A character type in which numerically increasing changes are allowed but each such change can only happen once on a tree; thus, multiple reverse changes (= losses) are allowed. This character type is favoured by those who feel that a complex structure (e.g. the insect wing) can only originate once, although it may be lost many times. This character type has been suggested for DNA restriction site data, because gain of a new site is much more improbable than loss of an existing one. By definition, a Dollo character is polarised in advance, making the use of an outgroup redundant (cf. ordered, unordered, irreversible).