The Gender domain captures all concepts about the sex of a person, denoting the biological and physiological characteristics. In fact, the Domain (and field in the PERSON table) should probably should be called “sex” rather than “gender”, as gender refers to behaviors, roles, expectations, and activities in society.
The domain contains only two standard concepts: FEMALE (concept_id=8532) and MALE (concept_id=8507). Many data sources contain other codes, such as “Unknown”, “Refused to tell”, “Hermaphrodite”, as well as transgender constellations (“male to female”, etc.). For the current purposes of the OMOP CDM, the gender concepts are used to stratify patients by their biological make-up or to adjust analytical results for the influence of the biological sex. Therefore, all those other genders are denoted as concept_id=0 (unknown information).
There are no relationships defined for Gender, other than mappings from source concepts. However, most ETL handle Gender Concepts directly without the help of mapping tables.