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