Glossary of terms used to describe coat colour genetics

The technical language of genetics can be difficult to decipher. Here are definitions of key terms to assist you.



A condition caused by the lack of melanin production in the iris, hair and skin. In humans, most often inherited as an autosomal recessive. Many different types of albinism exist in humans and other animals, caused by a variety of different genetic mechanisms. Some forms of albinism cause almost total lack of melanin, others have a limited effect.


Any of the alternative forms of a gene. These may be thought of as various different versions of a gene. There may be several possible alleles for a particular locus, a corresponding point on two homologous chromosomes. The older a gene is, the more time it has had to mutate, or develop new versions of itself. There can be many alleles for such a gene.


Animals that have a common origin and common characteristics that distinguish them from other groups of animals within that same species.


Thread-like structures found in the cell nucleus on which genes are carried.


An animal produced from parents of different breeds.

Deoxyribonucleic acid (DNA)

The chemical that is the basis of the gene.


An allele that overrides the expression of a corresponding recessive allele. In the heterozygous state, the dominant of the two will hide the instructions its recessive partner holds. For example, a grey horse has at least one dominant allele (G) at the grey gene locus. This allows grey pigmentation to form throughout the coat of the animal.


The basic unit of inheritance, transferred from parent to offspring.


The genetic makeup of an animal, which can be thought of as a recipe or set of building plans.


A state where two different alleles are found at a particular locus. A horse that is heterozygous for tobiano (Toto) has inherited the dominant To allele from one parent, and not from the other (to instead). This horse will have tobiano offspring half of the time, when bred to a mare not carrying genes that mask tobiano patterning.


A state where both the alleles found at a given locus are identical (see definitions that follow). For example, horses that are homozygous for a particular trait, such as tobiano patterning, carry two dominant alleles on the tobiano locus (ToTo). These horses will have tobiano offspring no matter what the colour of the other horse they arecrossed to, as long as that horse carries no genes which mask or alter tobiano, such as the grey gene (G).


Secretion from special glands which permits or encourages certain functions.


The mating of progeny produced by parents that are more closely related than the average of the population from which they come.

Incomplete Dominance

A gene that acts is partially expressed when one allele is present, and at a higher/stronger level of expression when the horse is homozygous for that allele. Examples in the horse include the cream dilution gene (Cr) and the leopard complex gene (Lp).

Lethal mutation

A form of a gene that when expressed causes death.


Genes associated because they appear on the same chromosome.


The specific site of a gene on a chromosome. For each matching pair of chromosomes, there are genes located at identical points on these chromosomes that are said to be homologous with each other. These matching locations are the loci for a particular gene.


Change in genetic material induced naturally, by chemicals or by radiation.


Used to describe when a specific cross appears to be successful.


Outward expression of the animal's genetic makeup.


Controlled by many genes.


A member of a group of animals which have been developed, through successive generations of closed breeding and selection for a specific set of traits, such that they are capable of consistently reproducing themselves (see definition of true-breeding).


Term describing an allele that is not expressed in the heterozygous condition. The expression of the recessive allele is overridden by the presence of a corresponding dominant allele. Homozygous recessive traits can only be expressed when no dominant allele is present. For example, a chestnut horse has two recessive alleles for the extension of black locus (ee), so that black pigmentation is restricted to the skin, and is blocked from entering the hair. It is important to note that recessive traits can only be expressed in the homozygous state.


The causing or allowing of certain individuals to produce the next generation. Both artificial selection by humans and natural selection by nature are operative in a population.


Condition seen more frequently in one sex than the other.


Condition carried on the sex chromosomes.


Characteristic or feature of an animal.


An animal that demonstrates prepotency by showing the ability to transmit its traits to its offspring consistently such that they clearly resemble the parent and each other. Homozygous dominant individuals are prepotent as a direct result of multiple generations of breeding type to type while selecting for a specified set of traits.