The difference between mirror and common carp is both genetic and visual - biologically they are similar. The mirror carp are a mutation of common carp, owing to two alternative genes that control scale development, the Ss pair of alleles and the Nn pair. The genetic term for a mirror carp is "ssnn" (all recessive). Common carp have an even, regular scale pattern, whereas mirrors have irregular and patchy scaling, making many fish unique and possible to identify by sight, leading to most carp in the UK over 40lb being nicknamed. This lack of scales is widely believed to have been bred in by monks in order to make the fish easier to prepare for the table. The current record (as of December 13, 2005) is known as "Two tone" due to its colouring, and is currently around 92lb. The current EU record stands at 88lb (40.188 kg), the fish was caught from a lake in Bordeaux, France and is the first in the world to weigh more than 40KG.
Contrary to popular belief, Leather carp are not Mirror carp without scales; but a variety with a distinct genetic difference. Leather carp have a few scales. However, the dorsal row of scales is either absent or incomplete. Leathers also have reduced numbers of red blood cells compared to mirrors, slowing growth rates.