Mobs with Genes
When two mobs breed, their offspring has a chance to inherit Genes from their parents. The chance is 50% per parent that has the Gene.
This can have interesting interactions with Genes like Bountiful!
Gene Requirements
Certain Genes may require other Genes be present in the host. If that Gene is not present, the dependent Gene will fade away.
For example, the Photosynthesis Gene requires Eat Grass and Thorns.
The only way to see Gene requirements currently is to check the DNA Helix's information recipes in EMI, or by looking at the source code here.
A way in-game will be made eventually. Probably.
Mutation Genes
Mutation Genes are especially strong Genes, that are generally acquired through special means, such as GMO Cells.
Additionally, mutation Genes often have prerequesites.