1. (in plant taxonomy) A plant specimen that is a duplicate of or very similar to the type specimen and can be used as a reference specimen if the type specimen is lost.

2. (in genetics) A variant product of a gene or gene cluster arising from the translation and assembly of alternative transcripts. For example, the classes of immunoglobulins (IgM, IgG, IgD, IgA, and IgE) are isotypes, each characterized by a particular constant region of the heavy chain, encoded by one of the immunoglobulin C-region genes.