Homology (chemistry)

In chemistry, homology refers to the appearance of homologues. A homologue (also spelled as homolog) is a compound belonging to a series of compounds differing from each other by a repeating unit, such as a methylene group, a peptide residue, etcetera.

A homolog is a special case of an analog. Examples are alkanes and compounds with alkyl sidechains of different length (the repeating unit being a methylene group -CH2-).

On the periodic table, homologous elements share many chemical properties and appear in the same group (column) of the table. For example, all noble gases are colorless, monatomic gases with very low reactivity. These similarities are due to similar structure in their outer shells of valence electrons.

Homology in medicine refers to being from the same family. For example, the gastrointestinal tract hormone gastrin is homologous to cholecystokinin (CCK), meaning they are from the same family of hormones.