Both cats missing teeth - genetic problem or something else?

Question
I have two cats, the oldest being about 3 years old and the younger cat 2 years old. I have noticed that both cats are missing a few of the very small teeth between their canines (the incisors?) There is no sign of infection or swelling, and  it doesn't seem to affect the cats' eating habits. The teeth actually look like they have been gone for a long time.

Both of these cats are strictly indoors, and normally have wet food (all the brands of dry food we've tried give them the runs, and our vet doesn't know why.) They do not often fight with each other either, so don't think they've lost them that way.

They are both purebred Abyssinians, and have very small jaws (their teeth aren't crowded together, though.) Could this be a birth defect or something caused by inbreeding? Or is it more likely to be a different problem?

Answer
Sincere apologies for the delayed response.

The age of the cats seems rather young for significant dental disease and resorption of bone.   Were you able to verify if they did have adult teeth in this area?  In some cases yes there are some born with the defect of not having them, and this can be seen running in family lines.

If however the teeth were there and have been lost I would suggest have them looked at by your veterinarian to make sure there is not underlying nutritional deficiency.