Genetics of Cross-breeding

The following explanation of genetics is pretty hard going if you are unfamiliar with the topic, so don’t worry if you have to read some bits over and over. If you’re interested in knowing why people breed cross-bred dogs rather than purebred dogs, it’s worth the effort.

All dogs were once mixed breed. Not in the sense that there were particular breeds that were then mixed, but in the sense that there were no particular breeds. Dogs all had a mixture of characteristics, with mating occurring between dogs that were in the same location for whatever reason, and who chose each other as partners. These earliest dogs that became friends of men and were bred for specific purposes were thought to have evolved from a now extinct wolf-type canine that was also the direct ancestor of the common grey wolf now in existence. caveman-and-dogThe exact timing of this evolutionary divergence is unknown, but it is thought to have been up to 40,000 years ago, and may have occurred during the time these animals became domesticated.

Different breeds of dogs were created by human intervention. The most likely purpose was to create dog breeds that were most suited to different types of work, for example herding sheep or retrieving prey during hunting. Creating a breed is done by repeatedly breeding dogs together who both share the desired characteristics. Once the characteristics of pups become predictable (i.e. both parents have the desired traits and all puppies born between them share these traits too), the breed is considered to be ‘pure’.

The process of creating dogs that are so similar physically can mean that a lot of their genes are very similar, or the same, as well. Having a lot of the genes the same in a breed of dogs is described as having a “small gene pool”.

Think about it as being like a community with young people wanting to marry, as depicted by these diagrams. In a moderate sized community there are a variety of people for them to choose from. In a small community, however, where a lot of people are related, it’s hard for young people to find someone unrelated to them with which they can marry and start a family. LargeGenePoolToo many of the people are too similar to themselves genetically, as depicted in the second diagram here. If you investigate purebred dogs, you will see that calculations have been made about the viability of their gene pools and the gene pools of many, like the golden retriever, are in danger of being so small they are no longer viable. SmallGenePool

Small gene pools means less variation in the dogs, which means predictability of the dog’s characteristics, which is great, but it can also cause problems too. Unfortunately not all the genes present in these gene pools are desirable. Some genes cause diseases or make a dog vulnerable to the development of health conditions such as allergies, growth deformities, blindness and cancer. With dogs who share a lot of their genes, the chances of them having health problems as a result of undesirable genes is higher. This is because the chance of having undesirable “recessive” genes expressed is higher. Thus pure-breed dogs can often have a lot of health problems because of the genes they have. To understand how this works, let’s look into how genes work.

Some genes cause diseases or make a dog vulnerable to the development of health conditions such as allergies, growth deformities, blindness and cancer.

The fundamental characteristics of a dog come from the genes it inherits from its parents. A gene is a portion of DNA that dictates what a particular characteristic will be like, for example eye colour.ChromosomeDNAGene2 Some characteristics are dictated by only one gene, but most are dictated by multiple genes. Each gene has different types (like different flavours) and each type is called an allele. A pup inherits two genes (or alleles) for each spot on their DNA, one that they get from their father and one from their mother. In a healthy gene pool there are a lot of different alleles for each gene, so each parent is likely to have quite different alleles, and therefore each puppy has a high chance of geting a good genetic mix. When the gene pool is smaller, the chances of a puppy getting the same allele for a particular gene from both parents are much higher. When a puppy has two alleles the same, it is said to be “homozygotic” for that gene.

In addition, each allele for a gene can be either dominant or recessive. Dominant ones will always be seen or evident (expressed) if they are present, and they mask the effect of recessive alleles. Recessive alleles are only expressed if there is no dominant allele present. So for example, if a puppy inherits the dominant black allele for the gene determining coat colour from the mother and a recessive allele for coat colour from the father, the dominant allele will dictate what colour the puppy’s coat is, and it will be black. It is only if the puppy inherits two recessive alleles for the coat colour gene that the recessive allele gets a chance to dictate the coat colour. Thus recessive alleles are only evident or expressed when they occur together. Allele Take a look at the diagram below. The capital “A” represents a dominant allele for gene A, and a lower case “a” is a recessive allele for the same gene. A puppy inherits one set of genes from the mum, one from the dad. A puppy that inherits two dominant alleles OR two recessive alleles for that gene is homozygotic for the gene (either AA or aa), and having one dominant and one recessive gene makes the puppy heterozygotic for the gene.

So lets think about how that affects inheriting bad genes. If there is a recessive allele for the “blindness” gene that causes a blindness disease, so long as a puppy has at least one “A” version or allele of the gene it will be ok. But if a puppy inherits two recessive alleles (aa), the puppy is said to be homozygotic for the allele causing the genetic blindness disease and will definitely develop the disease. Any puppy that inherits only one recessive gene for the disease will not develop the disease, but it will be called a carrier. This means it has the gene and can pass it on to its offspring. If the puppy is homozygous AA, it doesn’t have the disease causing allele at all, won’t get the disease, and won’t pass it on to it’s offspring either.

Most diseases that come from dominant alleles have been breed out of purebred lines. This has been possible because when the disease-causing allele is present the disease is always evident in a dog, and the dog is not used for breeding. Through this process, over a period of time, the disease becomes non-existent. Some genetic diseases involving dominant genes, however, only become evident when the dog is older, and has already been used for breeding, and the disease-causing genes have already been passed on. This is where doing gene testing for the disease-causing allele is important.

Some genetic diseases involving dominant genes only become evident when the dog is older, and has already been used for breeding

Diseases that come from recessive alleles are much more difficult to eradicate as they can be present but not expressed (seen in the dog), so nobody knows they are there, and the disease-causing recessive allele may be present in generations of dogs in the same family line without being expressed. The disease is only seen when a puppy is finally born that has two copies of the disease-causing recessive allele, or in other words, is homozygotic for the recessive allele. There is no dominant allele to mask the disease, and the disease is expressed (the disease develops, and the dog gets sick). Autorecessive This is illustrated in the diagram. Two adults are unaffected with a disease, but are “carriers” for the disease-causing allele. For each of their offspring they contibute one allelle. If they each contribute their disease-free allele, the puppy would be disease free and not be a carrier (25% chance per puppy). If one or the other of the parents contibutes the disease-causing allele, the puppy will be disease free but a carrier (50% chance per puppy). There is a 25% chance, per puppy, of both parents contributing the disease-causing allele to the pup, meaning the pup will develop the disease.

Thus the smaller a gene pool is for a particular purebred line of dogs, the more likely it is that a pup will be born homozygotic for a gene which results in a very undesirable trait, like a disease, being expressed. In these populations specifically, and in dog breeding in general, genetic testing for the disease causing allele is extremely important for diseases caused by a single recessive allele, as it can prevent dogs, who are carriers of the disease, being used for breeding, thus preventing the disease from being passed on to future generations.

To complicate matters more, (for genetic researchers, not for you!!) inherited diseases can be the result of multiple genes. So for the disease to develop a pup may need to have 3, 5, 10 or 20 specific alleles, either dominant or recessive, all present to develop a certain condition. It’s incredibly difficult to test for these types of diseases – the only option is to reduce the risk of them developing by having a large gene pool.

In addition to specific serious diseases that can occur in purebred dogs from small gene pools, these dogs may also inherit a myriad of less serious conditions, meaning they will be more likely to develop skin problems, allergies, and more. For this reason dogs from such breeds are described as being genetically less robust. For the owner, this means being more likely to have frequent trips to the vet, to have to care for a sick dog, and to have to choose between having their dog euthanased or watching it suffer, and then potentially die younger than expected for their breed. Breeders of purebred dogs have been attempting to reverse the problems of small gene pools for decades now. One of the ways of doing this is called “outbreeding”.

Outbreeding is a term used to describe breeding outwards from a particular gene pool by breeding with a dog or dogs who come from a different gene pool, thus increasing the genetic robustness of the offspring. For purebreds this may be achieved by importing a dog of the same breed from another country, where the gene pool is different enough to make it worthwhile. Outbreeding may be difficult to achieve for purebreds because of lack of alternative gene pools that are different enough, or other practical difficulties.

The effect of increasing the robustness of animals by crossing animals of different breeds is called heterosis.

Alternatively, outbreeding can be achieved by breeding two different purebred dogs together and creating cross-breed dogs. This type of outbreeding, involving the merging of two very different gene pools, most often results in genetically robust dogs due to the larger increase in genetic variability (more types of alleles, or flavours, for each gene). This means there is less chance of producing a puppy that is homozygous for an undesirable gene or genes, and therefore less likely to develop unwanted inherited health conditions. It also means there is less predictability of the characteristics of the dogs, as a pup may inherit characteristics from either breed. Characteristics of the pups are not totally unpredictable, however, as the pups will resemble either one or the other of the parent’s breed, or a combination of their characteristics, in every way.

So, as explained above, cross-breed puppies are much more genetically robust than their parents due to the larger gene pool that they come from. The effect of increasing the robustness of animals by crossing animals of different breeds is called heterosis. The effect of heterosis is greatest between pure-bred parents and their cross-bred offspring because you are comparing parents from a small gene pool with offspring from a much larger gene pool. The heterosis effect decreases between parents that are first-generation cross-bred dogs and their off-spring. This is because the first-generation cross-bred dogs already have a much healthier or variable mix of genes, so the difference between them and their pups isn’t as great.

If breeders start breeding cross-breed dogs from the same two breeds together, the heterosis effect decreases with each successive generation. After breeding like this for a minimum of 7 generations, you get the kind of predicatbility in characteristics that is required to have a dog breed classified as a pure-bred breed by a dog breeding club. These clubs have a written description of each type or breed of dog, and dogs of this breed considered to have a “good pedigree” are the ones who most conform to this description.

It is the heterosis effect that makes cross-breed dogs healthier on average than pure-bred dogs. This does not mean that you will have no health problems with a cross-bred dog, it just means the chances are less. The more different breeds that are present in a cross-bred dog, the less likely it is the dog will have health problems caused by its genes, but also, the less likely it is you will be able to predict how the dog will develop and what its adult temperament will be like.

A compromise between characteristic predictability and genetic health is to just cross two breeds. In this way, you know that the dog’s characteristics and temperament will be within the range of that of the two parent breeds, and that you will also get a reasonable amount of genetic robustness in the dog. A dog breeder concerned with maintaining a good level of genetic robustness will ensure that the dogs they breed from are not too many cross-bred generations removed from the original breeds, and that dogs who are too closely related are not bred together.

Whew!! Hard going eh? The simple message from all of that is as follows. The bigger the gene pool that went into making a puppy, the more genetically robust that puppy will be, and the less likely it is that the puppy will have inherited health problems, which is good. If the gene pool is too big, or unknown, it’s hard to predict what the puppy will grow up to be like, which is not so good. One way of getting robust genetic health while also being able to predict what the puppy will turn out like, to a reasonably good extent, is to cross just two different breeds together, and to keep the generations of cross-breeding low enough to maintain a good sized gene pool. That way you get better (but not guaranteed) genetic health with reasonable predictability of the dog characteristics. Not a pure-bred dog, but hopefully a healthier dog.

As a final note, it’s important to point out that cross-bred dogs as described here are only possible because of all the centuries worth of work that has gone into making purebred dogs, so to all you purebred dog breeders out there – thank you!!!