Here's the answer: A feature is genetic if the knowledge for the feature is the person's genes.
This is an example of the general purpose value of epistemology: epistemolgy is needed for solving problems in many other fields.
Let's consider an example. Jack is born and grows up to be a seven foot tall basketball superstar. Is his height genetic? Is his basketball playing genetic?
On both of these questions, some would get confused. They would say the basketball playing is a caused by a "gene-meme interaction" (or "gene-environment interaction"). That is true. Basketball coaches, and parents, are more encouraging to tall children. His cultural environment responds to tallness. And although many would overlook it, becoming tall is not purely genetic but requires environmental help, for example Jack must be fed regularly. The genes alone cannot make Jack tall without the meals.
Some would therefore conclude that both height and basketball skill, and pretty much everything else, are partially genetic.
But let's look at the knowledge. There is no knowledge about basketball in the genes. There is only knowledge about it in coaches, parents, other kids ... in Jack's culture. So basketball playing is not genetic, full stop. Note that this conclusion matches common sense, and also that it can give meaningful answers instead of just vaguely saying everything has multiple causes.
As to height, his parents don't know anything about making Jack tall, but his genes do have knowledge about how to construct a tall person. So height is genetic. Again the knowledge-based definition gets the common sense answer.
 There are some humans ideas about making people tall. You can stretch them. You can feed them "health food". You can bless them. To the extent the parents do those things — and they work — then the height is partially caused by knowledge in the parents.