Genetic ...

 

 According to the studies which were realised in the USA, they discovered that the folded ears came from a natural genetic change, exactly  from only one gene: the Fd gene. 

These studies also showed that the coupling of two cats fold (folded ears) is formally disadvised. The kittens resulting from such crossings were indeed, often victims of congenital osseous lesions

 The best way to get healthy kittens  was, consequently, the coupling of cat with folded ears (Fd), and a cat with right ears (fd).

 The kittens will be obligatorily "Fdfd", and perfectly healthy.

 In a range, we find, in variable proportion, of the kittens with folded ears (Highland Fold), and of the kittens with no folded ears (Highland Straight). The kittens are born like all the other kittens, with small round ears. It is only about the third week that they will be fold.. or not..

 It can happen that a kitten has the ears folded at the third week, then  they are raised a few times afterwards... However, at the age where the kittens are sold (in 3 months), there is no more doubt.