Dogs, like wolves from which they probably descend, have thirty nine pairs of Chromosomes. These are long thread-like structures found in each body cell and along the length of which are the genes. The number of chromosomes is fully documented but the number of genes quite unknown and likely to remain so in the foreseeable future. There are, however, likely to be many thousands and each one will influence a specific aspect of the dog or, in some instances, more then one aspect. A gene is found at a particular location on a particular chromosome and, as such it will influence a particular trait, Because a dog has two of each chromosome, one from the father and one from the mother, it has two of each gene. If the tvvo are identical, the genes are said to be homozygous and if they differ, they are said to be heterozygous for that trait. Two versions of a particular gene are called alleles. Thus we have the gene causing black pigment which comes in two versions One, designated B, causes black pigment to form in the dog while the alternative, b, prevents black pigment forming and instead leads to liver or chocolate pigment. The upper-case letter indicates a dominant allele vvhile the lower-case letter indicates a recessive. A dog carrying one dominant allele will show that it carries it whereas, for a recessive to be seen, the dog must carry two versions of the recessive allele. Thus BB and Bb will carry black pigment. Such dogs will have black noses, eyerims pads and so on, even if the coat colour is all white and they will be quite indistinguishable. Just looking at BB and Bb animals, one could not tell that they vvere genetically dissimilar However, breeding from the first dog will give rise only to black pigment-producing offspring because they all get a B from him or her. In contrast, the Bb dog can transmit either B or b. If he transmits a b and the offspring also get b from the other mate, they will be bb and thus cannot from black pigment. Such dogs will be liver or chocolate. In some breeds, e.g. Dobermanns, Labradors, and Springer Spaniels, such a colour is quite acceptable. In others, e g. GSD’s or Tibetan Terriers, such a colour is frovvned upon as undesirable. It is, however, important to appreciate that recessive traits such as bb are aesthetically undesirable rather than being physically damaging whereas some recessive traits are actually harmful to the dog. Genetic disorders can be classified into one of three broad categories:
1. Defects caused by a single mutant gene of large effect.
2. Defects caused by the interaction of a series of genes (polygenic).
3. Defects caused by an abnormality of chromosome number or structure.
This article will deal only with the first of these categories: Single gene defects of large effect. Such genetic defects originate usually as a mutation. This is a genetic change of the material inherited by a dog from its parents. Either there is a change of the material in situ or the gametes produced by that dog (as sperm or eggs) are altered and thus contain something which was not originally present in the dog itself. Mutations take place very rarely. They may occur as infrequently as once in a million or once in one hundred thousand gametes. It has been estimated for example, that albinism in man occurs twenty eight times per million gametes per generation as a mutation. Mutations are caused by such things as radiation, certain chemicals or even abnormally high and low temperatures. Sometimes they occur naturally without any obvious reason. Frequently, when a defect crops up in a kennel, the breeder dismisses it as a mutation because it has never been seen before. ln a strict sense it is a mutation, but the implication that it arose in that single generation may not be accurate. lt may have been lying in the breeding stock of that kennel for many years without coming to the surface. A German Shepherd kennel could carry the liver’ factor (b) for many generations without it being knovvn because the matings have aivvays been BB to Bb and thus livers did not appear. Suddenly, a Bb to Bb mating is made unwittingly and a proportion of liver marked dogs (bb) can crop up. The liver factor is truly a mutant but not of recent production. Recessive genes can lie hidden for generations and suddenly crop up when one least expects them. Some breeds, of course, have been based upon mutant genes. The achondroplastic breeds like the Dachshund are founded upon the occurrence of a mutation vvhich vvas developed by someone vvho saw a use for the animal. Many mutations are lethal, such that we are not even aware that they were present. A pup is conceived, carries a lethal mutation and is lost at an early embryonic stage without the breeder even knowing that it had existed. Sometimes the gene is not powerful enough to cause death at this early stage but allows development to full term and then brings about death in early post-natal life. Alternatively. the gene may not express itself until later life even though obtained at conception. Thus, some eye diseases do not become apparent until the dog is three to five years of age. Some defects cause severe problems to an animal, others are more aesthetic than biologically important. Some tend to be self-limiting because the bearer of the double recessive gene is sterile and thus cannot be bred from: in such cases, the defect is spread by carriers. This would be true of such disorders as progressive neuronal abiotroph in the Kerry Blue vvhere affected dogs rarely survive to adulthood, or pituitary dwarfism in the GSD vvhere the dw/dw animal is sterile. In the case of a defect like canine Von Willebland’s Disease, we have a dominant gene VWD) and the homozygous dominant VWD/VWD is dead at birth or soon afterwards while the heterozygote VWD/vwd is suffering from a blood clothing disease at factor VIII.