Why Ivory isn't like the Horse Cream-gene

written by Leah Patton

Ivory (BEW) genetics in donkeys are interesting, as thus far they do resemble the newly discovered "Barlink" and Perla genes in Paint and Iberian (Andalusian/Lusitano) horses. (These genes appear to be recessive, producing diluted horses from non-dilute parents, which is previously unheard of!)

Why do we suspect that BEW is not Cream? (You may see the spelling creme, but it is not correct).

A brief genetics lesson to explain:
The Cream gene can be found in horses in either heterozygous (meaning "One dose") or homozygous (meaning "two doses") form. All genes are paired - they have one half of the pair from the sire, the other half from the dam.

Genes, although really protein sequences, can be best visualize by thinking of a bank of light switches, with Off-On positions. ON (caps) means the gene is present, and if present, is expressed. *off* (in lower-case letters in a gene pair) means the gene is either not present, or cannot express because of the partner-gene.

A Sorrel (red, chesnut) horse is genetically ee - Black. A Black horse is EE or Ee. But for black there is another modifier, agouti. Black horses do not have agouti, so they are aa. Eeaa, or EEaa. If a horse has a gene for black, and a gene for agouti, it is Bay (EEAA, EEAa, EeAA, EeAa).

Now, add the horse cream-gene into the mix. Cream has an additive effect in horses. One dose of the gene lightens the coat a little, the second lightens - or the genetics term "dilutes" it, even more.

One dose of Cream on a sorrel horse (ee Crcr) produces a palomino. They have yellow-golden bodies and light manes and tails. Two doses (eeCrCr) produces a cremello, a horse that appears to be nearly white, with pink skin and blue eyes. Looks a little like an Ivory donkey. BUT... Breed a cremello back to a sorrel and 100% of your foals will be diluted - palominos.
                                                                            eeCrCr x eecrcr
                                eeCrcr(pal)                  eeCrcr                             eeCrcr                             eeCrcr

One dose of cream on a bay horse gives buckskin - golden body, black mane and tail, black lower legs. There is no true buckskin in donkeys. Some light-brown donkeys look close, but they do not reproduce like buckskin. Two doses of cream on a bay gives a perlino. Pale cream horse, slightly darker, reddish-cream mane, tail and lower legs. Once again, breed a perlino to a bay, and you will get 100% buckskin foals. Breed a perlino to a sorrel, and you will get all buckskin or palomino foals. The cream gene will always be visible on the coat of these animals, with rare exceptions where they have a modifier called sooty. This makes the coat appear very dark, but you can still tell by examining the parents, the offspring records, and by DNA test if horses have the cream gene or not.

Cream in a single dose does not always express on a black coat. Black can also have other "dampening" effects on some patterns (but that is much more complicated genetics). Suffice it to say you cannot tell by looking if a black horse carries the cream gene. You can test for it, by DNA, and by breeding. If he was bred to sorrel mares, and had buckskin or palomino foals (the A gene doesn't express on sorrel, so sorrels can carry A, in case you wondered where those bays came from) then you would know your black stallion had a cream gene. The mares don't have it - why? Because they are visibly sorrel.

Two doses of cream does express on black, and they look a lot like dark perlinos. They have two cream genes, so every one of their offspring would get a cream gene.

Misty, buckskin
photo courtesy of Leah Patton

mini cremello mare /buckskin filly
photo courtesy of Michele Johnson

perlino filly
photo courtesy of Michele johnson

smutty buckskin /perlino filly
photo courtesy of Michele johnson

There are no recorded cases of palomino donkeys, or of buckskins. While we are fairly certain there is a "dilution" gene in donkeys, it is not horse-cream. Ivory acts like a recessive - ie, hidden until it's mate comes along.

Sorrel formulates as a recessive - Ee (black horse) bred to Ee (Black horse) can have EE, Ee, Ee, or ee foals. What is that ee foal - Sorrel. Sorrel had to wait until e came along from both parents.
Ivory appears to be this way - both parents have to have ivory to produce an ivory foal.

Why do we think Ivory isn't cream? It doesn't express like cream. An Ivory donkey bred to a dark-brown donkey (that looks like horse bay) should produce a diluted foal. Instead we see dark-brown (dark bay) foals from these matings. If there was a DOMINANT gene, it would dilute the bay to a lighter color.

Now granted, a lot of donkeys (and much of the ivory animals) are black based, so that makes it more difficult. But there are several cases of dark-brown animals (if they are indeed equal to bay) resulting from ivory x gray-dun or ivory x brown matings. Therefore, until a DNA test proves otherwise, the base research thus far supports a recessive (ivory) gene.

We know genetics can be exhausting, but we need to stress the point as to why, thus far, ivory doesn't act like cream.

Actually there would be a test to prove the theory other than DNA. An ivory mini jack could be bred to a mare tested eeaa (sorrel with no agouti). If a palomino foal resulted, the cream gene could only have come from the jack. If a SORREL foal resulted - plain and simply, the ivory gene isn't cream.

There are other dilution genes in horses besides Cream. Champagne, Silver (Silver dapple) and Dun are also dilution genes. All have a visual effect on the coat in single doses. Unlike Cream, the other dilutes don't seem to have a cumulative effect in homozygous doses. They DO, however, have interesting cumulative effects when paired with other dilution genes (for example a cream +champagne gene will produce a light-colored horse - {formerly called ivory!} - that shows outwardly it is a double-dilute).

Need more genetics info? Email adms@juno.com with your name and mailing address, and ask for the color basic packet. A more extensive packet is available for $7 through ADMS.

Ivory Genetics basic introduction
Why Ivory isn't like the Horse Cream gene
Furball Acres

Last Updated: 04/19/2006
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