This article is from the Rhodesian Ridgebacks Breed FAQ, by email@example.com (Richard Gordon) with numerous contributions by others.
Ridgelessness is not a "medical problem" per se, but a genetic fault.
That is, some Rhodesian Ridgebacks are born without ridges. Again,
many breeders euthanize these puppies, a subject of some controversy.
A puppy's ridge is very visible when he or she is whelped, but may
sometimes become harder to judge for estimating "show potential" for
the puppy's first several weeks of life (i.e., the ridge remains
clearly visible but the "crowns" or "whorls" may become indistinct for
a few weeks). However, the ridge does _not_ disappear and reappear.
Further, a Ridgeback's ridge will _not_ develop later in life, so do
not believe the breeder that tells you the ridge will "come in later."
If you buy a ridgeless puppy or one with a short or imperfect ridge,
you should expect a lower price and a spay/neuter contract.