How much do you really know about genetics and genetic health?

 

As an experienced dog breeder, dog judge and teacher, I am going to try and explain the significance and importance of genetic health because it is clear that many people, including those who made the television programme, Pedigree Dogs Exposed, do not have a clear idea of how genetics affects the development of species in general and pedigree dogs in particular. I am afraid it might be boring – but it is important. You can find a video of this talk on my web log at http://uk.youtube.com/user/davidcavill

We all live with genetic defects. I am short sighted and suffer from a number of allergies. My mother is short sighted and suffers from number of allergies too. I have a friend who is epileptic – as was her mother. Another is diabetic and I have just heard that someone close to me has developed breast cancer. All these conditions have a high ‘hereditability’: that is, their genetic component is significant. If you are the direct descendent of someone who has any of these conditions you will not necessarily get them yourself – but the likelihood of them occurring is much higher that it would otherwise have been. Scientists have identified around 3,000 genetic defects in humans and about 300 hundred in dogs.

This is not the place for long explanations but there are some words which have been used wrongly in the recent furore surrounding the Pedigree Dogs Exposed programme and in the programme itself. They have emotional overtones that add to misunderstanding and bias so it might be helpful to go through some definitions.

Firstly a ‘genetic condition’ refers to a physical or mental disorder caused by an absent or defective gene or by a chromosomal aberration. The make up of the genetic material of the sperm and egg is such that the DNA of the resulting embryo is damaged and the development of some characteristics is distorted. Most damaged embryos are detected and are aborted but many are not, so the embryo survives and is born. At this stage the condition may mean the baby will not survive. In some cases the child is severely disabled but in most cases and more commonly the child become an adult and learns to live with the condition, often with the help of medication.

This is not the same as a ‘congenital condition’. This is where the original embryo was fine but suffers damage between fertilisation and birth. The mother smoking, drinking or taking recreational or prescription drugs may cause congenital conditions. The best-known example is the conditions directly caused by the drug thalidomide being prescribed to women who were pregnant.

A mutation is quite different. In genetics, a mutation is a permanent, transmissible change in genetic material as a result of a miscopying of a section of DNA. This can be caused by environmental factors (radiation, for instance) or by the age of the mother or the father because eggs and sperm deteriorate over the years. It is mutation which has produced us! It is the whole basis of evolution and without it we would not exist. But the important point is that is it entirely random. Most mutations have little effect but occasionally one turns up which improves the organism’s chances of survival. To describe any living creature with genetic disability as a ‘mutant’ shows deplorable ignorance from a scientist or anyone who has even a basic knowledge of genetics.

As humans have developed companion animals to be more and more useful there has been a conscious desire to ‘fix’ certain characteristics. At the simplest level it was breeding dogs which were faster to help with hunting and bringing down animals which humans could not catch on their own and later, as nomads become farmers, cows were bred to give more milk for longer periods, sheep for more coat to provide fleeces which would give more wool, chickens to lay more eggs and all them as well as pigs, goats and geese to give more meat. This process, called ‘selective breeding’ is common throughout agriculture

However, ‘fixing’ one characteristic which was desirable could also fix others which were less so. Random liaisons between animals tend to keep their DNA healthy (this is called ‘hybrid vigour’) although, as we have seen in humans, it cannot do so altogether. It is for this reason that most societies forbid marriage between those directly related to each other although many societies are comfortable with a cousin marrying a cousin and if both are fit and healthy there is not normally a problem. And there are plenty of examples where cousin-to-cousin marriages have been contracted down several generations without any difficulties. The reason is that what is called the inbreeding coefficient (the measure of how close two people are genetically related to each another) of a cousin to cousin relationship is just 6.25% and for most couples in this position there is little likelihood of serious genetic defects arising although, of course, there is always a risk. However, most relationships have an inbreeding coefficient of much less that 6.25% – but because of the complexity of human DNA many children are still born with genetic defects. Most are minor although there is always the chance of something very serious – but this can happen between people who are not at all related. Genetic defects are in our genes and will sometimes occur. This is very simplistic, of course and there is an array of mechanisms which result in specific conditions such as haemophilia (which is caused by a sex-linked recessive gene) and others where the recessive is expressed on both sides of the DNA.

Most pedigree dogs are ‘line-bred’; that is they are mated to members of the same family although seldom very closely. In fact, the average breeding co-efficient on the Kennel Club registry is between 5 and 10%. Inbreeding where fathers are mated to daughters or vice versa or mothers are mated to sons or vice versa) is not common: less than 1% of puppies on the KC register are the result of inbreeding which is defined as being anything mating with a breeding coefficient of over 25%. A common rule of thumb for dog breeders is ‘line-breed two generations and then outcross’. Basically this means that in each two-generation ‘layer’ of a pedigree you will find the sire and the dam of the puppies more than once but in every third ‘layer’ you would find a dog which was not repeated anywhere.

In dog breeding, responsible breeders will try to select dogs and bitches which are good examples of their breed and, these days, most will ensure that their stock is not carrying serious defects. This is not true of everyone, of course – and I am afraid that those who breed large numbers of puppies without the expertise and care the dogs deserve, often have little regard for their health and welfare.

However, what was not made clear in Pedigree Dogs Exposed was that there are two quite separate issues here. They are ‘genetic conditions’ and ‘conformation’. Although they are connected it helps if we look at them separately.

As we have seen with other companion animals, is relatively easy to change the conformation and performance of an animal by selective breeding. Frisian cows are bred primarily to give milk – their flesh is not very suitable for meat and most meat from Fresian cows ends up in meat derivative products such as pies, sausages and dog food because it would look very unappetising on a butchers counter. At the other end of the scale, Aberdeen Angus give exceptional meat but their cows give little milk.

When countrymen wanted dogs which would go to ground and flush out vermin they were able to produce dogs with the right conformation by selective breeding very quickly. In fact, it has been shown in experiments at the turn of the last century that, given a group of mongrels, a breeder can produce a creditable example of any breed within five generations. You like the idea of a hairy dog with a flat face? Just collect some small, hairy mongrels and mate them. Select those puppies with the shortest muzzles and mate them. Continue the process and you very soon have a Pekingese! The Dobermann and the Leonberger and our British Gundog and Terrier breeds were all ‘created’ in just this way. It is simple and it works. However, it is possible to take a specific characteristic too far as has happened with some breeds and this will inevitable lead to problems of conformation. This is not the same as deleterious, damaging genetic conditions. Bear with me.

As you change the conformation of the dog by selectively breeding them for shorter legs, longer bodies, more skin or deeper, wider and rounder chests, the rest of the dog’s anatomy is stretched or compressed to fit. Up to a point this does not matter. As I explained in my previous talk, so long as the dog can eat, breath, walk and run, mate and whelp normally then its conformation and head shape is unimportant. But once its conformation affects those natural behaviours then however attractive the breeders and potential owners find the look of the breed, their conformation should be modified. It is easy – you just reverse the process. You do not have to start again. Four or five generation is all it takes.

However, in getting a desired conformation there is a tendency to use the same small group of dogs and if they have a deleterious genetic condition, a disease, then this will become endemic within that breed. This is what has happened with dogs which should be perfectly sound from the point of view of their conformation. It is usually because breeders have relied on too narrow a gene pool – and in some of the smaller breeds this gene pool is very small. These conditions are more difficult to breed but it can be done by introducing dogs from outside the breed into the breeding programme.

This can and has been done (with Kennel Club approval, I must add) and I am sure that in the next few years it will be done much more often. But I must emphasise that although some of these genetic diseases conditions are serious and, of course, very distressing for the owner, the majority, like short-sightedness in humans, are a minor inconvenience which can be treated or managed. So the incidence of a genetic condition in a breed does not mean all the dogs are ill or damaged. Some will be, I am afraid but for many or most their dysplasia or patella luxation is nothing more than a minor inconvenience. Few dogs die of genetic disease. If is serious they do not survive beyond the womb in any case

And it is also important to note that for many or even most breeds, deleterious genetic conditions have a very low incidence. They are there, just as they are in humans and in other animals but, in general, they are as fit and healthy as any mongrel you might take home from a dog rescue centre.

Go to a dog show and see for yourself

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One Comment on “How much do you really know about genetics and genetic health?”


  1. Im a huge Dog fan and I really liked your website. Thank You


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