Although this mutation is more common with canines, it does occur with felines as well. The mutation basically mean that they don't have ears. They have the cavity, with a fully functioning ear drum and can hear and all, but simply don't have the external part. This would be visible since birth, as there would be a cavity in the place of the ear cartilage.

This does mean, however, that their ears can get infected much easier than usual, and that can lead to death in the cub stages. This would be completely random, however, and would not occur always. Some text that could pop up if this is the case would be:
"You had to call (Cub) name several times today for them to look at you. Strange."
"You groomed (Cub), but noticed a foul smell coming from where their ears should be."
"(Cub) is complaining that their ears are hurting a lot. You take a look, but see nothing."

I don't know, this is just a pitch. Let me know what you think!