Changes to AffectedStatus in Pedigree Element

[azankl]

Referring to Documentation here:

1. Prefer "UNKNOWN" instead of "MISSING". The description of the term also uses 'unknown' ("It is unknown if the individual has the affected phenotype").
2. Do the allowed terms have to be all-caps? I think capitalisation should be ignored, i.e. "Affected", "affected" and "aFFecteD" are all valid, and this needs to be mentioned. Alternatively, if terms have to be all-caps, that should be mentioned.