Inbreeding, Line breeding and Cross breeding in Dogs

Inbreeding, Line breeding and Cross breeding in Dogs

Written by Kate Schoeffel Bsc(hons)BVsc

Some animal breeders, including many dog breeders, make a distinction between inbreeding (mating mother/son, father/daughter, brother/sister) and line breeding (mating say grandparent/grandchild, aunt/nephew).

To a geneticist linebreeding and inbreeding only differ in degree. Inbreeding occurs when animals are bred to their relatives. The closer the relationship the higher the “inbreeding coefficient”. Inbreeding coefficients measure the degree of inbreeding an animal shows relative to a randomly breeding population.

Inbreeding reduces fertility, vigour or overall health and mental stability. Inbred animals are more prone to diseases such as infections and cancer, and more likely to be “highly strung”.

To understand why this happens we need to consider basic genetics:

All animals, including people and dogs, carry two copies of each gene – one from our mother and one from our father. These genes are unique sequences of DNA, each of which codes for a unique protein. Changes in the DNA code (called mutations) change the structure of the protein produced by the gene and as a result change the way the protein works.

Because evolution has for millions of years selected for perfection of performance most changes or mutations are less effective than the original gene copy. The chance of having an abnormal copy, or mutation, of any one particular gene is low, but because we have so many genes we all carry some harmful genes. These genes are usually hidden because we have one good copy of the gene to carry us through and this gene produces a normal protein which can perform the tasks required. When we have two different copies of a gene we are said to be HETEROZYGOUS for that gene and if one gene copy is hidden by the other, the hidden copy is said to be RECESSIVE.

If both copies of a gene are the same then we are HOMOZYGOUS and if the copy is “bad” then that gene won’t work normally and we will be to some degree less healthy. Some single genes are so important that affected animals die, or suffer debilitating disease and some have only minor effects – affecting for example jaw structure or coat colour, the efficiency of an antibody molecule, the structure of a neurotransmitter or the shape of a red blood cell.

As animals are mated to their relatives, however distant, simple mathematics will show that the likelihood of any one gene becoming homozygous will increase. As homozygosity increases, variation among offspring decreases. The dog breeder takes advantage of this in line breeding to produce a breed which “breeds true” and conforms to a “breed standard” and within the breed to produce offspring that are like peas in a pod. Breeders look for a “prepotent” sire or bitch ( ie one that always throws pups very similar to itself). These animals come from a “good linebred pedigree” – that is one that is inbred so that the animal is homozygous for as many as possible of the characteristics that the breeder regards as desirable.

Unfortunately this search for perfection and uniformity comes at a cost. Undesirable genes also become increasingly likely to be homozygous and so affect the health of the animal. Most of these genes have minor effects which gradually accumulate. There are many genes involved in traits like fertility, immune competence and mental stability and accumulation of homozygous recessive “bad” genes gradually diminishes the function of these systems.

If inbreeding increases homozygosity, crossbreeding is its opposite and maximises heterozygosity. Crossbreeding is the mainstay of most farm animal and plant production – it takes advantage of aphenomenon which is widely talked about but poorly understood: HYBRID VIGOUR (or for the geneticists – HETEROSIS) is the term used to describe the burst of fertility, good health and growth that is seen in the progeny when two unrelated breeds are mated. The longer that these breeds have been separated, and the greater the differences between them, the stronger will be the resulting hybrid vigour.

Hybrid vigour is not a theory, it is the name given to describe something that happens repeatedly throughout all species in the animal and plant kingdom. When unrelated breeds of any animal species are mated the offspring in the first generation will be more healthy, fertile, and (in animals) mentally stable than either parent breed. This first F1 (Filial1) generation as the geneticists call it, will be intermediate in characteristics to the parent breeds and the offspring will resemble each other. Let’s look at how this works using a simple model with only 9 genes. (Note that a convention in genetics is that small case letters are used to denote recessive genes and large case letters are used to denote dominant genes.)

Say we have a dog of breed (A) which always has has long ears (aaBB), Short legs (CCdd), black coat (EE) and, by chance because of years of breeding within a small gene pool, has the “fertility” genes FF gg hh II. The bitch of breed (B) has short ears (AAbb), long legs (ccDD) a gold coat (ee) and, again by chance, the “fertility” genes ff GG HH ii.

The cross would look like this:

aa BB CC dd EE FF gg hh II
Producing an F1
AA bb cc DD ee ff GG HH ii
Aa Bb Cc Dd Ee Ff Gg Hh Ii
These offspring will have medium ears (A-B-) and medium length legs (C-D-). They will be black (Ee) because only one gene is involved and black is dominant to gold, and they will be more fertile than either parent. Genes for mental stability and “vigour” – like these fertility genes – become more homozygous with inbreeding and in the first cross will be as heterozygous as it is possible with that breed combination.

This example oversimplifies matters – in reality thousands of genes are involved, dominance/recessive relationships are not that simple and different breeds have many genes in common, however it does demonstrate the way in which hybrid vigour works and why the F1 progeny are similar to each other. The degree of similarity in the F1 progeny will, as in pure breeding, depend on how homozygous the parent breeds are.

Certainly if two animals of different breeds are mated and if both parents are highly strung, and if this nervous behavious is genetically based, and if the same genes are involved in both parents, then crossbred progeny will probably also be highly strung or neurotic. Clearly however this is less likely than if these two animals are of the same breed and related in some way.

This example can also be used to show why hybrid vigour only works in the first generation and why “mongrels” or “mutts” come in all shapes and sizes. While our theoretical F1 – produced from highly homozygous parents, has only one genetic combination possible, if an F1 was crossed with another F1 there would be many hundreds of possible combinations produced in the “F2” generation. These offspring would range from almost the same as one grandparent to almost the same as the other, with every possible combination in between and while most of these would be more heterozygous than the grandparent generation most of them would be less heterozygous than the parent F1s.

It is possible to prevent this deterioration in health and vigour by limiting linebreeding and by selecting rigorously for highly fertile, vigorous, long lived, mentally stable animals. Responsible, intelligent dog breeders do just that. Unfortunately the the problem arises – what does one do with the less healthy, vigorous, and mentally stable animals? The answer in livestock breeding is that they are sold to an abattoir – in purebred dog breeding they are usually sold as pets.

Another feature of crossbreeding used in livestock breeding is “complementarity” -the term used to describe the way two breeds can be combined to overcome defects in one or other breed – the way two bloodlines “nick” in dog breeding terms. The example which is commonly applied in cattle is the crossing of a dairy cow with a beef breed to produce a calf which has hybrid vigour and will be fertile healthy and fast growing. The heifer calf will also have better muscling than the dairy parent and better milk production than the beef parent and in this way the two breeds are complementary. Should the heifer calf be retained for breeding she could be backcrossed to either parent breed or – as is more commonly practiced – bred to a third breed in a “three breed rotation” which will retain the benefits of hybrid vigour, complementarity and reliable uniformity in the offspring.

I am a country veterinarian and for a long time I have been aware of the trouble that purebred dog breeders are unintentionally getting their animals into. The practice of closing the stud book once a breed is “recognised” is, to me, a practice which benefits dog breeders but works to the detriment of the breed. I am not a traditional dog breeder and my bias is towards breeding dogs as life enriching companions for people rather than breeding dogs to preserve animals of an idealized type in perpetuity.

I have chosen to breed Labradoodles for reasons outlined elsewhere. I believe that there is scope for other crossbred dogs to be deliberately bred. I realise that this approach to dog breeding is frowned on but I think that by creative application of crossbreeding new breed combinations can be produced which bring together desirable characteristics from different breeds.

There is no reason why breeds of dogs should be treated as if they are separate (and increasingly endangered) species as is presently the case. The challenge is to develop checklist of desirable characteristics and then find the breeds most likely to complement each other. Complementary breeds should not share common structural problems, such as Hip Dysplasia, or Luxating Patellae and should have temperament characteristics which combine well. Single gene problems such as Haemophilia or Progressive Retinal Atrophy would still need to be screened for if they occurred in both parent breeds but carriers are not be a problem in this system because the progeny are not be bred from.

Remember that the second generation is likely to be more variable and have less hybrid vigour than the F1 so breeding F1 to F1 is unwise unless the breeder is trying to create a new breed and doesn’t mind breeding a lot of funny looking , and possibly unsatisfactory, dogs on the way. I don’t want to create a new breed or produce puppies which are all identical – only puppies which are all healthy, intelligent, full of personality and with attributes which make them good family pets. LINKS

If you want to get a feel for the amount of crossbreeding that occurs in other species I suggest a search for HETEROSIS. Hundreds of thousands of references appear in an extraordinary range of species. When I first Googled heterosis and dogs (some time ago) I could find no references to applying this well recognised and accepted breeding technique to dogs. Now there are many thousand – most of them controversial because crossbreeding is still universally condemned by purebred dog breeders.

The Canine Diversity Project This project is an admirable undertaking in trying to introduce a rational modern approach to dog breeding.

A most informative web site covering a wide range of canine genetic issues.

Jeffrey Bragg is Chair of The Working Canine Association of Canada, breeds Seppala Siberian Sleddogs and resides in Whitehorse, Yukon Territory. He is a constant voice of reason against closed stud books in his chosen breed the Siberian Husky.

Examples of breeders extolling the virtues of inbreeding – no mention of inbreeding depression only the risk of single gene defects.

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