Homozygous is a Genetics term to do with Genes. A Gene is a part of DNA which acts as a code to control part of a cells chemistry. In this excerpt we are focusing on the gene(s) that produces color.

Genetics control how our bodies, Human and Animal are built and how we operate. Genetics is controlled largely by our genes; notwithstanding environmental influences. Each cell in our body carries two sets of genes, one from the mother, and one from the father.

The equivalent genes in each set don't have to be the same. In other words, one set might carry the gene for blue eyes, and the other set may carry the gene for brown eyes.

The different possible genes that could be carried for a specific characteristic are called Alleles. Allele means any of the alternative forms of a gene that may occur at a given location. Alleles for eye color could be green, blue, brown, hazel etc., and each person or animal carries some combination of two of those alleles.

Homozygous means that each set carries the same gene for that characteristic meaning that both sets carry the gene for blue eyes. The term homozygous is commonly used when talking about color and if progeny will inherit if from a parent.

Only one set of genes get passed on to the offspring. If a parent is homozygous for say the eye color blue, that parent will definitely pass that gene on to their offspring as that is the gene for eye color that they carry in either gene set.

The opposite of Homozygous is Heterozygous Hetero meaning different. This means the two alleles the parents carry are different. The Mother may carry the gene for brown eyes, and the Father the gene for green eyes.

With a homozygous individual, it is obvious that if they are homozygous for blue eyes, the offspring will have blue eyes. But what about the individual who is heterozygous for eye color and carry two different kinds of genes like both the green and brown genes. Will the offspring’s eyes be green, brown, both, or a mixture of the two? That depends on how the genes interact. One allele might be dominant over all other alleles which means that in a heterozygote animal, that is what the appearance the animal will be.

Let us say for argument sake that brown is dominant over all other alleles. In our green-brown animal, its eyes will be brown since brown is dominant. Green would be known as recessive, in this case, because it is not shown in the animal’s appearance. This is called phenotype.

Other forms of interaction between alleles are co-dominance, where both alleles show in the phenotype. This would be eyes that are both green and brown e.g. in patches of each color; or incomplete dominance where the phenotype is a mixture of the two allelels e.g. the whole eye is greeny-brown.

This has to do with gene pairs. If the genes are identical, you have Homozygous. This means guaranteed to throw a certain type of color.

The term homozygous of course is often used in relation to alleles without the terminology recessive or dominant attached when the gene is commonly known it will be dominant and will show as the phenotype in offspring.

There are other factors that apply as in the allele responsible for black coat color known as the recessive Agouti gene. A foal must be aa to be black. A stallion may pass one a on to his offspring, but the mare must also contribute an a as well. A is the dominant Agouti allele.

If an owner of a stud horse advertises it as homozygous black for example, you are guaranteed a black foal. However, it depends on what they are homozygous for. If they are homozygous for recessive alleles then the other parent may contribute a dominant allele which would over-ride that.

So, if you were to buy a homozygous black Stud, he would throw a black foal only if he is homozygous dominant. This also applies to homozygous anything else too.

Also see these for specifics on Homozygous Genetic Traits:

• Pleasure Gait Farms

• Newzeland Horses Online

• Old Mill Farms Gypsies & Drums