Genetics
The fold is inherited as an autosomal dominant trait, possibly with incomplete penetrance. A cat with folded ears may have either one(heterozygous) or two copies(homozygous) of the fold gene (Fd). A cat with normal ears should have two copies of the normal gene (fd).
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Mating a homozygous fold with any cat will produce all folds, but because homozygous folds are prone to severe health issues, breeding for them is generally considered unethical. A homozygous to normal mating will produce only heterozygous folds but presumably in ethical breeding programs, there will be no homozygous cats available to breed from.
| Straight-eared | |||
|---|---|---|---|
| fd | fd | ||
| Heterozygous fold | Fd | Fd fd | Fd fd |
| fd | fd fd | fd fd | |
The only generally accepted breeding gives a 50% chance of producing heterozygous folds and 50% chance of producing normals.
| Heterozygous fold | |||
|---|---|---|---|
| Fd | fd | ||
| Heterozygous fold | Fd | Fd Fd | Fd fd |
| fd | fd Fd | fd fd | |
There is suspicion that some non-fold litters are genetically heterozygous folds but because of very low expression of the gene, appear to be straight-eared. Such kittens may develop folded ears initially which then straighten back out. Because of this there are suggestions by some breeders to avoid mating Folds with straight-eared Scottish Folds but only use British Shorthairs(BSH) as outcross. If Scottish Shorthairs are to be used, they should be test mated to a BSH to make sure that they are not genetically folds. If such apparent straight-eared cats are mated with a fold, there is a 75% chance of folds(25% homozygous folds, 50% heterozygous folds) and 25% chance of straight ears.
Read more about this topic: Scottish Fold