Not sure what to call it but it's basically the skin type maybe?
There are 4 genes for it:
Feathers, scales, fur and hairless
Fur is the most common and it's automatically in the mutation menu
It gives you +1 cold resistance
For feathers you invite a wanderer with it
+2 flying if you have 2 wings
Maybe 1 cold resistance too
For scales you invite a wanderer with it
Not sure about what it would do
For hairless you spend some time in a hot climate
Disables cold resistance and gives you +2 heat resistance
Maybe disables fur color too or it's just a lighter version of the fur color