Allele: An alternative form of a gene. A gene may have several alleles (be multi-allelic) but a dog can only have two alleles of any gene and these may be identical or not. 

Dominant: Relates to an allele of a gene which, when present in a single dose, will mask the presence of another. Usually depicted by upper case letters. 

Gene: The unit of inheritance. 

Genotype: The genetic structure of the animal. What an animal carries in its genetic makeup but which may not be apparent in his appearance. 

Heterozygous: Strictly different -- used to indicate dogs that carry different alleles of a gene at a specific locus. Thus Aa and Bb are examples of heterozygous combinations. 

Homozygous: Strictly the same -- used to indicate dogs carrying the same allele of a gene in duplicate. Thus AA and aa are homozygous animals. 

Locus (plural loci): A particular point on a specific chromosome. Any gene is always found at the same locus. 

Phenotype: The visible expression of a character. Thus BB and Bb would be different genotypes, but the same phenotype (black). 

Recessive: Refers to an allele that needs to be present in duplicate in order to indicate its presence. In a single dose it will be masked by a dominant allele. 

Bibliography

"Genetics of the Dog", by Malcom B. Willis. Howell Book House, New York. 1989.
"The Inheritance of Coat Color in Dogs", by Clarence C. Little. Howell Book House, New York. 1957.