Partial Dominance
by C. Williams

Partial Dominance is where there are two alleles at a locus, but the heterozygous individual, is different from either homozygote. The best example of this, is the Cream gene which produces both Palomino and Buckskin colors.

There are two alleles at the Cream locus, and they are usually annotated as:

C = Normal color
C^Cr = Cream color

The Cream gene is known as a dilution gene, because it's phenotypic action is to dilute the body color. Specifically, the Cream gene works primarily to "dilute" or lighten hairs with red pigment (called "pheomelanin").

The Cream gene will cause a chestnut horse to become Palomino and a Bay horse to become Buckskin. For purposes of this example, I'm going concentrate the discussion primarily on Palomino coloring.

Homozygote CC	Homozygote CCrCCr
Heteroozygote C CCr

One "dose" of the C^Cr allele will cause the body color to be lighter... resulting in palomino. If both alleles at the C locus are C^Cr, twice the "bleaching" action occurs, and the result is a white horse with pink skin.

These homozygous "cream" horses, are known as "Pseudo Albinos". Current theory is that true albino does not exist in the horse, or at least this gene/locus is not responsible for it. Not only do "Pseudo Albino" horses not have red eyes (as albino rats and mice do), they do actually have some pigment remaining. You can sometimes make out white markings on the face or body, which are whiter (and the pink skin is often a shade lighter under those areas.)

Homozygote CC	Homozygote CCrCCr
Heteroozygote C CCr

The result of the Cream allele on Bay color is a Buckskin. In the heterozygous form, the Cream allele lightens only the red body color, leaving the points black.

Some years back, the Palomino breeders, wanted to establish a breed of palomino colored horses. However, they soon found out that there was no way for Palomino color to "breed true"... that is, where palomino horses would always produce palomino offspring.

The following chart will show you why:

Palomino X Palomino		Sire
		Allele #1 C	Allele #2 CCr
Dam	Allele #1 C	CC Chestnut	CCCr Palomino
	Allele #2 CCr	CCCr Palomino	CCrCCr Cream

As you can see, only half of the offspring are palomino. Because the coloring is based on the heterozygous condition, it is impossible to establish a trait such as this that will "breed true" in all the offspring.

Based on the chart, the relative "probabilities" to obtain Palomino color from two Palomino parents are:

• 25% Chestnut (non-Cream) (CC)
• 50% Palomino(CC^Cr)
• 25% Cream (Pseudo-Albino) (C^CrC^Cr)

Unfortunately, some associations do not accept Cream (Pseudo-Albino) color for registration. The very allele which is responsible for often-prized golden coloring, is rejected when in it's homozygous form.

Let's take a look at two more charts:

Palomino X Chestnut Sire Allele #1 C Allele #2 CCr Dam Allele #1 C CC Chestnut CCCr Palomino Allele #2 C CC Chestnut CCCr Palomino	The Value of Homozygous Cream Sire Allele #1 CCr Allele #2 CCr Dam Allele #1 C CCCr Palomino CCCr Palomino Allele #2 C CCCr Palomino CCCr Palomino
50% Palomino 50% Chestnut (non-Cream)	100% Palomino

As you can see from the above examples-- you can obtain the same number of Palomino offspring, without ending up with a white foal, by crossing a Palomino to a Chestnut. The only way to increase your odds of Palomino color, is to cross non-cream individuals to homozygous Cream (Pseudo-Albino). Unfortunately, that is not an option with some breeds.

Not all Pseudo-Albino or Homozygous Cream horses look alike. If the underlying base color is Chestnut, the resulting white/cream color may also be referred to as "Cremello". When the underlying base color is Bay, the individual may have smudgy or "dirty" points... that is, lower legs, mane and tail... and that shade is often referred to as "Perlino".

In its heterozygous form, the Cream allele has only a minimal effect on black pigment (eumelanin). A black horse carrying a Cream allele, may appear to be "Smoky Black"... rather than an intense black. This may not always be noticed (may look dark bay or brown)... and may result in the horse "surprising" everyone by producing buckskin (and possibly palomino) offspring.

Homozygous Cream on Black results in "Perlino" type coloring... pink skin, blue eyes, cream coat possibly with darker cream points.

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Views expressed herein are those of the writers and compilers of the various information. Reference sources are cited where applicable. Copyrights are the property of the respective authors.