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The difference between mirror and common carp can largely be reduced to the presence of the large mirror-like scales on the former. The mirror-scale phenotype is caused by a genetic mutation for which there are two alleles (i.e., forms of a gene) denoted as the S and the N alleles. The genotype coding for the recess mirror scales 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 individual fish by sight, leading to most carp in the UK over 40 lb being nicknamed. Mirror carp usually belong to the common carp's nominate subspecies, "C. c. carpio".