For a century or more the costs and benefits of selective close breeding have been recognized, even if the mechanisms weren’t fully understood. Breeders knew from experience that there were risks and trade-offs. The Inbreeding Coefficient was developed specifically to serve as a tool to quantify these in a way that could be used in making breeding decisions.
Inbreeding increases homozygosity (genetic similarity). The inbreeding coefficient is the probability of inheriting two copies of the same allele from an ancestor that occurs on both sides of the pedigree. These alleles are identical by descent. The coefficient is the fraction of all the genes of an animal that are homozygous (two copies of the same allele), so for a mating that would result in offspring with an inbreeding coefficient of 10%, there is a one in 10 chance all the genes in an animal will be homozygous.
The original purpose of the coefficient was to give breeders a number that would indicate both the amount of benefit to be gained from inbreeding as well as the magnitude of the risk of deleterious effects they could perhaps expect. The skill for the breeder is to weigh the benefits and risks of a particular mating and judge what is an acceptable balance, offsetting the benefit of homozygosity and consistency in type, with those of potential deleterious effects of similarity e.g. higher risk of disease due. Too much inbreeding can lead to ‘inbreeding depression’, where the deleterious effects begin to outcompete the positive ones. This is characterised by small size, failure to thrive, a higher risk of disease (both genetic and infectious) and fertility problems. A low coefficient will have a low risk but will also only have a modest benefit; a high coefficient would produce more consistency and prepotency (predominance) in the offspring, but there will also be increased risk of a significant loss of vigour and health.
The GCCF is very concerned about close matings and thus requires any kittens bred from these types of matings to be placed on the non-active register in order to prevent the inheritance of any detrimental genetic traits by subsequent generations, unless there is a specific purpose which has been advised by veterinary or genetic counselling and is supported by the relevant BAC and/or approved by Genetics Committee. Exceptions will only be permitted for specific purposes, e.g. very low numbers of cats in the breeding pool due to scarcity etc:
i) Mother to son; ii) Father to daughter; iii) Full siblings.
Levels of Coefficients of Inbreeding (COIs)
| Percentage | Level | Comments |
| 0 to 10% | Low | COIs which fall within this low banding are ideal |
| 10 to 20% | Fair | COIs which fall within this banding are reasonable and acceptable |
| 20 to 25% | Close | COIs which fall within this banding represent close matings approaching the higher end of what breeders should normally consider. |
| 25 to 40% | High | Matings producing higher COIs than the first degree matings should only be undertaken by experienced breeders for very specific reasons. If offspring are retained for breeding they should only be used for matings producing low COIs. |
| 40%+ | Not advised | The welfare and health of cats with such high levels of inbreeding is highly likely to be compromised and such matings should not be undertaken. |
