Genetic Diversity and the Siamese breed group

The Siamese breed group is in a state of crisis — too many instances of cats dying young, too high kitten mortality, too many health issues. Breeders have been talking amongst ourselves for a while, but until relatively recently, we had only guestimates and hints and ideas as to precisely why.

Now we know. The Siamese breed group is one of the groups of pedigreed cats with the lowest genetic diversity. The average diversity for a random bred cat is 38.8%, for a pedigreed cat is 33.4%, and for the four breeds in the Siamese breed group: Balinese 24.3%, Oriental shorthair, 24.4%, Oriental longhair, 26.0%, and Siamese, 23.9%. Numbers were taken from Optimal Selection Genetic Diversity test as of September 2018.

What is genetic diversity, and why is low genetic diversity an issue? First things first… when a daddy and mommy cat love each other very much… 🙂 Each parent cat gives their kittens one copy of a gene at each location or loci. If both parents give a kitten the same variety of gene at the same location, they are said to be homozygous for that gene. If each parent gives the kitten a different variety of gene at that same location, they are said to be heterozygous for that gene.

Genetic diversity is the measure, across a large number of marker locations, of what percentage of the genes are heterozygous. It’s a really simple concept that impacts vastly more things than you would imagine.

Low genetic diversity has some extremely awesome effects in breeding of purebred animals — it makes them look more and more alike, it makes them act more and more alike, and in the end creates a ‘type’ which is pretty much entirely what purebred breeders are looking for. This is why breeders have used things like inbreeding and linebreeding to ‘set type’ since the Victorian age.

Unfortunately, it has some really, really, really nasty side effects. discusses this better than I could, but I’m going to steal a quote:


lower fertility
lower “vigor”
birth defects
smaller size
fewer offspring
slower growth
higher offspring mortality
shorter lifespan
increase in genetic diseases
reduced “genetic potential” (ability to improve a trait)

Low genetic diversity also has a harmful effect on the immune system .

Those links above are about dogs, but inbreeding depression (the bad side effects of low genetic diversity) have been found and studied in all sorts of creatures, plants and animals both. It’s just that the dog people tend to write the best descriptions for non-geneticists. Low genetic diversity is a well known problem in domestic animal populations.

Now back to our cats. In early 2018, one of the first tests for genetic diveristy in cats came to the market. Practicalcats volunteered to join the TICA State of the Cat Study and health and genetically tested all of the breeding cats in our household — and we were horrified at the results. Our cats are, in many cases, above average for the breed group, but they’re still exceptionally low compared to most other breeds, and outrageously low compared to random bred cats. The average genetic diversity in our cattery, as of August 2018, is 24.3%.

How do we fix this?

There are two ways to increase genetic diversity in a purebred animal. One is to outcross within allowable breeds, and the other is to outcross with unrelated breeds. The first wave of genetic testing through Optimal Selection has found that the best outcrosses for the shorthair breeds are the longhair breeds, and vice versa. But in the end, they can’t add new genes to the breed group, only preserve what is there.

There are some awesome studies that show how closely the various breeds are related, which gives us a great idea of good outcross candidates, and specifically this image.. Thankfully, many of the scientific minded cat registries allow outcrosses under specific rules. Both FiFE and TICA have a registration program for outcrosses and cats, after a certain number of generations, are considered purebred and showable.

At present, I know of outcrosses with domestics, Maine Coons, Turkish Angoras, and Russian Blues. In every situation, even the first generation outcrosses look shockingly Oriental in type, but come with a huge increase in genetic diversity.

Preserving the genetic diversity that we have within the breed group and increasing it with intelligent outcross programs while working to maintain show type and temperament is our plan for the future. We have brought in our first longhair outcross, and will be importing a boy from Australia as another option for new genetic diversity. We will be introducing outcrosses to our cattery as we can, and will link to the genetic diversity results of all of our cats on their pages, as well as list estimated genetic diversity for all our litters. We will continue to test our cats for genetic diversity and breed to increase or maintain what we have.

We will, as we always have, keep track of our kittens and our breeding cats and their health and longevity, and continue to use that information as well as information from scientific research, veterinarians, and other breeders to try to make the healthiest cats that we can, ones that terrorize their people for as many years as possible.

The breed group is in crisis… but we have the tools to fix it.