The Differences Between Inbreeding, Line Breeding, and Outcrossing

Katia Kingston

A good breeder should have a goal in mind. A primary goal is the one everyone sees—a distinct, recognizable appearance. After all, this is the number one reason breeders choose to breed for a specific look in any breed and why pet owners choose to buy from a specific breeder. However, when the focus of a breeding program is solely on appearance, that is not necessarily what is best for the breed. Breeders should also remember that buying pedigreed cats is about having a known, purposefully selected health history.

In general, breeders face the choice of using inbreeding, line breeding, or outcrossing to achieve their goals. One way or another, a responsible breeder must be mindful of genetic diversity because, if it becomes dangerously low within a breed, it can lead to higher levels of illness and, ultimately, extinction. Is it better to address genetic diversity before it gets too low? Absolutely! Is it possible to increase genetic diversity in a compromised breed? Absolutely, but not through inbreeding! So, what are the differences between inbreeding, line breeding, and outcrossing within the same breed?

Inbreeding

Inbreeding reduces genetic variability within a breed. It involves mating close relatives—full siblings (brother and sister), parents and offspring, second-degree relatives (grandparents and offspring, half-siblings, and relatives such as uncles/aunts and nephews/nieces), and even double first cousins.

Inbreeding increases the probability that animals will share desired traits, such as physical appearance, because they are genetically similar. However, the negative effects of inbreeding can accumulate over time. Inbred animals are more likely to have genetic defects and inherited diseases, which can be extremely detrimental to their health and welfare. In other words, they may not live long, healthy, and happy lives.

Some examples include Polycystic Kidney Disease (leading to kidney failure) in Persians and Siamese, Hypertrophic Cardiomyopathy in Maine Coon cats, mitral valve disease, and spinal abnormalities in dogs. Additionally, inbreeding can lead to inbreeding depression, which can cause fertility problems, reduced offspring survival, and weakened immune systems. This is where breeders can experience many heartbreaking situations while raising their kittens.

Line Breeding

Line breeding is typically done to lock in desirable traits. It seeks to pass outstanding genetics from one generation to the next. Line breeding involves mating more distantly related animals, often choosing parents who share a common ancestor but are not as closely related as those used in inbreeding.

The art of reading pedigrees is crucial when selecting the right parents. Breeders can use information from their ancestors to help predict potential outcomes. The aim is generally to improve or maintain specific physical traits within the breed. However, line breeding still carries risks of health issues and welfare concerns, as described above.

Outcrossing

Outcrossing in cat breeds is the practice of mating cats from different lines to create offspring. It is a planned process aimed at increasing genetic diversity and potentially improving the health and genetic fitness of the breed.

This is where the real art of reading pedigrees comes into play. For example, many Siamese catteries in the USA work with pedigrees that are completely different from one another. There should be no repeated cats within at least five generations.

For example, if Cattery A works with pedigrees that are completely different from those of Cattery B, and the two catteries work together, this can create a foundation for successful outcrossing and promote significant genetic diversity.

Mating two unrelated Siamese cats can produce healthy, beautiful kittens with outstanding personalities. Some breeders say that we all started with the same cats. That does not matter! Each cat comes with its own genetic makeup, and each new breeding creates new genetic combinations.

As long as the inbreeding coefficient is below 6.25%, it may be considered an acceptable level for a breeding decision, although the coefficient alone does not guarantee that a breeding will be free of genetic disease. A lower coefficient means that the new kittens are less likely to inherit two copies of the same allele that are identical by descent.

Ultimately, breeders understand that health, beauty, and temperament are genetically influenced and inherited, while environmental factors also play an important role. Litter size and kitten viability are influenced by numerous genetic and environmental factors and should not be attributed to heterozygosity alone.

Inbreeding

• Involves mating parents and offspring, siblings, or other close relatives.
• Reduces genetic variability within the cattery.
• Increases the likelihood of genetic defects and inherited diseases.
• Can contribute to inbreeding depression, including reduced fertility, reduced offspring survival, and impaired immune function.

Line Breeding

• Involves mating animals that are more distantly related than those used in close inbreeding.
• Is a strategic way to lock in desirable traits.
• Increases homozygosity within the cattery.
• Is used to develop animals with specific physical traits.
• Can lead to underlying health issues if the breeder is unaware of health conditions in some of the cats within the pedigrees.

Outcrossing

• Can increase genetic diversity and heterozygosity.
• Can contribute to hybrid vigor.
• Can reduce the probability that offspring will inherit two copies of the same harmful recessive variant.
• May contribute to improved genetic fitness and, in some circumstances, longevity.
• Outcrossed kittens can have unique physical characteristics.
• It can be more difficult to predict which bloodlines will work well together in a cross.

Vocabulary

Allele – A different form of the same gene.

Genetic defect – An inherited medical condition caused by a genetic or DNA abnormality.

Homozygous individual – An individual that carries two identical alleles of the same gene.

Heterozygous individual – An individual that carries two different alleles of the same gene.

Hybrid vigor – Also called heterosis, a phenomenon in which offspring produced from genetically different parents may demonstrate improved biological performance or fitness compared with the parental populations. The effects of hybrid vigor vary depending on the traits and populations involved.

Inbreeding coefficient – A measure of the probability that an individual carries two alleles at a particular locus that are identical by descent.

Inherited disease – A genetic disorder that can be passed from parents to offspring. Some examples in cats include heart conditions such as hypertrophic cardiomyopathy (HCM), urinary and digestive diseases such as FLUTD and pyloric stenosis, progressive retinal atrophy, and deafness.

Immune system – The body’s system that protects it from infections and other diseases.

Pathogen – A microorganism or other biological agent that can cause disease in a host organism.

References

Avalyn, H. (2014). Inbreeding, outcrossing and line breeding. American Classic Pedigrees. www.americanclassicpedigrees.com

Charlesworth, D., & Willis, J. U. (2009). The genetics of inbreeding depression. Nature Reviews Genetics. https://pubmed.ncbi.nlm.nih.gov/

Raven, P. H., et al. (2013). Raven biology (10th ed., AP Student Edition). McGraw-Hill Education.

Vogt, D., Swartz, H., & Massey, J. (2021, March). Inbreeding: Its meaning, uses and effects on farm animals. University of Missouri Extension. https://extension.missouri.edu/publications

Windig, J., & Doekes, H. (2009). Limits to genetic rescue by outcross in pedigree dogs. https://pubmed.ncbi.nlm.nih.gov/