All animals are made up of billions of tiny cells. The nucleus of these cells contains all the information to regulate
the activity of the cell and therefore the form and function of the particular body tissue, and ultimately to form the individual
animal. This information originally comes from the parents of the animal, with approximately one half from the mother and
one half from the father.
This information from which all life develops is in the form of DNA (deoxyribonucleic acid). A
gene is a portion of a DNA molecule, carried on a chromosome. Think of a chromosome as a long string of genes. Hundreds and
even thousands of genes may be carried on a particular chromosome. Chromosomes occur in pairs in the cell nucleus, except
in the egg and sperm where they occur in half pairs. When an egg is fertilized by the sperm, the resulting cell from which
the animal will develop has complete pairs again. This is the way in which one half of the genetic information comes from
The dog has 78 chromosomes, in 39 pairs, on which approximately 100,000 genes are located. This makes up
the animal's genotype. The phenotype is what you actually see in the animal, and this can be influenced by both environmental
and developmental factors. For example, a dog's size as an adult is determined partly by his or her genes, but is also influenced
by environmental factors such as its health as a puppy and the food it eats.
Each gene in a chromosome pair has a partner
at the same position (or locus) on the matching chromosome. Each member of a gene pair is called an allele. A gene can have
many alleles within a population but an individual animal will have only 2 alleles which influence a particular trait. If
the 2 alleles are identical (AA or aa for example), the individual is homozygous at that locus; if the alleles
are different (Aa), then heterozygous.
If the allele is dominant, only 1 copy is required
to express the trait; if recessive then 2 copies. Upper case letters are traditionally used to represent
dominant traits, lower case letters for recessive traits. Thus for a dominant trait, either AA or Aa will express the particular
characteristic, while for a recessive trait only aa will express the characteristic. The heterozygote (Aa) will be a carrier
- clinically unaffected but able to pass the harmful allele to the offspring.
Sex-linked characteristics are slightly different. Females
have a pair of X chromosomes (XX) while males have 1 X and 1 Y chromosome (XY). Thus 1 dose of a recessive X-linked trait
(x) will cause the expression of that characteristic in a male, while a female with only 1 dose(Xx) will be a clinically unaffected
carrier . The bleeding disorder hemophilia is probably the best known example of a sex-linked condition.
Defects: inherited or not?
A disease condition or abnormality
may be caused by many factors. Some of these are genetic; that is the disorder is a result of a mutation in a gene that carries
particular information. Some mutations are spontaneous, such as a mutation caused by toxins consumed by the mother during
pregnancy. An inherited defect is one in which the defective gene has been inherited from one or both of the parents.
conditions that have a well-documented hereditary basis may also have other causes. For example, there are several forms of
hereditary cataracts, but cataracts may also occur as a result of injury, toxins, or a disease such as diabetes. In trying
to determine whether a disorder is inherited, your veterinarian will look at many factors, including the age the disorder
becomes evident, whether littermates or other relatives are affected, and whether the defect is known to occur in that breed.
It is very important that inherited disorders be identified so that information can be relayed back to the breeder, and on
a larger scale, so that breeding programs can be designed to reduce or eliminate these debilitating conditions in dogs.
Patterns of inheritance
The specific pattern of inheritance
has not been established for many of the disorders that are believed to be inherited. Where the mode of inheritance is not
known, breeds that have an increased risk relative to other dog breeds are said to have a breed predisposition for a particular
The following describes known patterns of inheritance.
1 copy of the gene, which may be inherited from either parent, is required to produce the trait. The parent with the dominant
trait will pass the affected gene to approximately half its offspring, and the trait will be apparent in both the parent and
the affected progeny. These conditions are uncommon because, as long as it is of early onset (i.e. becomes apparent before
breeding age is reached), the disorder can be readily eliminated by avoiding the breeding of affected individuals.
many instances however, there is incomplete dominance. The trait may be dominant with variable expressivity, which means that
if either parent is affected, all puppies have a susceptibility to the disorder but not all will be affected equally. Alternately,
a dominant trait may have incomplete penetrance. If penetrance is 75% for example, only about 3 quarters of the pups who inherit
the trait will express it.
This is the most common mode of inheritance for
genetic conditions in dogs. Progressive retinal atrophy (PRA), which causes blindness in many breeds, is such a trait. To
be affected, the animal must inherit 2 copies of the gene (genotype pp), 1 from each parent. Dogs with the genotype PP (normal)
or Pp (carrier) will be clinically normal but the carrier will pass the affected gene to approximately half the offspring.
As long as carriers (Pp) are mated to normal animals (PP), the offspring will be unaffected but some will remain carriers.
If 2 carriers are mated, some of the offspring (approximately 25%) will be affected.
As long as the frequency of a gene for a recessive disorder remains low in the population, the particular gene may
be passed along for many generations before by chance 2 carriers are mated and affected individuals are born. However, the
gene frequency may become unusually high due to breeding of close family members, or because of the "popular sire"
effect , where a sire with a harmful recessive gene is mated frequently because of desirable traits.
Because the recessive
gene is carried in the population in outwardly normal animals, it is very difficult to eradicate these traits. However the
incidence can be reduced by identification of carriers through test matings or through various tests that have been developed,
and the conscientious use of this information in breeding programs. Veterinarians, dog breeders, and breed associations must
all work together for substantial progress to be achieved.
In these traits,
the gene is located on the X chromosome. Males have 1 X chromosome from their mother, and 1 Y chromosome from their father,
which carries little information other than maleness. Females have 2 X chromosomes, 1 each from their mother and father. So
if a mother who is a carrier for a harmful recessive gene (Xx) passes the recessive gene (x) to her daughter, the daughter
will be an unaffected carrier, but her sons who receive that gene will be affected.
The bleeding disorder hemophilia
is the best known of the X-linked traits, which are uncommon in the dog. Control programs are possible because carrier females
can be identified through blood screening.
The above-mentioned traits are inherited in a straightforward manner. Many
others are inherited in a more complex fashion. In fact, most traits that are selected for in the dog are the result of the
interaction of many genes. Modifying genes may influence how other genes are expressed. As mentioned above, a trait may be
dominant, but with incomplete penetrance so that it is not always expressed. Epistaxis occurs when alleles at one locus mask
the action of another pair of alleles.
Polygenic traits are controlled
by an unknown number of genes. The gene expression is influenced by a variety of factors including gender, nutrition, breed,
rate of growth, and amount of exercise. These traits are quantitative traits - that is, there is a wide range within the population.
Such traits include height, weight, character, working abilities, and some genetic defects. Heritability varies within different
breeds and within different populations of a particular breed.
Because it is virtually impossible to determine the exact
genotype for such traits, it is difficult to control defects with a polygenic mode of inheritance. The best attempts at control
are based on a grading scheme for identification of the defect and a breed policy of recording and publishing the results
for as many dogs as possible. Canine hip dysplasia is a polygenic trait that remains a problem in most large breeds of dog,
despite efforts to control this condition dating back to the 1960s. Breed organizations and veterinarians in various countries
have developed control programs that rely on radiographic evaluation and a central registry of dogs. Thoughtful selection
by breeders, using this information, has greatly reduced the incidence of hip dysplasia in those breeds in particular countries.
© 1998 Canine Inherited Disorders Database