Higgins, Des The original author of the CLUSTAL suite of programs for performing automated, progressive multiple sequence alignment. email: des@ebi.ac.uk
 

Homolog This is a gene or morphological character that shares a common ancestry with a different gene or morphological character.
 

Homologous Descended from a common ancestor.  Defined in 1843 by Richard Owen, the British Palaeontologist as the "same organ under every variety of form and function".  For molecular sequence data, it is taken to mean that two sequences or even two characters within sequences are descended from a common ancestor.  This term is frequently mis-used as a synonym for 'similar', as in "two sequences were 70% homologous".  This is totally incorrect!  Sequences show a certain amount of similarity.  From this similarity value, we can probably infer that the sequences are homologous or not.  A statement that is attributed to Walter Fitch is that (I'm prarphrasing here) homology is like pregnancy.  You are either pregnant or not, two sequences are either homologous or they are not.
 

Homology see Homologous.
 

Homoplasy The existance of characters that have been subject to reversals, convergences or parallelisms.
 

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Any Questions? email: James Mcinerney