At any single gene, diversity can be measured as the degree of homozygosity (the inheritance of the same version of a gene from both dam and sire) which is observed. The more homozygous a population the less diverse it is. One of the main factors leading to increased homozygosity or loss of diversity is inbreeding.
Mate Select from the Kennel Club
Ever increasing demands are being placed on today’s dog breeders and never have their activities been subject to so much external scrutiny. For many years now, the responsible dog breeder has been striving, through the appropriate selection of compatible dams and sires, to produce litters of puppies with the correct breed type, good temperament and the best chance of living a long and healthy life. To this latter end, breeders have been harnessing an ever-increasing list of health checks to which potential dams and sires can be subjected. Going back forty or fifty years, these health checks were clinically based and breeders were able to use this clinical information to try to gauge the genes that a dog carried with respect to a particular inherited disease, its genotype, and therefore predict what genes a puppy might inherit if the screened dog was bred from.
These clinically-based health screening programmes still form an important part of today’s breeder’s armoury to help produce healthy puppies. More recently, over the last ten years or so, more sophisticated DNA tests for single gene mutations known to cause inherited disease have been evolving, giving breeders a precise way of predicting an individual dog’s genotype with respect to a particular inherited condition. The volume of health screening procedures now available to breeders of certain breeds is such that the network of information required to make the best breeding decisions is growing increasingly complex, and things will only get more complex as we understand more about the genetics of inherited disease in the dog.
Some years ago now, the Kennel Club took the decision to try to better understand the genetic population structure of the breeds that it recognises. Initially this work was undertaken in collaboration with research scientists at Imperial College in London. More recently, this work has been continued by Dr Sarah Blott and her colleagues at the newly established Kennel Club Genetics Centre at the Animal Health Trust (AHT). Moving forward we need to look at ways to manage the genetic diversity in the dog population to try and prevent breeds from becoming genetically homogenous. One way of achieving this will be to ensure there is a greater number of individual dogs contributing to the genetic population.
Why should breeders worry?
Why should breeders worry about these trends in their breed’s genetic population structure? Increases in a breed’s average inbreeding coefficient, and indeed on the average inbreeding coefficient of their own dogs, means that the chances of genes becoming homozygous, i.e. the chances that a puppy will inherit the same copy of a gene from both its dam and sire, will increase. This is of course true for genes that have a beneficial impact, and this has traditionally been the reason for breeders practising what they call ‘line breeding’. But sadly it is also equally true for those genes which have a potentially deleterious and sometimes very serious impact if they too become homozygous. There is absolutely no way that we can make precise predictions about the impact that increases in average inbreeding coefficients will have on a breed, but what we do know is that, as the inbreeding coefficients increase, the risk of these having a serious and deleterious impact on the breed will also increase.
The simple case would be a single recessive mutation that might cause a new inherited disease in the breed. As the level of inbreeding increases, then the risk of a dog inheriting a copy of this recessive mutation from both parents will increase, causing the dog to become clinically affected. More complex is a group of, as yet, anonymous genes which we believe make a contribution to a breed’s genetic fitness and vitality. As levels of inbreeding rise, the risk of more and more of these genes becoming homozygous for deleterious recessive mutations increases, resulting in what is known as inbreeding depression. The most commonly seen consequences of inbreeding depression are reductions in the average litter size for a breed and an increasing inability for breeders to get their bitches pregnant. In the future, breeders will undoubtedly have to manage these risks by managing the increases in inbreeding from generation to generation.
How the Kennel Club is helping
To this end, the Kennel Club has launched this new service for dog breeders, known as Mate Select, initially to help breeders manage inbreeding in their future breeding programmes. Mate Select provides breeders with three particular features. Firstly, a breeder can access a dog’s individual inbreeding coefficient, which indicates the chances that a dog will have inherited the same copies of genes, whether ‘good’ or ‘bad’, from both its dam and sire; the higher the dog’s inbreeding coefficient, the greater the chances of this happening. Secondly, it provides the average inbreeding coefficient for each individual breed recognised by the KC. Finally, and most importantly, it allows breeders to perform hypothetical matings on paper, and predict the inbreeding coefficients of the resulting puppies.
Matings 'On Paper'
It is this latter feature that enables breeders to better manage inbreeding in their breeding programmes. Most serious, responsible breeders wishing to breed from their bitch select a number of potential sires that might be considered. In drawing up this short list the breeder will have considered health screening results of the dogs and the likelihood that the puppies will be of the correct type with correct temperaments, as well as other criteria. The availability of Mate Select changes none of these considerations, but it allows the breeder to run through all of the hypothetical matings between the bitch and the short list of potential sires and predict the inbreeding coefficient of the resulting puppies, thus allowing the breeder to choose the dog(s) from this short list that will have the lowest risk in terms of inbreeding, for example, the dog(s) that will produce puppies with inbreeding coefficients that are less than the breed’s average inbreeding coefficient.
A service and an opportunity
Mate Select represents an opportunity for breeders to gain a better understanding of the impact that a proposed mating might have, not only on the puppies that might be produced, but also on the broader population structure of their chosen breed. It does not compromise or replace the traditional art of the dog breeder, but its availability will give added confidence to breeders that the selections they make will be to the long term benefit of the breed as a whole.