Genetic Basics

Horse coat colour genetics can be a complex thing to learn about to somebody who is not familiar with basic genetics or with horse colours. To simplify a genetic code resulting in a horse colour you can think of somebody building a house; you start with the foundation (the Extension and Agouti), add the walls (dilute genes) and stick on the roof (modifiers), remove parts of the walls for doors and windows (pinto genes), get a modern arts architect in there to totally remodel things (the LP and pattern genes) and you get an unique house (or a very colourful horse). This way of thinking of colour genes as building blocks is a great way to better understand a horse’s colour.

Genetic Terms:

Gene: A gene is a specific sequence of DNA (deoxyribonucleic acid) forming a section of chromosome. It is often refer to as the section of coding that relates directly to the inheritance from parents to offspring.
Genome: A genome is referred to as the entire sequence or code of an animal’s hereditary information.
Chromosome: Chromosomes are structures created by DNA and proteins. Horses have 64 chromosomes whereas humans have 23 chromosomes.
Locus: A locus is the location of a gene on the chromosome. Loci is the plural term.
Allele: An allele is a mutation form of a gene which is found in specific locus. There can only be a maximum of 2 alleles found at one locus. Often people, including myself, will mistakenly use the term gene in the place of allele and among the basic genetic world it seems to be an acceptable mistake.
Heterozygous: One allele present in a locus.
Homozygous: Two of the same alleles present in a locus.
Phenotype: A phenotype is a characteristic or trait influence by a gene or outer trait. Here it is used as a term reflecting how the horse looks with the specific set of genes it carries.

Mendel’s Theory of Inheritance

Gregor Mendel discovered the basic mode of inheritance via breeding pea plants and other flowers. He discovered that when breeding different colour plants together the resulting plants were not a mixture of both colours but rather was either colour of the parents. He bred many generations resulting in interesting results which led to the theory (and discovery) that each living creature has two spots to inherit a gene for a specific trait; one from the mother and one from the father. This area is called a locus and the codes for the traits are called alleles. When one spot is filled for a trait it is called heterozygous, when both spots are filled for the same trait it is called homozygous. Through this he also discovered dominant and recessive alleles that are present at these loci. A dominant allele expresses it’s trait with only one copy of the gene needed at the locus. A recessive allele needs two copies of the gene to express it’s trait at a locus, when in a situation where there is a heterozygous genome with 1 dominant and 1 recessive allele at one locus the dominant allele will win and show it’s trait (this is seen in the extension locus in horses).

Punnett Squares; Dominant and Recessive Genes

Punnett squares are used to discover and predict the outcome to offspring thanks to the Mendelian theory of having a location (or locus) of two spots for alleles, one spot to be filled from the mother and the other spot to be filled by the father as well as having dominant and recessive alleles within these loci. Listed on the top and the left side are the parents genome to the trait that you are trying to calculate, in the center of the square is the possibilities of the offspring to carry that trait. The most basic Punnett square is only calculating for one trait’s chances. A dominant allele is written in a capital letter and a recessive allele is written in a lower case letter. You will see here three different Punnett squares showing three different genome combinations and therefore the different odds using homozygous and heterozygous parents. Assuming in these situations that the green g is a recessive allele and the yellow Y is a dominant allele. First you see two homozygous recessive green g’s breeding together results in 100% green g recessive offspring. This would be the same when two homozygous dominant yellow Y’s breed together resulting in 100% dominant yellow Y offspring. When a homozygous recessive green breeds with a homozygous dominant yellow it results in 100% heterozygous yellow Yg. Yellow being the dominant allele it will show it’s traits with just one copy and over powers the one recessive green trait. Two heterozygous yellow Y’s breeding together results in 25% chance of getting a homozygous yellow Y, 50% chance in getting a heterozygous yellow Yg and 25% chance of getting a homozygous green gg.

Incomplete Dominant Genes

Horse coat colour genetics can be easily predicted using Punnett squares providing you know the correct colour genes your horses carry and of course knowing what genes are dominant and what genes are recessive. Horse genes are more complex then that however, horses also carry what is called an incomplete dominant gene. An incomplete dominant gene is a gene that when heterozygous (one copy of the allele) gives a different result then when homozygous (two copies of the allele). In this Punnett square we will see that blue B is an incomplete dominant colour. Homozygous B is bred with homozygous g results in 100% heterozygous Bg, however as there is only one copy of the incomplete dominant B allele in the locus resulting in the B being lighter and less bold in it’s trait.