“Breeding true” means that the offspring look like their parents. It is achieved by “line breeding” which is a form of inbreeding which ideally does not transfer undesirable traits together with those traits that the breeder wants such as the correct phenotype (appearance as per the breed standard). It is a fine balance being inbreeding which can cause health problems labelled under the umbrella term “inbreeding depression” and creating beautiful cats time and again based on excellent foundation cats.
In the words of the experts1: “As inbreeding causes the loss of genetic variation, the more intense the inbreeding is, the greater the loss becomes. Therefore, the breeder must consider the rate of loss. It is a question of how much of a loss can be tolerated.”
Possible definition of breeding true:
Breeding true means creating offspring that have the same appearance as their parents because breeders engage in line breeding which is a form of controlled inbreeding with the intention of avoiding inbreeding depression but ensuring that offspring are phenotypically correct as per the breed standard at all times.
[Incorrect? Please tell me in a comment and provide a better definition!]
Breeding true means that the parents are homozygous for every trait. Sometimes it is impossible to meet this goal as is the case with the Scottish Fold cat breed. When the dominant gene that causes the folded ears is present in homozygous form the offspring have serious health problems due to cartilage problems. Dr. Desmond Morris describes the homozygous form very nicely, as a gene which is present in “double strength”.
So if you mate one Scottish Fold to another Scottish Fold the kittens all have folded ears but they are also liable to suffer from two serious defects which you can read by clicking on this link. These defective offspring are an example of breeding true but the outcome is undesirable and so breeders have to not breed true which results in some of the offspring having straight ears while the others have folded ears. That’s why you see breeders of the Scottish Fold trying to sell Scottish Fold cats with straight ears. They are called Scottish Straights.
On my understanding of cat breeding, the goal of all purebred cat breeders is to breed true provided the starting point is as near as perfect as they can get it. Then they want the offspring to be of the same appearance. I guess if their line breeding is producing cats that are not of the right type with reference to the breed standard then they have to make adjustments and not breed true until they have created a bloodline which meets the requirements.
I am not a cat breeder so therefore I am writing about this as an outsider based upon my knowledge of the cat fancy. As mentioned, the great difficulty is avoiding inbreeding depression, which I have written about as well before. If you want to read some articles about inbreeding then please click this link. In short, inbreeding depression results in a cat which is inherently unhealthy with a compromised immune system and a shortened lifespan. In the words of the book Robinson’s Genetics for Cat Breeders and Veterinarians:”The animals produced are then below par in growth and health, as well as in breeding.”
If inbreeding results in a major problem with for example infertility careful outcrosses to other breeds should solve the problem.
1. Robinson’s Genetics for Cat Breeders and Veterinarians.