Gene:  1) hereditary determinant for a trait. 2)  Piece of DNA with code for a protein.   For example, genes are responsible for hair and skin color by directing the production of varying amounts and types of the protein melatonin. 

Genome:  The complete set of genes and associated regulatory DNA.  The study of the genome is called genomics.

Chromosome:  Large molecule that contains DNA (complexed with proteins) in a cell.  Each horse cell has 32 pairs.  Human cells have 23 pairs.

Allele:  Form of a gene. For example, alternate forms of the gene called MC1R produce red or black pigment in hair.  (Sometimes alleles are referred to, colloquially as genes, as in the gene for red hair versus the gene for black hair. Technically, allele is the correct term.)

Homozygous:  All animals have two gene copies, one from the mother and one from the father. If both copies of the gene are the same, then the individual is said to be homozygous for that gene.

Heterozygous:  All animals have two gene copies, one from the mother and one from the father. If both copies are different, then the individual is said to be heterozygous for that gene.

Dominant gene:  An allele which is expressed whenever it is present.

Recessive gene:  An allele which is expressed only in the absence of a dominant gene.  Horses with one copy of a dominant allele and one copy of a recessive allele are said to be carriers of the recessive allele (gene)

Carriers:  Individuals which do not possess a trait but can pass it to their offspring are said to be carriers for that genetic trait. This is characteristic of recessive genes.

Gene locus: The DNA site for a gene.  For example, there is a locus for the grey gene and its two alleles are the presence of grey and the absence of grey. 

DNA:  The molecule that contains hereditary material and the major constituent of chromosomes.  DNA is composed of only 4 molecular units (bases) but the organization of the units is unique among genes and species. A gene may have 1000 to 2000 bases long.  The whole genome of any mammal is composed of approximately 3 billion units of these 4 bases.

Mutation:  A genetic variation in the gene that causes and alternative appearance of a trait within a species.  This can also be called a genetic variant. Mutations can be identified as changes in the composition of the DNA at a unique site. 

Proteins:  Product of genes.  Genes usually act by creating proteins which act as enzymes or which can be structural parts of cells.

Gene Map or Genome Map:  The known order for a set of genes or DNA markers at intervals along a chromosome.  A minimum density map will have 300 markers while a high density gene map will have 10,000 markers.  Denser maps are more effective research tools.

Genome Sequence: The complete DNA sequence determined for all chromosomes in a cell for an individual, including all 20,000 genes and approximately 3 billion DNA bases.