From Creatures Wikia
The gene header is a set of data at the beginning of each gene that tells about when the gene comes into use.
It contains the life stage that the gene switches on at, and whether the gene affects males, females, or both sexes.
It also controls whether the gene can be duplicated (copied), whether it can mutate, and whether it can be deleted or made dormant.
Degree: The degree of mutation probability - 0 is less mutable, 255 is maximum mutability.
Variant: Used in Creatures Adventures to influence norn personality. While it exists in the C3 genetics kit, according to the official help file, it has no function. Testing of the feature, however, has shown that it does work in C3/DS: if a creature has a genome which contains different variants of genes, then the game will randomly choose one variant for it to express.
Organ: If the gene belongs inside an organ, it will be listed.