The Pros and Cons of Inbreeding in Cats

Inbreeding is the mating together of closely related cats, for example mother/son, father/daughter, sibling/sibling matings and half-sibling/half-sibling. It is the pairing of animals which are more closely related than the average population. For breeders, it is a useful way of fixing traits in a breed - the pedigrees of some exhibition cats show that many of their forebears are closely related.

For example, the name of Fan Tee Cee (shown in the 1960s and 1970s) appeared in more and more Siamese pedigrees, sometimes several times in a single pedigree, as breeders were anxious to make their lines more typey. Superb specimens are always much sought after for stud services or offspring (unless they have already been neutered; cloning may solve that problem in the future) having won the approval of show judges.

To produce cats which closely meet the breed standard, breeders commonly mate together animals which are related and which share desirable characteristics. Over time, sometimes only one or two generations, those characteristics will become homozygous (genetically uniform) and all offspring of the inbred animal will inherit the genes for those characteristics (breed true). Breeders can predict how the offspring will look. "Line-breeding" is not a term used by geneticists, but comes from livestock husbandry. It indicates milder forms of inbreeding. Line-breeding is still a form of inbreeding i.e. breeding within a family line and includes cousin/cousin, aunt/nephew, niece/uncle and grandparent/grandchild. The difference between line-breeding and inbreeding may be defined differently for different species of animals and even for different breeds within the same species. It is complicated by the fact that a cat's half-brother might also be her father!

However, inbreeding holds potential problems. The limited gene-pool caused by continued inbreeding means that deleterious genes become widespread and the breed loses vigor. Laboratory animal suppliers depend on this to create uniform strains of animal which are immuno-depressed or breed true for a particular disorder e.g. epilepsy. Such animals are so inbred as to be genetically identical (clones!), a situation normally only seen in identical twins. Similarly, a controlled amount of inbreeding can be used to fix desirable traits in farm livestock e.g. milk yield, lean/fat ratios, rate of growth etc. In human terms, inbreeding is considered incest; cats do not have incest taboos.

Outcrossing is when the two parents are totally unrelated. In pedigree animals, this often means where a common ancestor does not occur behind either parent within a four or five generation pedigree. In animals with a small foundation gene pool, this condition is difficult to meet. First I will define some terms used by animal breeders. In general I've avoided specialist terms, but you will meet these terms outside of this article.

"Homozygous" means having inherited the same "gene" for a particular trait from both parent e.g. for fur length. Barring random mutation, 100% of the offspring of a homozygous individual will inherit that gene. Inbreeding increases homozygosity by "fixing" a particular trait. Purebred animals display a high degree of homozygosity compared to mixed breeds and random-bred animals. The idea of purebred animals is that they should "breed true". When one purebred is mated with another of the same breed, the offspring will have uniform characteristics and will resemble the parents.

"Heterozygous" means having inherited a different gene for a particular trait from each parent. For example one gene of long fur (recessive) and one gene for short fur (dominant). 50% of a heterozygous individual's offspring will inherit one form and 50% will inherit the other. Carefully controlled "out-crossing" increases heterozygosity for selected traits by introducing new genes into the hybrid offspring.

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