TERMINOLOGY RELATED TO COAT COLOR GENETICS:
GENE: The genetic coding or DNA sequences that determine horse coat color. These 'Genes' are located on strands of protein located in the nuclei of cells.
GENOTYPE: What the genes or specific genetic coding is.
PHENOTYPE: What the horse actually looks like.
HOMOZYGOUS/HETEROZYGOUS: Refers to the gene pairs at a specific location on the protein strand. If the genes are identical, they are Homozygous. If the genes are different, they are Heterozygous.
HOMOZYGOUS BLACK EXPLAINED:
Black And Red are the two basic pigment colors of the horse. Black is dominant. Red is recessive. The Black and Red pigment genes or the "E" Genes are symbolized as follows:
EE - HOMOZYGOUS BLACK
Ee - HETEROZYGOUS BLACK
ee - HOMOZYGOUS RED
Each horse possesses one pair of "E" (Extension) genes or, basic pigment color genes. The offspring that result from the mating of two horses inherits one basic pigment color gene from each parent. Currently there is a DNA-based test called the Red Factor Test, which will determine which genes are present. See the VGL-UC DAVIS RED FACTOR TEST for additional information.
A Homozygous Black (EE) can only pass on the dominant "E" black gene, while the Heterozygous Black (Ee) is able to pass on either the dominant black gene "E" or the recessive red gene "e". A horse must inherit the recessive "e" gene from each parent to be red. Therefore, it is genetically impossible for a Homozygous Black to produce a red foal since there is no recessive red "e" gene to pass on.
Other coat color variations and shades of color occur as a result of the effects of separate, "modifying" genes located at other "spots" or "loci" on the DNA strand. For example, the "A" (Agouti) gene effects the expression of black coat color in horses at times. The "A" gene does not visibly effect red horses since there is no black pigment.
A (Dominant) - Causes the black pigment to be restricted to the points of the horse (mane, tail, legs) with red expressed across the rest of the body. The horse is Bay.
a (Recessive) - Does not restrict the black pigment. The black color is fully expressed across the entire body. The horse is black.
A Homozygous Black (EE) horse will always produce black offspring, not bay, when mated with a sorrell/chestnut whose sire and dam were both black. A black crossed with a black will always produce a black, not bay, if at least one parent is Homozygous Black (EE). If both sire and dam are Heterozygous (Ee), the chance of a black offspring is 75% with a 25% chance for red.
Here is one of the sources on The Minstril: The Minstril | Black Arabian Stallion Homozygous | Arabian Foals Black & Homozygous Black