Norwegian Buhund


Genetics – a talk by Dr Jeff Sampson  Reproduced with permission

In September 2000 Dr Jeff Sampson,   the Kennel Club Canine Genetics Coordinator, gave a talk to the club on   Genetics and Genetic testing.  Genetic testing may be a future way forward   to eliminate hereditary cataract in the breed.  His talk is reproduced   here with grateful thanks.

Genetics is very much a young science, as sciences go. The roots of   genetics are to be found in the work of this monk, Gregor Mendel. Along   with his duties as an abbot of his monastry, he also carried out   experiments on breeding pea plants. He was particularly interested in the   various forms that these plants could adopt, and just how the forms were   inherited from generation to generation. He eventually published the   results of his investigation in a scientific paper in 1856. Unfortunately   the impact of this paper went largely unnoticed. This work was eventually   ‘rediscovered’ in the very first years of the 20th. Century. Mendel’s   results of his experimentation on pea breeding were set to form the   foundations of a science called ‘genetics’, a term coined by William   Bateson. During the remainder of the 20th. Century this fledgling science grew at a   rate that far surpassed the rate of knowledge increase in the more   traditional sciences. In 1953 James Watson and Francis Crick deduced the   chemical structure of DNA, the chemical that makes up genes. From this   point our understanding has increased at a rate that has certainly   astonished even the more enthusiastic observers. The 21st Century begins   with the prospect of shortly knowing each and every gene that is required   to make a human, what the genes are, what they do, how they work and what   happens when they don’t work properly. This information will be highly   significant for the dog, because humans and dogs share very many of the  same genes.

Let’s take a closer look at Mendel’s proposals. He made two very simple   simple deductions: Firstly, he proposed that the characteristics of an individual, the way it looks and behaves, in fact everything about an individual, are inherited from its parents. He was the first to coin the term ‘gene’ and define it   as a unit of inheritance and a determinant of a given characteristic. Secondly, he proposed that both male and female contribute to the   characteristics of the offspring So, what Mendel was saying is that everything about an individual is  inherited via the genes that it receives from both its mother and father.

Today we know a great deal about genes and how they work. The gene   contains a genetic plan or blueprint that an individual requires to   express a particular characteristic or trait. For example, the colour of   your hair or eyes, how tall you are going to be, whether you can roll your   tongue, in fact everything about an individual. Genes are made of s   chemical called DNA and the plan is actually stored in the chemical   structure of the DNA molecule that makes up a gene. Cells use the genetic   plan embedded in a gene to make molecules called ‘proteins’ and it is the   action of these proteins, working either individually or in groups, that   actually determine the characteristics. The colour of your hair is   determined by a group of proteins that manufacture the pigments that are   responsible for hair colour. So it is the proteins, called enzymes, that   make the pigments and determine hair colour, but an individual could not   make the proteins necessary without first having the plans for their   manufacture that are stored in the genes for these proteins. Current estimates suggest that we need somewhere in the region of 75,000   different genes (the word ‘genome’ is used to describe this collection of   different genes) to produce a perfectly healthy dog. A similar estimate   has been suggested for the genes required for a human. In fact, very many,   possibly the vast majority, of the genes needed to specify a human are   also required to specify a dog. Owners not only look like their dogs, they   are also genetically very very similar to their dogs. One of the important   features is that the dog possesses two copies of each and every gene in  its genome.

Every cell in the dog’s body ( and there are something like one million   million cells in total) contains two copies of each and every gene in its   genome. The only exception to this are red blood cell, which actually   possess no genes at all, and the reproductive cells, eggs and sperm, which   only have one copy of each and every gene (see later). The genes are   stored in a special area within the cell called the nucleus where they are   assembled into structures called chromosomes. The dog has 38 different   chromosomes (numbered 1 – 38) and these are shown above. Each chromosome   carries genes along its length. The larger chromosomes , like chromosome 1   will have anything between 4,000 and 5,000 different genes arranged   side-by-side along its length. Smaller chromosomes, like 38, carry fewer   genes, say between 1,000 and 2,000. In fact, running along the length of   each and every chromosome is a single molecule of DNA and different   regions of this molecule actually represent the different genes. Remember,   there are two copies of each and every gene and this means that there must   be two copies of each chromosome. This picture quite clearly shows the   different chromosome pairs; a pair of chromosome 1, 2 3, all the way down   to a pair of chromosome 38. The dog also has a special pair of chromosomes that determine the sex of   an individual. If a dog has two copies of the X chromosome, as is  portrayed in this picture, it will be a female.

This picture shows the chromosomes taken from a male dog. It has two   copies of each of the chromosomes 1 -38, just like the female seen before.   However, its sex chromosomes are different. Instead of possessing two   copies of the X chromosome it has one X chromosome and one Y chromosome.   This is what makes it male. So, sex determination in the dog is just as it   is in humans. Females have two copies of the X chromosome and males have   one copy of X and one copy of Y. In order to distinguish these sex   chromosomes from the rest we refer to them differently. Collectively, we   call chromosomes 1-38 the autosomes; the X and Y chromosomes are called   the the sex chromosomes. Let’s recap at this stage. Each and every cells in the female has a pair   of each of the autosomes (chromosomes 1 – 38) and these contain the two   complete copies of the 75,000 genes needed to make a dog. Every cell in   the male has exactly the same complement of autosomes ( chromosomes 1-38)   but now has an X and a Y sex chromosome. This has effectively covered the first of Mendel’s propositions; the gene   as a unit of inheritance and determinant of a given characteristic or   trait. What about his second proposal, that both male and female contribute to  the characteristics of the offspring………?

The process of fertilisation results in the passage of parental   characteristics to offspring. Bitches produce eggs and into each egg the bitch deposits one complete set   of autosomes, I.e. one copy of each of chromosomes 1 – 38. Remember, each   and every gene required for the dog will be present somewhere on one or   other of these chromosomes. The egg therefore possesses a complete   compliment of maternal genes. In addition, the egg contains just one copy   of the X chromosome. Dogs, males, of course produce sperm. Each sperm head contains one of each   of the males autosomes (chromosomes 1 – 38) and thus a complete set of   paternal genes. However, each sperm head must also contain a sex   chromosome. This means that there are in fact two different classes of   sperm that can be produced. One will have the 38 autosomes and the   paternal X chromosome, the other class will have a complete set 0of   autosomes and the paternal Y chromosome. There will be approximately equal   numbers of sperm in each class. At fertilisation the egg and sperm fuse and in this process the genes in   the sperm head are injected into the egg which now becomes a fertilised   egg. The fertilised egg now has two complete sets of autosomes and thus   two complete sets of genes, one maternal and the other paternal. However,   in terms of the sex chromosomes, there will be two types of fertilised   egg. The egg fertilised by the sperm carrying an X chromosome will now   have two X chromosomes and will develop into a female. The egg fertilised   by the sperm carrying the Y chromosome will have one X and one Y and will  develop into a a male.

This process of fertilisation explains Mendel’s second proposition and   demonstrates how genes are passed from parents to their offspring. The   fertilised egg contains two complete sets of chromosomes: a maternal set   with one copy of each and every maternal gene and a paternal set with each   and every paternal gene. This fertilised egg then ultimately produces the fully formed puppy. This   requires a colossal amount of cell division. The fertilised egg is just   one single cell, but it ultimately has to give rise to the million,   million cells that are present in the puppy. The fertilsed egg initially   grows and divides into two daughter cells. Each of these then grow and   divide to give two of their own daughter cells, i.e. four cells in total.   Each of these four divide to give eight cells in total, then 16, 32 64 and   so on until the required cell number is achieved. Immediately before each and every one of these cell divisions every gene   has to be copied so that the two daughter cells produced will still have   two copies of every gene ( the maternal and paternal set) and two sex   chromosomes. This represents a phenomenal amount of gene replication   during the development of the puppy from a fertilised egg. Very early on in this process of cell division the cells produced appear   to be identical and collected together into a ball of cells. However,   soon, new cells produced by cell division take on different appearances   and functions, a process known as cell differentiation. Cell   differentiation is driven by the genes. Liver cells arise because they   express a subset of genes that are required for liver cell to form; kidney   cells express kidney genes, muscle cells muscle genes, and so on.   Accompanying this process of cells differentiation and complicated cell   movements so that slowly but surely the initial ball of cells grows and  begins to take on the physical appearance of a puppy.

Sometimes the chemical structure of DNA within a particular gene becomes   altered. Since the DNA structure stores the plan embedded in a gene, this   alteration of a gene’s chemical structure will alter the plan. We call the   alteration of the structure of DNA within a gene a MUTATION. Mutations essentially occur randomly in the DNA and so gene mutations are   a random process and can in fact occur in any of the 75,000 different   genes. Mutations are not common events so still the vast majority of genes  in an individual will be unaltered.

We don’t know all of the causes of gene mutations, but we know of some: RADIATION. Most forms of radiation are known to cause structural damage to   DNA and can lead to a gene mutation. For example, ultraviolet light in   sunshine is hypothesised to cause gene mutations in some skin cells that   can ultimately cause skin cancer. Other forms of radiation known to cause   DNA damage include nuclear radiation. It is known, for example, that   Russians who remained in the vicinity of Chernobyl following the nuclear   accident there have 1000 times more mutations in their DNA than a control   set of Russians that were nowhere near Chernobyl. CHEMICALS. Certain types of chemicals have also been shown to alter the   structure of DNA and lead to gene mutations. Although both radiation and chemicals can be responsible for gene   mutations, probably the vast majority of mutations come about as a natural   consequence of the way that life has evolved. Two slides ago we discussed   the colossal amount of gene replication that is required during the   development of a fertilised egg into a puppy. This copying of the DNA   structure within a gene is not 100% efficient and mistakes do occur.   Thankfully the vast majority of copying errors are detected and corrected,   but even this proof reading is not 100% efficient and, as a consequence,   some copying errors go undetected and the DNA structure within a gene  becomes mutated.

Mutations in genes do not necessarily spell disaster. In fact, the vast   majority of gene mutations are neutral and don’t affect the plan embedded   in the mutated gene. Sometimes the mutant gene affects the cell in which it first occurs. It   could kill the cell or confer new, unwanted properties on the cell. Cancer   is a good example of this. It is now generally believed that a cell has to   accumulate mutations in a number of different genes before it becomes a   cancer cell. It is known, for example, that 5 different genes need to be   mutated before a colon cell becomes cancerous and gives rise to colon   cancer. The mutations are in fact conferring the new property of   immortality on the cell and it simply grows and grows in an essentially   uncontrolled fashion. Such mutations are clearly critical for the cell;   they are also critical for the animal in which the cell resides and can,   ultimately cause the animal to die. But the mutation dies with the animal. The only mutations that are relevant for inherited disease are those that   occur in genes where the altered plan causes a particular disease state   and the mutant gene is put into either of the two types of reproductive   cells, eggs or sperm. If a mutated gene that can cause a disease is put   into an egg as part of bitches set of genes, or a sperm as part of the  dog’s set of genes, then we will have the basis for an inherited disease.

It’s time to look at a specific example to attempt to draw together   everything that I have been saying thus far about genes, inheritance, gene   mutation and inherited disease. The example that I am going to use is that   of sight, just one of the many characteristics of the dog. The   biochemistry of vision in the dog is nearly identical to that in humans. The light that passes into the eye is focused onto the retina at the back   of the eye. The cells that make up the retina are designed for just one   purpose; to take this light information and convert it into a nerve   impulse that passes down the optic nerve to the centre of the brain   devoted to sight. There the nerve impulse is interpreted back to a picture   by the neurons in the brain and we ‘see’ the image that the light entering   the eye carried. We now know that inside each of the retinal cells is a group of proteins   that are all required if sight is to be achieved. Each of these proteins   will have a corresponding gene that is inherited at conception and used by   the retinal cells to allow them to manufacture the protein. The number of   different proteins required for sight is relatively large and could   approach 50. There will be a corresponding number of vision genes that   will need to function normally to allow the retinal cells to assemble a   complete complement of retinal proteins. In this example I have chosen to   reduce the complexity by assuming that only four different proteins are   required ( proteins 1 -4), but remember this is a simplification to aid  description.

The retinal cells contain a group of proteins ( 1- 4, for simplicity) that   are required for vision. Normally these proteins are not active inside the   retinal cell. You could imagine each protein to be a switch and all of   them are switched off as a default situation. When the light signal hits the retinal cells it actually switches on   protein 1 and activates it. This is shown by a change in shape here.   Circular protein 1 is switched off and inactive. The energy in the light   is used to alter the shape of protein 1, switching it on and giving it a   new property. Once protein is switched on, it, in turn, switches on protein 2, i.e.   activated protein 1 converts inactive protein 2 to active protein 2. Activated protein 2 then switches on protein 3 which then switches on   protein 4. Once protein 4 is switched on a nerve impulse is generated that   passes from the retina down the optic nerve to the brain. The generation   of the nerve is represented here by switching on the light bulb. So, in summary, each retinal cell contains proteins required for sight and   they all have to work in a precise sequence in order to generate a nerve   impulse. The light activates the first protein in the chain, this then   activates the second which then activates the third and so on until the  final activated protein in this sequence generates a nerve impulse.

Now lets see what will happen if the gene for one of these vision proteins   becomes mutated. Let’s assume that the gene for protein 3 in this pathway   becomes mutated. I have represented this by changing the shape of protein   3 from a circle to a triangle. Let’s also assume the mutation actually   alters the plan significantly so that retinal cells with the mutation   actually make an altered protein 3 that cannot be activated. So, what happens now? The light will activate protein 1 which will   activate protein 2. However, now protein 3 is altered so that it cannot be   activated no matter how hard protein 2 tries. The sequence becomes blocked   and because protein 3 cannot be activated, protein 4 remains in the   switched off mode and a nerve impulse cannot be generated (the bulb   remains switched off). The brain will therefore not know that light has  passed into the eye. Sight will be lost.

Now let’s turn to looking at the different types of inherited disease.   Broadly speaking, there are diseases that result from a single mutant gene   and those that involve mutation of more then one different gene   (polygenic) Single Gene Diseases Again there are two types of single gene disorders. To understand the   difference you have to remember that there are two copies of each and   every gene. When a mutation first occurs in a gene it only affects one of   the two versions, the other gene copy remains normal. Once the mutation   has occurred in a gene it will not be corrected and the mutant version   will be passed on to future generations by breeding. One of the two types   of disease mutation is called a recessive mutation and the other is termed   a dominant mutation Recessive Mutations A disease caused by a recessive mutation will only ever be expressed if an individual inherits two copies of the mutant gene, one from its mum and the other from its dad. A dog that has one recessive mutant gene and a perfectly normal counterpart gene will not be clinically affected, but it will be a CARRIER. Dominant Mutations An animal need only inherit one copy of the dominant mutation to be affected. The dominant mutation is expressed even in the presence of   a perfectly normal gene copy. So, to summarise, to be affected you need two copies of a recessive mutant   gene, but only one copy of a dominant mutant gene. Polygenic Diseases Here more than one different type of gene is involved. The expression of   the disease results only when an animal inherits mutations in more than   one different gene. The classic example is that of hip dysplasia; we don’t   know how many different genes need to be mutated for the disease to occur,  but it is certainly more than one gene.

This slide represents the possible outcomes of a mating between two   carriers of a disease caused by a single recessive mutation. The white   circle represents the normal, non-mutated gene copy, the black circle the   recessive mutant gene responsible for the disease. The bitch is a carrier   and has one mutant gene and one normal gene copy (she is clinically   clear). She will produce two types of egg, one carrying a full complement   of genes including the normal gene and one carrying a full complement of   genes but this time carrying the recessive mutant copy. The dog is also a   carrier and produces two types of sperm, one carrying all the genes   including the normal gene version and the other carrying all of the genes,   but this time having the recessive mutant version of the gene. Remember,   fertilisation is a random process so either class of egg can be fertilised   by either class of sperm, so there are four different combinations that   can occur. In this diagram I am attempting to use different colours to   relate the outcome of a particular combination of egg and sperm at   fertilisation. Consider the fertilisation of an egg by the sperm connected   by the blue arrow. The puppy that develops from such a fertilisation is   boxed in blue. The fertilisation depicted by a red arrow gives progeny   that are boxed in in red, and so on. You can see from this that when you mate two carriers of a single   recessive mutation you will expect to get 25% of the progeny that are both   clinically and genetically clear (they have inherited a normal gene copy   from both of their parents), 50% of the progeny that are carriers (these   will still be clinically clear) and 25% of the progeny will be clinically   affected because they have inherited a recessive mutant gene from both of  their parents.

Now let’s do the same for a disease caused by a dominant mutation. Again,   the white circle represents the normal gene copy and the black circle the   dominant mutated version of the gene. You only need to have one copy of   the dominant mutation to be clinically affected. This slide demonstrates the possible outcomes of mating an affected bitch   (one normal gene copy and one dominant mutant gene copy) to a clear dog   (two copies of the normal gene copy). The colour convention used here is   exactly the same as described in the previous slide. Different types of   fertilisation are related to the outcome of that fertilisation by virtue   of using the same colour. You can see from this that if you mate the affected to a clear you would   expect to get 50% of the progeny to be both clinically and genetically   clear and 50% of the progeny will be affected because they have inherited   their mother’s dominant mutant gene. A Note On These Expected Outcomes. The %s used in this slide and the previous slide are what we expect to   happen in terms of how progeny will be distributed. However, I have   stressed previously, fertilisation is completely random in terms of what   type of sperm will fertilise what type of egg. This means that actual   ratios of progeny in a litter may not be exactly what you expect. You   expect from a carrier carrier cross to obtain 25% normals,50% carriers and   25% affecteds. These are simply probability estimates, in reality the   distribution of pups in the litter might well be different. If you   repeated the mating often enough and get a large number of puppies then   the observed distribution would approach the expected distribution.   However, in a single litter you could get an entire litter of clears, and   entire litter of affecteds and anything else in between these two  extremes. The %s quoted are simply a guideline.

This slide depicts the theoretical outcomes of matings between dogs that   may or may not have a recessive mutant gene that causes a clinical   disease. The symbols used here are new and therefore need some   explanation: Squares represents males (dogs), circles represent females   (bitches). A horizontal line connecting a dog and bitch represents a   mating and the progeny that come from such a mating are connected by a   vertical line. The symbols can be split to represent the genetic makeup.   So, of the symbol is completely white the animal has two normal versions   of the gene and is genetically and clinically clear. If one half is white   and the other black, the animal has one recessive mutant copy and one   normal copy of the gene. It is a carrier and will be clinically clear. A   wholly black symbol represent an affected because it has two copies of the   recessive mutant gene. Obviously, clear to clear is ideal and all offspring will be clear, both   clinically and genetically. A clear mated to a carrier will give expected   ratios in the progeny of 50% clears and 50% carriers. We have already seen   the outcome of a carrier carrier mating. Mating an affected to a clear   will give you an entire litter of carriers (this will always occur and   doesn’t depend on probabilities). If you mate a carrier to an affected you   would expect to get a litter in which 505 are carriers and 50% are   affecteds. Finally, mating two affected can only give you an entire litter  of affecteds.

This slide shows the expected outcomes from various matings for a disease   caused by a dominant mutation. Remember, you only need one copy of the   dominant mutant gene to be clinically affected. On the top we see the outcome of a clear to a clear, as expected all   progeny will be clear. Moving to the second row, on the left, if an affected is mated to a clear   you would expect half the litter to be clear and half to be affected. This   is assuming that the affected only has one dominant mutant version of the   gene. Occasionally, but a lot less common, an affected will actually have   two copies of the dominant mutation. If such an affected is mated to a   clear the outcome will be different (second row, right hand side). In this   case all of the puppies will be affected. The bottom row depicts the outcome of mating an affected to an affected.   On the extreme left we what happens if both of the affecteds have a single   copy of the dominant mutation. You would expect to get 75% of the litter   affected, but 25% both clinically and genetically clear. The other two   examples show what happens if one or both affecteds actually have two   copies of the dominant mutant gene. In both cases (bottom row middle and  extreme right) all of the progeny will be affected.

Inherited Disease in the Dog

Now let’s look at the types of inherited disease that have been recognised  in the dog.

There are now 370 recognised inherited diseases in the dog. This sounds   like a large number, but contrast the same estimate in humans where there   are now in excess of 4000 different inherited conditions. Of the 370 canine inherited diseases, 167 have yet to completely defined   and we have no information on their precise mode of inheritance. Of the   remaining 203 conditions where we know the mode of inheritance, 66% are   the result of a single autosomal recessive mutation, 15% by an autosomal   dominant mutation, 5% by an X-linked recessive mutation and 20% are   polygenic. The only class of mutation that we haven’t yet covered are the X-linked   mutations. These refer to mutations in genes that reside on the   chromosome, one of the chromosomes that determines the sex of a dog.   Diseases caused by mutations of genes on the X chromosome have a very   characteristic mode of inheritance. The disease is normally passed down   the female but is usually only expressed in male offspring. Consider a   recessive mutation of a gene on the X chromosome. The gene causing   haemophilia A is one such X gene. Females that have a haemophilia mutation   on one of the X chromosomes are not normally affected because the   condition is recessive and their other X chromosome will have a normal   copy of the gene. So, they can either pass the X chromosome containing the   disease gene to their son or their other X chromosome containing the   normal gene copy. Sons receiving the maternal X carrying the disease gene   will become affected because their other sex chromosome is Y, inherited  from dad, and it will not have a normal copy of this mutated gene.

The problem with data like that provided in the previous slide is that it   is largely gathered by veterinary specialists who see nearly all of the   affected cases. Of course, unaffected dogs don’t go to the vet, so it is   not really possible to say just how relevant a condition is to a   particular breed. It is not sufficient simply to say that an inherited   condition has been reported in a particular breed, we need to also know   how frequently the condition occurs within a breed. Breeders are clearly ideally placed to provide some insight into the   prevelance of a given condition within the breed. They are the ones that   breed the puppies and they are the ones that will know when puppies are   affected with an inherited condition and how frequently such puppies   appear. It is therefore important that breed clubs and councils have   health surveillance in place in order that they can gather information on   disease prevelance. From a recent questionnaire designed to discover just how many breed clubs   or councils in this country have some kind of health surveillance, we   estimate that only around 20% of the clubs and councils in this country   have established health surveillance. It is important for those clubs that   haven’t done so already to establish either a health coordinator or a  health committee.

This slide shows the kind of things that those involved with health   surveillance in a breed can do and the sort of information that they can   provide in order to help breeders construct better breeding programmes   that might limit the spread of inherited disease. The information above is  fairly self explanatory and so we don’t need to elaborate any further.

It is important that breeders screen their breeding stock before they are   used for breeding so that they have a better idea of what genes they carry   and what matings are likely to be compatible and avoid producing affected   offspring. This is not an exact science because we cannot yet test for   many of the genes that cause inherited disease. However, it is certainly   possible to ascertain whether a dog is clinically affected with an   inherited condition. Since many of these will result from recessive mutant   genes, the identification of a clinically affected individual immediately   identifies both of its parents as carriers of the condition (provided the   disease is caused by a single recessive mutant gene). The parents of dogs   that are clinically affected with a polygenic disease are also suspect,   because both must contribute to the clinical status of their offspring,   although not necessarily equally. Health screening breeding stock should become an accepted part of breeding   and all breeders should be encouraged to check their breeding stock before   they use them for breeding. The results of health screening and their   incorporation into breeding programmes can make a significant contribution  to limiting the spread of inherited disease to future generations of dogs.

There are three health screening programmes that have been developed and  run jointly by the Kennel Club and the British Veterinary Association: The BVA/KC/ISDS Eye Scheme: Checks for inherited conditions that affect the eye. The BVA/KC Hip Scoring Scheme that checks for the quality of a dog’s hips in relation to Hip Dysplasia. The BVA/KC Elbow Scoring Scheme that checks for Elbow Dysplasia

Identifying dogs that are affected with an inherited disease and not   breeding from them will certainly, over a period of years, significantly   reduce the incidence of the disease within the breed. For conditions that   result from a dominant mutation, not breeding from affecteds will   essentially remove the disease gene from the breed’s gene pool. However,   most inherited disease in the dog results from recessive mutations.   Unfortunately just removing affecteds from the breeding programme will   have significantly less effect on the overall breed incidence of the   condition. This is because health screening does not identify carriers.   Carriers are usually perfectly normal clinically. Removal of affecteds   will reduce the carrier frequency, but not to the same extent as the   affecteds will go down. These carriers act as a silent reservoir of the   mutant disease gene. The only way to make significant in-roads into recessively inherited   disease is to be able to identify carriers and take note of this   information when constructing breeding programmes. What is needed is a   really good way of identifying carriers. This could be done if we were   able to identify the genes responsible for inherited disease. Once we can   identify individual mutant genes, simple DNA tests of breeding stock will   tell us whether they are genetically clear, a carrier or an affected..   This information will then be invaluable to the breeder who, over a number  of generations, could remove the mutant gene from their line.

The most straightforward way of identifying genes responsible for   inherited disease, although not always possible, is the CANDIDATE GENE   APPROACH. Many inherited disease in dogs have equivalent diseases in man,   mouse and other species. When scientists discover genes that cause disease   in these species, this often acts as a clue as to the dog gene involved.   Two examples should suffice to illustrate the power of this approach. PRA IN THE IRISH SETTER is clinically very similar to a human disease   called Retinitis Pigmentosa (RP) and a blinding disease in mouse caused by   a mutation known as rde. When it was discovered that a mutation in the   same gene caused one form of RP in man and the rde condition in mouse,   scientists checked to see if the same gene was involved in PRA in the   Irish Setter, and it was. The gene responsible for PRA in the Irish Setter   had been identified and a DNA test for this mutant gene quickly followed. CLAD IN THE IRISH SETTER is an inherited disease where affected dogs have   a severely compromised immune system. CLAD stands for Canine Leucocyte   Adhesion Deficiency. Humans (LAD) and cows (BLAD) suffer from a very   similar inherited condition. The same gene mutation causes LAD and BLAD   and this information allowed us to check whether the same gene was   involved in CLAD in the Irish Setter. It was and so a DNA test for CLAD   quickly followed. Irish Red and White Setters also suffer from CLAD and   the DNA test developed for Irish Setters was also shown to work for the  CLAD gene in this breed.

Identifying candidate genes in other species is not always going to be   possible. We therefore need another way of identifying mutant, disease   genes. The recently developed Canine Genome Map will be an invaluable   resource to this end. The Genome Map has resulted in easily identifiable   marker DNA sequences being placed along each and every chromosome in the   dog. Each marker uniquely identifies just one region of a chromosome. So, how will this help to identify disease genes? Well, although we don’t   know where a gene is located, we do know that it will be on one or other   of the canine chromosomes and will have one of the markers that make up   the genome map on either side of it. If we could identify the DNA markers   on either side, we will have located the gene to a relatively small area   of just one of the chromosomes. It then becomes a much simpler task to   look at the genes known to be present in this newly identified chromosome   region and see if any are likely to be candidate genes for the disease. Identifying the markers that flank a gene is relatively straightforward   and relies on the fact the regions of DNA close to each other on the same   chromosome tend to be inherited together. By analysing DNA from   individuals from a pedigree where one or more of the offspring are   affected we can identify the markers that lie either side of the disease  gene.

The markers that lie on either side of a disease gene are said to be   LINKED to the disease gene. As we saw on the previous slide, identifying   markers linked to the disease gene locates the gene to one small region of   one of the chromosomes. However, the identification of linked markers   allows the development of a DNA test for the disease gene. Essentially,   dogs that have the linked marker are very likely to have the disease gene   as well. So, a linked marker test can be developed once the linked markers   are known. Such tests can be of great value to the breeder, but they are   not 100% accurate, a fact that also needs to be taken into account.   However, if used carefully, breeders can use linked marker tests to begin   to construct better breeding programmes. Eventually linked markers will develop into Gene based tests, because the   linked markers will greatly accelerate the discovery of the disease gene.   Candidate gene approaches immediately identify the disease gene and lead   to the immediate development of gene based tests. These tests are the  Rolls Royce of DNA testing because they have 100% accuracy.

The great value of having DNA tests available for disease genes is that   dogs can be prescreened before they are used for mating. Breeders will   know that there dog is clear (two perfectly normal gene copies), a carrier   (one normal gene and one mutant gene) or is affected (two copies of the   mutant gene) before it is used for mating. This information will greatly   improve breeding decisions. For example, breeders will be able to avoid   mating carriers to carriers and the possibility of producing affected  offspring.

Once a DNA test is available for the disease gene is available, breeders   will be able use their carriers for breeding. This is vitally important.   In the past carriers have been identified by other means. But the advice   has always been not to breed from carriers. Whilst this has to be the best   genetical advice, it might not be the best for the breed as a whole. Not   breeding from carriers will certainly halt the further spread of the   disease gene, but it will also deny the breed the possibility of   inheriting the positive features and genes that a carrier might provide.   By not breeding from any proven carriers could well be throwing the baby   out with the bath water. An available DNA test for a disease gene makes   the mating of carriers possible. So, that really good specimen that just   happens to be a carrier can now be bred from and the breed as a whole can   benefit from the positive things the dog has to offer. DNA testing allows carriers to be bred because it allows the owners of the   carrier to choose a genetically normal partner. If a carrier is bred to a   clear, approximately half the litter will be carriers and the other half   will be clear. Furthermore, the DNA test can be used to screen the puppies   to identify the carriers and the clears in the litter, allowing clear  progeny to be bred on from.

In the absence of an available DNA test, breeding from known affected   should be discouraged. However, once a DNA test is available, breeding   from affected should be possible under appropriate circumstances. Again,   the DNA test will allow the owner of the affected dog to choose a   genetically clear mate. All of the litter will be carriers, but one of the   bitch puppies could eventually be mated to another clear dog and this   second generations will be approximately 50% clear and 50% carrier. Again   DNA testing the progeny in this second generation will identify clear   puppies that can eventually be used to extend the line to future  generations.

The Kennel Club recognises the potential role that it can play to   encourage the use of future DNA testing and the subsequent reduction in   the disease burden imposed by inherited disease. It has accepted that the   results of DNA testing can be incorporated into the registration process.   Once a gene based test becomes available, the Kennel Club, in   collaboration with the breed clubs and councils concerned, will draw up a   Control Scheme for the condition. The control scheme for CLAD in the Irish   Setter is given above as an example. Essentially it states that following   the introduction of a new gene based test for a mutant gene, the breed   should very quickly move to a situation where all breeding stock are DNA   tested. With such information, breeders will then be allowed to breed   their carriers provided that a carrier is only mated to a clear dog. If   such a mating takes place, all progeny should be DNA tested to identify   the carrier puppies and the clear puppies. Breeders will have a window of   time, 5 years was the window decided by the Irish Setter breeders, during   which carriers can be mated to clear dogs. However, after this window of   time has elapsed, dogs will only be registered if they can be shown to be   clear of the condition, either by direct DNA testing, or by virtue of the   fact that both their parents were tested clear, I.e. the dog is   hereditarily clear. After this window of opportunity has closed, no   carriers will be registered by the Kennel Club. This will form the model   for all future gene based tests, provided that the breed clubs and   councils agree. The only major difference will be the window of   opportunity during which carriers can be mated to clears. The Irish Setter   breeders decided that 5 years would be sufficient. Other breed clubs might   decide that this window should be either lengthened or shortened as   appropriate. This type of breed control system will not be invoked by the Kennel Club   if the DNA test developed is of the linkage type. This less accurate test  is not considered robust enough to base decision of registration on.

There are only DNA tests available for a relative handful of inherited   disease. Much more research is required to develop new breed-specific DNA   tests for inherited disease. This inevitably, means that more research   will be needed, research that will somehow need to be funded. That is why   the Kennel Club has recently established the Kennel Club Health Foundation   Fund. Formally part of the Kennel Club Charitable Trust, the Health   Foundation Fund will provide ring-fenced funding to allow breed clubs and   councils to develop further DNA tests that will benefit their breed. The   idea is that clubs and councils will raise part of the funding toward the   cost of a research programme and the Health Foundation Fund will provide   top-up funding to allow the research programme to go ahead. The precise   level of support raised by the club will not be an issue. In this way it   is hoped that the numerically small clubs will have as much access to   top-up funding as the numerically larger clubs with their increased   ability to raise funds of their own.