Facial expression genes are used to determine what expressions creatures use when they feel certain drives, helping the player to read their creatures' needs. The facial expressions in Creatures 3 are described as 'hungry', 'scared', 'happy face', 'sleepy', 'tired', 'angry' and 'poorly' (in pain).
The weighting option seen in the gene controls which face is used when a creature has mixed feelings.
Mutations or other alterations to the facial expression genes can make it unclear what drives a creature is feeling. Some breeds may intentionally obfuscate a creature's needs by altering facial expressions: for example the Cry Norn genetic breed, which always has a sad expression.
The original genomes included in Creatures 3 did not change expressions properly. The Expressive Norns fix was later provided.