism responsible for the contrast that characterises the breed.
How it works
The bird is born with dark plumage. After the juvenile plumage, a progressive loss of melanin-producing cells begins in the body — which lightens with each moult. The wings and tail, however, keep their melanin, creating the dark-on-light contrast that defines a good London Fancy.
It is not spangling
It is essential not to confuse the two: Progressive Greying is not the same mechanism that produces spangling in the Lizard. They are different mutations with different results. It was precisely this distinction that Hein van Grouw, Senior Curator of Birds at the Natural History Museum in Tring, documented in his work on colour aberrations in birds.
One genetic family
Melanics, Spangled and Classics are all expressions of the same Progressive Greying mutation, at different stages of selection. They are the same genetic family — which is why they cross with one another successfully, without ever resorting to the Lizard.
Related: How to pair London Fancy · The Manifesto