Genetic diversity

© Therese Rodin

Background: Almost all breeds are currently in crisis due to inbreeding depression. The vast majority of breeds have been created by allowing a limited number of individuals from the country of origin to be used in breeding. Then the studbooks were closed and the dogs that were accepted as representatives of the breed (i.e. those registered in the studbooks) soon became relatives. This means that although we may have a population of several thousand dogs (2017 5479 Pugs were registered with the Swedish Board of Agriculture), all are relatives further back in the lines.

Genetically, kinship between different individuals means that they have the same genes, or rather the same alleles, i.e. variants of the same gene, both in each locus of a gene, and in each gene pair. The latter means homozygosity and is something that characterizes purebred (= inbred) dogs. Each gene usually has many different variants (alleles), and it is the variants that create variation between individuals. Purebred dogs is a new phenomenon that originated in the West in the 1800s, when studbooks for dog breeds were created and then closed for the intake of new individuals. Only the few individuals who were registered in the studbooks and their offspring were allowed to breeding. Breeding of purebred dogs aims at disposing of alleles that give properties that you do not want, such as a color, or ear shape, so that the individuals in the population become more similar to each other. All breeds have been bred by mating close relatives with each other, as relatives have similar genes (alleles). In this way control over the offspring’s looks and behavior is created.

However, pure breeding with closed studbooks causes many problems. Since genes are inherited in “packages”, so-called haplotypes, it usually means that when you opt out an allele that you do not want, you lose many other alleles of other genes as well. This means that you also fix other features that you did not want. The reason why there are so many genetic diseases in purebred dogs today – something that also grows explosively – is that a lot of gene variants have been cleared away so that many dogs have the same defective recessive allele in a gene pair. Then, when two dogs that carry this recessive defective gene are paired and produce puppies who have two defective genes in a gene pair, the puppies become sick. In a population where you have great variation in the gene pool, this hardly happens at all. Even we humans carry defective recessive alleles, but the risk that we would meet someone who has the same defective allele is incredibly small because of the large gene variation in humans.

“The most efficient way to produce puppies that will be homozygous for a recessive allele is by breeding related dogs. In a population of registered, purebred dogs, ALL the dogs are related if you go back far enough, and the more recently ancestry they share, the more genes – good and bad – they will have in common.

So genetic disorders are a predictable – even guaranteed – consequence of inbreeding in dogs […]. There is always a risk, and you can know that level of risk by calculating the inbreeding coefficient.” (Carol Beuchat 2018)

In addition to the loss of variation that increases the risk of defective alleles being expressed, pure breeding also means a loss of “fitness”. Purebred dogs with a high inbreeding coefficient are simply not as viable as dogs with a low inbreeding coefficient. This is manifested by reduced fertility, fewer puppies in the litter, increased puppy mortality, various diseases of the dogs and shortened life span. It is well known in the research that when two purebred dogs from different breeds are paired, “hybrid vigor” occurs, i.e. great vitality because the mating gives a new gene set (e.g. Sponenberg et al. 2017: 87f.).

By adding new genes to a breed, one can thus increase health and vitality. However, in order to maintain the vitality, it is not enough to make one crossbreeding and then breed back to the original breed for many generations. Regular crossbreeding has proved to be the most effective way to deal with inbreeding. Neither a single nor a few crossbred litters are enough; the breed must regularly have crossbred litters. Approximately 5% of all litters born during a span of 25 years must be the result of crossbreeding according to a new study (Windig and Doekes 2018).

Below you see a Retropug (left) and a Rassmo (right) who contribute to a genetic variety of the Pug breed. Otto (left) is a Retro Pug with 25% Parson Russell Terrier and Otto’s mother Ghana (right) is a Rassmo, i.e. 50% Pug and 50% Parson Russell Terrier. Both of them are top fit from nose to tail.

Photo: Nina Tissen

How does the inbreeding percentage look like in the Pug? Dreger et al. (2016) have made a review of the degree of inbreeding in 112 breeds, looking at the degree of inbreeding genetically. (The kennel clubs are still looking at levels of inbreeding in databases with pedigrees, and for example The Swedish Kennel Club only calculates the inbreeding coefficient in the pedigrees over five generations (SKKs calculation of COI), but it is misleading since all pure-bred dogs are related further back in the lines.) The inbreeding coefficient (COI) for the Pug in the analysis of SNPs (gene markers the researchers used to investigate the percent of genetic inbreeding) was an average of 44.2%. It should be compared with the pairing of two half siblings which leads to an inbreeding coefficient of 12.5% in the puppies, and with pairing of two whole siblings where the inbreeding coefficient of the puppies is 25%. One can thus say that the Pug as a breed suffers from tremendous inbreeding depression, which we believe can be an important cause for all the various diseases that appear in Pugs, such as epilepsy, PDE, PM and so on. Research has shown that health problems due to inbreeding begin when the inbreeding coefficient exceeds 5% (Beuchat 2016).

Goal: To be able to preserve the Pug as a breed, we need to increase the genetic variety. This can only be achieved by carrying out crossbreeding. The goal is that all Pugs eventually will have a proportion of “new” genes, on about 5 to 20 percent. We also need individuals who have a higher share of genes from another breed. The latter will become valuable in the work to improve the Pug and can be combined with Pugs that have a very high proportion of Pug genes.

Genetic diversity will enable us to come to terms with many of the Pug’s current health problems as well as problems regarding fertility etc.

Strategy: As it currently seems that the kennel clubs are generally not prepared to start crossbreeding programs, we recommend each individual breeder to ensure that all their Pugs have a share of 5-20% from a donor breed and also have dogs with a larger genetic share from that breed, that can be used in breeding. When choosing a donor breed it is important to choose a healthy breed that completes the weak sides of the Pug. In Germany, Parson Russell terrier has been successfully used as a donor breed. Miniature Pinscher is a further breed that has been used successfully to improve the Pug.

The Pug Theo is 87.5% Pug and 12.5% Parson Russell Terrier
Photo: Liane Seemann, Mopszucht vom Ilexwald

Sources and further reading:

Beuchat, Carol. 2016. “Inbreeding of purebred dogs determined from DNA”. Blog article, Institute of Canine Biology. Accessed 110419.

Beuchat, Carol. 2018. “Managing genetics for the future”. Course at the Institute of Canine Biology.

Dreger, D. L., M. Rimbault, B. W. Davis, A. Bhatnagar, H. G. Parker and E. A. Ostrander. 2016. “Whole genome sequence, SNP chips and pedigree structure: building demographic profiles in domestic dog breeds to optimize genetic trait mapping. Disease Models & Mechanisms 9, 1445–1460.

Malm, Sofia “Släktskap och inavel: om avelsmetoder och structurer”, in: Sofia Malm and Åsa Lindholm. 2013. Hundavel i teori och praktik. Svenska kennelklubben, 128–147.

Sponenberg, Phillip, Alison Martin and Jeannette Beranger. 2017. Managing breeds for a secure future. Sheffield: 5m Publishing.

Windig, J. J. and H. P. Doekes. 2018. “Limits to genetic rescue by outcross in pedigree dogs”. Animal Breeding and Genetics 135, 238–248.

Chapters in Strategies for the breeding of Healthy Pugs