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Genetics Primer, Part 2

Each gene has a dominant form, usually signified by a capital letter, and a recessive form, signified by a lower case letter. For many genes, having one dominant copy will cause that gene to influence coat color or pattern. Some genes have to have the same copy of the gene to affect the coat color or pattern. This usually happens when it is the recessive form of the gene that causes the effect.

When both copies of a gene are the dominant form or both are the recessive form, the gene is called homozygous--meaning both copies are the same. If one copy is the dominant form and the other is the recessive form, the gene is called heterozygous--meaning the copies are different.

The technical term for a variation of a gene is "allele," so we would speak of the dominant allele and the recessive allele.

Now we get to specific color genes, and things get really interesting and sometimes quite complicated. Don't expect to master all this with one reading. Note that in the game, when you color test a horse, the genes which are affecting its color and pattern will be reported, but if the horse doesn't carry the color affecting allele of a gene will probably no show up.

Two genes are the primary determiners of a horse's coat color.

The first is called Extension and is identifed by E for the dominant allele and e for the recessive allele. E is a complete dominant, which means that a singe copy of the dominant allele will color the horse's hair black. A horse with two recessive extension alleles, "ee", will have a red coat (chestnut).

The second primary color determiner is called Agouti, and is identified by the letter A for the dominant allele and a for the recessive allele. What this gene does is to push any black pigment into the "points" of the horse--the mane and tail and the lower legs.

If the horse has even one dominant allele of Agouti and homozygous or heterozygous dominant E, this displacement of the black pigment will occur and the horse will be a Bay.

If the horse has any dominant alleles of Agouti and homozygous recessive e, the horse will still be chestnut, but it can pass on the dominant Agouti to its offspring So if a foal gets a dominant E from its other parent, it will be a bay.

In order for a horse to be black, it has to be at least heterozygous for dominant E and homozygous for recessive Agouti.

So, the primary variations in coat color are:

EE AA--homozygous dominant for both genes= a Bay horse.
Ee AA, Ee Aa, EE Aa, Ee, Aa will all produce a Bay horse.

ee AA, ee Aa, ee aa will all produce a Chestnut horse

EE aa, Ee aa will produce a black horse.

Genetic Primer Part 4

Sooty and Dilution Genes

Sooty is a gene that causes black hairs to appear on a horse from the top down, as if a bucket of soot had been dumped over it. The absence of this mutation is written S, its presence is written Ssty,

Homozygous Sooty will have more effect than heterozygous Sooty. This characteristic is called incomplete dominance. If you compare the brown horses above, you can spot the difference between homozygous recessive Sooty, heterozygous Sooty, and homozygous Sooty.

Black horses can carry Sooty, but it will not show because their coat is already completely black.

There are 4 dilution genes that cause different amount of lightening of the coat color. They may also have other effects, depending on the gene and whether or not is is homozygous or heterozygous. Cream is an incomplete dominant gene, which means that there is a distinct difference in the affects when it is heterozygous or homozygous. The other three dilution genes, Dun, Silver, and Champagne are complete dominant genes. They have the same effect whether they are heterozygous or homozygous.

Cream has three alleles. C, no mutation; Ccr, considerable lightening of the coat; Cprl, less lightening of the coat, and must be homozygous to affect coat color. However when combined with the Ccr allele, Cprl will cause additional lightening to the coat. This combination of alleles is called "pseudo-double dilute."

Cream primarily dilutes red colored coats (chestnuts and bays). It has to be homozygous to have an easily seen effect on black coats.
Homozygous Ccr causes extreme lightening of the coat and blue eyes.

Chestnut plus Ccr creates Palomino horses. Chestnut plus CcrCcr creates Cremello horses.

Palomino, note that the mane and tale are lighter in color.

http://www.huntandjump.com/horse.php?horseid=2765353

Cremello, note the eye color; the skin is pink.

http://www.huntandjump.com/horse.php?horseid=2867287

Bay plus Ccr creates Buckskin horses. Bay plus CcrCcr creates Perlino horses.

Buckskin

http://www.huntandjump.com/horse.php?horseid=2709741

Perlino--note dilution of mane and tail, blue eyes, and pink skin

http://www.huntandjump.com/horse.php?horseid=2748293

Black plus Ccr creates Smokey Cream horses that appear to be black. Black plus CcrCcr creates Smokey Creme horses that have diluted coats.

Smokey Black--note that any dilution of the coat color is scarcely visible

http://www.huntandjump.com/horse.php?horseid=2829677

Smokey Creme--note that the dilution of the coat is not as extreme as with Cremellos and Perlinos, but the eyes are blue and the skin pink here as well.

http://www.huntandjump.com/horse.php?horseid=3351658

Wow! I had no idea it was so complex!! I might need a degree in genetics for this game. I will be coming back to this post very often to figure out more stuff. Personally I like the appys, paints, and bays myself so I am going to try and learn what I will need to make my barn full of appys, paints, and bays! Thank you SandyCreek Acres for all the help!