[3]: 413  The duck may be of any colour; the crest results from the action of an autosomal dominant gene and varies widely in size, from imperceptibly small to a diameter of approximately 13 cm.

[3]: 413 A bantam version of the breed, the Crested Miniature, was bred by John Hall and Roy Sutcliffe in the United Kingdom in the late twentieth century; it was recognised in 1997.

[9]: 90 A crest of feathers may appear in any duck breed or type deriving from the wild Mallard, Anas platyrhynchos, with a frequency of some 0.0001%–0.001% of all eggs hatched.

[9]: 91  The crest allele is associated with a number of abnormalities in the structure of the brain and skull, including larger-than-expected brain size, abnormal cranial bone formation, fontanelles or perforations of the skull, intracranial lipomas or fat bodies, and reduced size of the cerebellum, the olfactory bulb and the prepyriform area;[11]: 1251  with physical deformities including shortening of the body, wry neck and wry tail[9]: 92  and with neurological disorders including impaired motor co-ordination, with symptoms such as an unsteady walk or inability to get up after a fall.

[11]: 1252 Researchers under the Bruno-Dürigen-Institut at the Wissenschaftlichen Geflügelhof of the Bund Deutscher Rassegeflügelzüchter [de] at Rommerskirchen have developed a test to assess the extent of motor impairment in individual ducks: the bird is placed on its back on a flat surface, with its feet in the air, and the time it takes to stand up is measured.