Defining Inbreeding in Cats
Inbreeding refers to the mating of closely related cats, such as siblings or parent/offspring. There are two main types of inbreeding:
Linebreeding involves mating cats that have common ancestors several generations back, such as cousins. This concentrates desirable genes from those ancestors.
Close inbreeding is the mating of cats that are closely related, such as siblings, parent/offspring, or half-siblings. This can quickly expose harmful recessive genes.
The key difference is that linebreeding focuses on concentrating desirable traits from earlier ancestors, while close inbreeding can rapidly increase homozygosity.
Risks of Inbreeding
Inbreeding in cats increases the risk of genetic defects and health issues. When closely related cats are bred, harmful recessive genes are more likely to be expressed. These genes can lead to conditions like heart defects, immune system disorders, kidney disease, cancer, and hypothyroidism (Casal et al., 2022).
Inbred kittens may also have physical defects like crooked tails, abnormal eye placement, cleft palates, and deformities of the nose and jaw. The risk increases with higher levels of inbreeding, as rare defects become concentrated in the limited gene pool (Problems with Inbreeding Cats).
Inbreeding also negatively impacts fertility and litter sizes in cats. Tight inbreeding often leads to smaller litters, failure to conceive, low sperm quality, and increased neonatal mortality. Studies show significant declines in fertility once the inbreeding coefficient surpasses 6.25% (Casal et al., 2022).
Acceptable Levels of Inbreeding
The coefficient of inbreeding is a common way to measure the degree of inbreeding in a cat’s pedigree. It represents the probability that an individual cat inherits two copies of the same gene from an ancestor that appears on both sides of the pedigree.
According to the OFA (https://www.ofa.org/diseases/other-diseases/inbreeding-coefficient), an inbreeding coefficient of less than 6.25% (equivalent to a mating of first cousins) is generally considered low risk for inherited disorders in cats. However, some cat breeders and registries aim for even lower coefficients.
For example, the Cat Fanciers Association (CFA) recommends breeding cats with no more than a 3.125% inbreeding coefficient, which is equivalent to a mating between second cousins. The European Burmese Cat Society suggests coefficients below 1% are ideal. Other registries or breeders may set limits between 0-10%.
In general, inbreeding coefficients under 10% are unlikely to cause immediate health issues in the offspring. However, coefficients above 10% significantly increase risks for inbreeding depression and inherited disorders. Therefore, responsible cat breeders take steps to minimize inbreeding and prioritize outcrosses when possible.
Managing Inbreeding
Responsible breeders use several strategies to minimize the risks of inbreeding while maintaining desired breed characteristics.
Outcrossing involves breeding a cat to an unrelated individual, usually from another cattery. This increases genetic diversity and reduces the probability of inheriting two copies of a detrimental recessive gene. Breeders should plan outcrosses several generations in advance when creating their breeding program [1].
Health testing allows breeders to screen for genetic diseases before breeding. Tests are available for many inherited diseases in cats, including heart anomalies, blood disorders, and metabolic defects. Cats who test positive for genetic disorders should not be used for breeding [2].
Breeding organizations often recommend limits on the degree of inbreeding. For example, maximum inbreeding coefficients may be set at 5-10%. Following these guidelines reduces the accumulation of detrimental genes over generations [3].
With careful planning and health screening, breeders can work to minimize risks while preserving desired breed traits and phenotypes.
Inbreeding in Purebred Cats
Inbreeding is quite common in purebred cat breeds in order to maintain desirable traits and conform to breed standards. Certain breeds like Persians and Exotic Shorthairs have very high rates of inbreeding with coefficients ranging from 1.98% to 5.5% (https://journals.sagepub.com/doi/full/10.1177/1098612X221118755).
Breeding practices like linebreeding and inbreeding are used to “set” desirable traits and create uniformity. Popular breeds like Ragdolls, Bengals, and Sphinx cats have limited gene pools and high inbreeding rates. Breeders often mate close relatives like siblings or parent-offspring.
Cat registries like TICA and CFA restrict some first degree relative matings like father-daughter or mother-son. However, matings between siblings, half-siblings, aunts/uncles with nieces/nephews are allowed after registration. There are no limits on coefficient of inbreeding for registered cats (https://labgenvet.ca/en/cat-genetics-4-0-evolution-breeds-breeding-strategies-and-inbreeding/).
Inbreeding in Non-Pedigreed Cats
Backyard breeders and pet cat owners may not keep detailed breeding records, increasing the risk of accidental inbreeding. According to https://animals.mom.com/problems-with-inbreeding-cats-5105419.html, closely related cats such as siblings or parent and offspring have a higher chance of being mated, since cat owners may not know the genetic history. This elevates the risk of inherited diseases and other issues.
Feral cat colonies can also experience high rates of inbreeding due to limited genetic diversity within the colony. Unneutered feral cats will breed with any available partner, regardless of relation. Over multiple generations this leads to increased homozygosity and heightened health concerns like heart defects and low fertility rates (https://journals.sagepub.com/doi/full/10.1177/1098612X221118755). Trap-neuter-return (TNR) programs help control inbreeding by preventing unwanted litters.
Inbreeding Depression
Inbreeding depression refers to the reduced biological fitness and vigor that occurs in a population as a result of breeding of related individuals.[1] In cats, inbreeding depression can have detrimental effects on health and viability. Specifically, it increases homozygosity, which can increase the chances of expressing deleterious recessive alleles.[1] This can result in decreased litter sizes, higher infant mortality rates, reduced fertility, compromised immune function, and increased prevalence of genetic disorders.[2]
The effects of inbreeding depression tend to compound over generations of continued inbreeding. With each generation, individuals become more homozygous and deleterious recessive alleles have a higher probability of being expressed.[3] This leads to a gradual reduction in fitness and health. Pedigreed cat breeds are at particular risk due to closed registries and the limited genetic diversity in many breeds. However, outcrossing to unrelated individuals can reintroduce genetic diversity and restore vigor.
Balancing Inbreeding and Genetic Diversity
Cat breeders must find a delicate balance between maintaining desired breed traits through inbreeding while also introducing enough genetic diversity to avoid inbreeding depression. This is done through careful outcrossing, which refers to breeding between two unrelated individuals that are members of the same breed. Outcrossing reintroduces genetic diversity, which leads to improved health and vigor, known as hybrid vigor or heterosis.
When inbreeding to set breed type, breeders should limit it to only the amount needed to establish the distinctive traits. Extensive inbreeding solely to preserve bloodlines can concentrate negative recessive genes. Once the breed characteristics are fixed, occasional outcrosses with unrelated breeding stock can improve health and fertility [1]. Outcrossing once every 4-6 generations can reduce inbreeding depression while mostly retaining desired phenotypes [2].
For non-pedigreed random bred cats, acquiring breeding cats that are as genetically unrelated as possible promotes hybrid vigor in the kittens. Responsible cat breeding requires carefully balancing inbreeding with outcrossing to safeguard genetic diversity and cat health.
Ethical Considerations
Responsible cat breeding requires considering animal welfare and genetics.
According to the European Commission’s “Responsible Cat Breeding Guidelines” document, breeders should avoid mating cats that are closely related, which can increase harmful genetic conditions. Mating cousins or other close relatives should be avoided. The guidelines recommend breeding cats with a coefficient of inbreeding below 6.25%.
Breeders should also follow ethical practices like health testing, ensuring cats are the proper age for breeding, avoiding overbreeding females, and properly socializing kittens. Kittens should stay with the mother cat until at least 12 weeks old and receive proper veterinary care. Breeders should also carefully screen potential owners.
Following these responsible breeding practices can help ensure the welfare of breeding cats and their kittens while also maintaining genetic diversity.
The Bottom Line
A low level of inbreeding (10-20% COI) is generally acceptable for cats. However, higher levels (above 25%) are associated with increased health risks and should be avoided or carefully managed. Purebred cats usually have higher COIs due to closed breeding pools, so breeders should focus on diversity and outcrossing. For regular house cats, any deliberate inbreeding should be reconsidered due to ethical concerns and risks. Bottom line – some limited inbreeding may be unavoidable, but higher levels should raise red flags.
Cat owners and breeders should:
– Calculate COIs for potential pairings and aim for less than 20% similarity. Monitor COI trends over generations.
– Prioritize genetic diversity through outcrossing and introduction of unrelated cats. Avoid repeatedly using the same studs.
– Screen breeding cats for genetic issues and do not breed affected cats.
– If inbreeding depression occurs, reevaluate the breeding program.
– When acquiring a new cat, ask about its COI and health testing.