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Lewis & Clark

Mustang History, part 2

A Quick Overview of Horse Genetics

picture from www.rothamsted.bbsrc.ac.uk	DNA is the basic stuff of inheritance. DNA strands are organized into segments called CHROMOSOMES. Chromosomes are made up of strings of paired GENES. Each genetic segment controls a specific effect - or it may influence more than one effect (Example: Frame Overo itself is a color effect resulting from the "Fr" gene, but the gene also causes Lethal White under certain conditions.)
Genes come in pairs, one inherited from Mom and the other inherited from Dad.	This illustration shows a DNA strand breaking apart during meiosis, the form of cell division that creates the egg and sperm for reproduction. Each gene pair separates, with one gene of each pair  going into one reproductive cell (egg or sperm) and the other going into another one. So each egg or sperm contains 1/2 the parent's genes - one gene from each pair.

If you want to know more about DNA, Genes, and Chromosomes, click here for a great website that explains genetics simply and clearly: eurekascience.com

And this one from Glaxo Smith Kline: genetics.gsk.com/ overview.htm

HORSE COLORS:

All other color effects are "add-ons" to the base colors. Here are some examples:

Base coat is RED with Appaloosa (Lp) genetic complex to add the spots)

Base coat is BLACK with Tobiano paint/pinto gene

Base coat is RED with the DILUTION Gene "Creme" which "bleaches" red to gold body with pale gold or white points: Palomino

Base coat is BLACK with the Agouti gene, which turns black to bay, then further diluted by the Creme gene, to create Buckskin

DOMINANT AND RECESSIVE GENES

Most color genes except Red and Flaxen are either dominant or mixed dominant.

Dominant means that the horse only needs one copy of the gene for the effect to appear.

Mixed Dominant means that one gene will create one effect, while two genes will intensify it to create yet another. An example is the Creme Gene. One creme gene dilutes the red base to create golden Palomino. Two creme genes intensifies the "bleaching" or "diluting" effect to create the nearly-white, blue eyed Cremello.

Genetically, "DOMINANCE" has nothing to do with aggressiveness or strength. Some of the recessive genes can be very "aggressive" and can dominate a life - consider hemophilia - but they are RECESSIVE because both genes on the pair must be the same for the effect to appear.

HOMOZYGOUS & HETEROZYGOUS

Genes always come in PAIRS - one from each parent. If both genes are the same (whether recessive or dominant), the animal is called "homozygous" for that particular gene. If different, the animal is said to be "heterozygous" for that trait.

Because they come in pairs, genetic coding is much like digital coding for computers. Either a gene is "turned on" or "turned off." For an effect to be present, the gene needs to be turned on. This happens if at least one gene in the pair is a dominant gene. (The exception here is the Extension Gene - "turned on" means "black or red" and "turned off" means "red only.")

The only time a recessive gene's action is felt is when it is paired with another just like it (in other words, when it is homozygous). All Red (chestnut, sorrel) horses are homozygous for the recessive Red gene. If they weren't, they would be black, as a single black gene will dominate a red one. Flaxen is the only other known recessive color gene. So flaxen chestnuts are homozygous for both red and flaxen. Thus they always breed true - they have no hidden recessives to pass on to their offspring.

ACTION OF GENES

Genes may be categorized by their actions upon base coat colors:

• Dilution (example: Creme, Champagne, Dun, and Silver Dapples)
• Restriction (example:  Agouti)
• Reorganizing: Brindle
• Additive (examples:  Sooty, Roan) or
• Blocking (example: White patterns on Pinto)
• Overtaking and Replacing (Grey)
• Modifying (example: Pangare)

Genes (Technically, Genetic Allelles*) may be:

• Dominant (examples: Gray, Roan) If the dominant gene is present, the horse will be exhibit that trait, regardless of the presence of any other genes) Most color genes are dominant.
• Recessive (example: Red.) A recessive gene will only be expressed if it has no other genes to overpower it. Red horses are pure for red. They are incapable of passing on any other color, since it is all they have. (When a red horse produces a foal of a different color, you can be sure that color came from the other parent)
• Partially Dominant (example: the Creme gene, which produces palomino and buckskin by partially dominating, or diluting, the red or bay)

  DOMINANT	RECESSIVE	PARTIALLY DOMIINANT	UNKNOWN
  Most color and color pattern genes other than red and the dilution genes	RED	Creme	Sooty
  	Flaxen	(One creme creates palomino on a red base, buckskin on a bay base, or smokey black on a black base; Two cremes create cremello, perlino, or smokey cream.)	Brindle

GENETIC NOTATION

In speaking of genes, we use abbreviations - one or two-letter codes for each gene (known or suspected; Some genes have been definitively identified, and others are still theoretical. New breakthroughs are coming every day, as we learn more and more about the equine DNA code!)

In the case of the two Base Colors, red and black, these are both determined at the same place (Latin term "locus" plural "loci") on the DNA strand. This "Locus" is called the Extension Locus, and it is signified by the letter "E". The dominant form, black, is called "E" and the recessive form, red, is "e".

Actually, the dominant form of the Extension Locus "E" allows either red or black, but unless there are other genes present (such as Agouti, which restricts black to the mane, tail, & lower legs) the color will be black, since black is dominant.

Since genes come in pairs, all red horses are "ee" while black ones can be either "Ee" or "EE". (And thus, they can pass either an "E" or an "e" to their offspring)

A List of GENETIC NOTATION terms can be found HERE

GENETIC TESTING FOR COLOR

As of April 21, 2003, The University of California at Davis can test for Extension (red/black), Agouti (it is actually identified--so we can tell you if your chestnut horse carries agouti, or if your very dark horse is actually black or bay), Cream (Leah S. Brault did that research--it's called the MATP gene), and Frame Overo (EDNRB gene). All of these genes have been identified and we can test for the mutations in them that produce the appropriate colors. We have NOT identified the gene that causes Tobiano. When we test for Tobiano, we're actually testing for the alleles of certain genetic markers that are near tobiano...but we don't know what tobiano itself is.  But because these two markers are closely linked to tobiano, we can tell with reasonable certainty if a horse is homozygous or heterozygous. 

As the DNA code is "cracked", more tests will come onto the market. At present these tests target genes that can be masked or hidden, or expressed too minimally to be sure if the horse actually carries it. This has economic implications for breeders of horses desiring a certain color or color pattern. In the case of Frame, it the simple test can prevent the tragedy of Overo Lethal White Syndrome.

For more info, see http://www.vgl.ucdavis.e

Other topics in this website's COLOR section:
A Quick Overview of Horse Genetics | Horse Color Genetics Charts 2 | Equine Base Colors | Dominant Horse Color Genes | The Dilution Genes | Recessive Color Genes

Up | A Quick Overview of Horse Genetics | Horse Color Genetics Charts 2 | Equine Base Colors | Dominant Horse Color Genes | The Dilution Genes | Recessive Color Genes

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