Autosomal Recessive Inheritance
Each cell in a dogs body contains 39 pairs of chromosomes. One chromosome (an allele) from each pair is inherited from the bitch and one is inherited from the sire. The chromosomes contain the genes that are inherited from the parents. When a sperm and egg come together to form a new set of DNA, the two halves combine, so that each puppy has two copies of every gene.
For example, in humans, for the gene that determines eye colour, you may inherit a brown-eye gene from your mother and a blue-eye gene from your father. In this instance, you will have brown eyes because brown is the dominant gene. The different forms of genes for eye colour are caused by changes (mutations) in the DNA code.
Dominant and recessive alleles
Alleles (one half of the chromosome) can be said to be either recessive or dominant.
A recessive allele is only expressed (influences the characteristics of the dog) if both alleles are the same.
A dominant allele on the other hand is always expressed, even if it is accompanied by a different allele.
The same is true for medical conditions. There may be a faulty version of a gene that results in a medical condition, and a normal version that may not cause health problems.
Autosomal dominant inheritance
Some conditions are inherited in an autosomal dominant pattern. In this case, if either dog has the mutation for the condition, there is a 50% chance of the puppy inheriting it.
Autosomal recessive inheritance
Some conditions can only be inherited in an autosomal recessive pattern. This means the condition can only be passed on to a puppy if both parents have a copy of the faulty gene – both are "carriers" of the condition.
If the dog only inherits 1 copy of the faulty gene, they'll be a carrier of the condition but won't have the condition.
If the bitch and sire both carry the faulty gene, there's a 25% chance of each puppy they have inheriting the genetic condition, a 50% of being a carrier and a 25% chance of being clear of the condition.
If one of the parents is clear and the other a carrier, there is a 50% chance of the puppies being a carrier.
Distinguishing between clear, carrier and affected dogs
Clear dogs have no copies of the mutant gene responsible for the condition and will neither develop the condition nor pass the gene on to their offspring.
Carrier dogs have one copy of the normal gene and one copy of the mutant gene; they will not develop the condition, but will pass a mutant gene on to approximately half of their offspring.
Affected dogs have two copies of the mutant gene that causes the condition and will develop the disease.
The term unaffected is a dog that is untested or a carrier of a condition.
This is a great, clear article on the use of carriers for breeding purposes - Cathryn Mellersh - Breeding with Carriers
The recent power point presentation by Clumber Spaniel Health is available here.