Definitions
Locus
A 'spot' in the DNA that contains information of a property. Plural: loci.
Gene
Is used with two meanings: locus or allele.
Allele
The possible content of a locus. If only one allele would exist for a locus, the effect would hardly (or not at all) be attributed to the gene and would not be interesting from a breeders point of view. In a limited population (e.g. a breed within a species) it happens that only one allele for a locus exists (e.g. only smooth coated dogs exist). The gene is 'fixed' in this case. For several diseases this would be a desirable situation. For most of the known properties there are at least two alleles.
Homozygous
Both alleles on a locus are equal. Offspring always gets one of these alleles.
Heterozygous
On a locus both alleles are different. A dog is for example heterozygous for black/brown (Bb). Half of its offspring gets the B allele, the other half the b allele.
Dominant
The allele dominates the other alleles at this locus. Often marked by a upper case letter (unless there is range of alleles with a relative hierarchy). A heterozygous animal (e.g. Xx) can not be discerned from a homozygous dominant animal (e.g.. XX). See also: incomplete dominant, recessive.
Recessive
The allele is dominated by the dominant allele at the locus. Often marked by a lower case letter. Only a homozygous recessive animal (e.g. xx) can be discerned from a heterozygous (e.g. Xx) or homozygous dominant animal (e.g. XX). See also: incomplete dominant, dominant.
Incomplete dominant
The allele dominates the recessive allele, but not completely. The phenotype of a heterozygous (e.g. Xx) animal is a stage between homozygous dominant and homozygous recessive.
Phenotype
The perceptible type as a result of a genotype, in other words: the type that you can see or measure. A phenotype can relate to a colour, but also to the ability to produce a certain substance or to the resistance to a disease. One phenotype can be caused by several genotypes. If the imaginary coat colour purple is caused by the dominant gene P, the phenotype purple can by caused by the genotypes Pp and PP.
Genotype
The genetic foundation of a phenotype. Several genotypes can result in the same phenotype, but one genotype can never result in more that one phenotype.
DNA
DesoxyriboNucleicAcid. A compound made of phosphates, sugars (deoxyribose) and nitrogen bases that forms long chains in the cell. Appears in double strands that have the form of a helix. This substance forms the carrier of genetic information. DNA contains information for building numerous proteins, e.g. enzymes. Several coat colours can be 'explained' by the fact that a certain gene (locus) contains information for the enzyme that regulates the production of a pigment.
Heritability index
Number to express the extent of heredity of a certain property. It gives an impression of the extent of the influence of the environment on the expression of genetic properties. A high heritability index indicates that environmental factors are less important than genetic material.
$$h^2 = { V_{genetics} \over V_{genetics} + V_{environment} }$$
\(h^2\) : heritability index
\(V_{genetics}\) : Variance caused by genetic influences
\(V_{environment}\) : Variance caused by environmental factors
The calculation of a heritability index is often used with polygenic transmitting properties for which no clear genetic base exists. These properties occur in various degrees and can be influenced by the environment. By calculating the heritability index one can guess the result that is to be expected from selection and the result from changing the environment.
Examples of this kind of properties are: protein content of milk, hip dysplasia in dogs, fertility.
Polygenic
Property determined by multiple genes (loci). A property like hip dysplasia is not a yes/no property. The severity of hip dysplasia is — as far as the genetic influence is concerned — determined by multiple genes (loci).
This kind of properties is often influenced by the environment. See also: Heritability index.