The passage of traits or characteristics from parent to offspring does not occur in a haphazard manner. It is a matter of genetic inheritance. The science of generics traces back to the work of an Austrian monk named Gregor Mendel who discovered that there is a reliable method of inheritance that can be predicted. Mendel found that small units, called genes, are present in the cells of all individuals.
These genes control the development of the organism's characteristics. They are present in pairs, with one half of each pair being inherited from each parent. When each half of the partner genes was found to affect the organism in an identical manner, the pair was called homozygous. When each half of the pair was found to affect the organism in a contrasting manner, the pair was called heterozygous.
Mendel discovered that in heterozygous gene pairs containing contrasting characteristics, one half of the pair suppresses the characteristics contained in the other half of the pair. He termed the characteristic that expresses itself dominant, and the one that remained hidden he termed recessive. There are instances, however, when neither gene is completely dominant over the other. This is known as incomplete or partial dominance and the result is blending of the characteristics from the pair. A few examples of traits that are genetically dominant and recessive in dogs are as follows:
To put these principles to work in breeding better dogs, breeder must understand that what a dog looks like physically, or on the outside, does not directly reflect what he looks like genetically, on the inside. His outward appearance is his phenotype, and all the traits he has inherited from his ancestors that he can pass on to his offspring form his genotype. Phenotype is not a guarantee of genotype. In other words, what a dog looks and acts like is no guarantee that he can pass his traits on to his offspring. This is even further complicated by the fact that some traits (phenotypes) are the result of several genes and therefore difficult to predict.
- Two dominant genes: he is pure for that trait, having received a dominant gene from each parent. This is termed dominant/homozygous.
- Two recessive genes: he is pure for that trait, having received a recessive gene from each parent. This is termed recessive/homozygous.
- One dominant and one recessive gene: he is mixed for that trait, having received a dominant gene from one parent and a recessive gene from the other parent. This is termed heterozygous.
By applying these (simplified) principles, there is a ratio that can be applied to help predict the genotype of the offspring of a mating. If a parent is homozygous for a trait (1 and 2), he will pass on that trait regardless of which half of his gene pair he gives to his offspring. If a parent is heterozygous for a trait (3), he will pass on either a dominant or recessive gene to his offspring - a 50/50 chance. These ratios apply for both parents, and whether or not an offspring exhibits a trait depends on the number of dominant and recessive genes he receives from each half of the inherited gene pairs. There are six ways that a pair of genes can unite, based on the possible combinations of dominance and recessiveness present in the genotype of the sire and dam.
The task facing a dedicated breeder is to determine what genes are present in his breeding stock and to plan matings that will bring forth offspring which exhibit and carry desirable genes. The goal is to eliminate unwanted traits not only from the phenotype but also from the genotype for future generations. In this way the overall quality of the breed should improve. The breeding out undesirable characteristics can be successful if sires and dams are selected carefully, using various breeding strategies to find mates that complement each other. It is important to choose a mate who shows strengths where his partner shows weaknesses, an animal whose background is compatible with that of his partner's.