View topic - Equine Genetics Guide Pt 3: Cream

My gallery · by **Arkinhallow** » Wed Apr 14, 2021 1:58 am

If you have not yet read parts one and two of this series, they are necessary for understanding much of the content here. I recommend you read those first beforehand if you are new to or unfamiliar with horse genetics.

────────────────────────────── Genetics Terminology ──────────────────────────────

Incomplete dominance: Where organisms that are heterozygous (one dominant copy, one recessive copy, e.g. Ee) for a trait have an appearance that is intermediate between the two homozygous phenotypes (two dominant copies or two recessive copies, e.g. EE or ee).
Using __ in genotypes: You may see genotypes written with an underscore "_" in the place of a letter occasionally. This is a commonly used notation that indicates that any allele from that gene can go there and the horse will still look the same. For example: "E_/aa" shows that regardless of whether the second allele of extension is dominant "E" or recessive "e", the way the horse looks (its phenotype) will be the same. In this case, both "Ee/aa" and "EE/aa" will result in a black horse. If you see two underscores, (e.g. "ee/__") this indicates that no matter what either of the alleles are, the horse will look the same. In this case, "ee/aa", "ee/Aa", and "ee/AA" will all be chestnut horses. You'll pretty much only ever see these underscores for extension or agouti.

───────────────────────────────── Cream Gene ─────────────────────────────────

Cream seems to confuse a lot of people, likely because it displays incomplete dominance unlike most other genes. Dun, for example, looks the same whether there are one (Dd) or two (DD) copies. Cream results in different coat colours depending on whether there are one or two copies of it.

To start off, let's go over the alleles of the cream gene.

For the moment, we will only discuss the cream coat alleles - Cr and cr.

A horse that is heterozygous for cream ("Crcr", one copy of cream and one of non-cream) has traits that are halfway between a homozygous double cream ("CrCr") and a homozygous non-cream ("crcr"). For example, a palomino (heterozygous for cream: "Crcr") has traits that are halfway between a chestnut non-cream ("crcr") and a cremello ("CrCr"). The most obvious of those intermediary traits is the golden colour of the body, which is halfway between a red chestnut body and the pale creamy body of a cremello.

You might also like to think of cream as having a "dosage effect", as it is sometimes described. That is, one copy has a small effect, and two copies have a big effect. This is clearly seen when you compare how one copy of cream can cause golden palominos or buckskins with dark skin and eyes, and two copies cause pale, blue-eyed and pink-skinned cremellos and perlinos.

The genotypes for cream horses look like this:

Red based

Black based

Bay based

Horses with one copy of cream are referred to as single dilutes. These include palominos, buckskins, and smoky blacks. Horses with two copies are called double dilutes. These are cremellos, perlinos and smoky creams. The cream gene dilutes only pheomelanin in its heterozygous form. This is why buckskins retain their black points, and why black horses are minimally affected or not affected at all by one copy of cream. On the other hand, palominos are diluted over their entire body because they have no black pigment. Double dilutes can be indistinguishable without genetic testing. My examples above show the possible variation, and when designing horses I usually make their colours distinguishable, but in real life it is usually impossible to tell the difference just by looking.

───────────────────────────────── Pearl Gene ─────────────────────────────────

As shown before, pearl is an allele that exists on the same locus as cream. It is not its own gene. In the same way that seal is (theoretically) on the agouti locus but causes quite a different look to bay, pearl is on the cream locus but causes a different coat colour.

Unlike cream, pearl is recessive. That means it needs two copies to have any effect. However, if cream is combined with recessive pearl, you end up with a coat colour that mimics a normal double dilute. These are called pseudo-double dilutes. In addition to the genotypes shown earlier, these genotypes are also possible:

Red based

*Note: Because pearl is on the same locus as cream, the allele that causes no change to colour is the same one: "cr". It is sometimes written as "Prl", but this can cause confusion that leads to people thinking they are separate genes when they in fact are not.

Black based

Bay based

Sources: (for above two sections)

wikipedia.org "Cream gene"
animalgenetics.us "Cream dilution"
vgl.ucdavis.edu "Cream"
colorgenetics.info "Dilutions in Horses: Cream, Dun, Champagne, Pearl and Silver"
wikipedia.org "Pearl gene"
vgl.ucdavis.edu "Equine Coat Color Genetics"
vgl.ucdavis.edu "Pearl"

────────────────────────────── References And Overlap ──────────────────────────────

Palominos
reference gallery
Buckskins
reference gallery, one, two
Smoky blacks
reference gallery
Double dilutes
Double cream: reference gallery, one
Cream-pearl: one, two, three

pearls reference gallery
Gold pearls

Bay pearls

Black pearls
one

Image sources:

Star Andalusians PRE stud "Mares"
Star Andalusians PRE Stud "For Sale"
Dilutes Australia "Shades of Pearl"

Further references

Foal references

My gallery · by **faentofheart** » Wed Apr 14, 2021 3:23 am

So double pearl on black gives dark brown points while bay has none at all? 😳😳 ive been doing pearls wrong this whole time

My gallery · by **Arkinhallow** » Wed Apr 14, 2021 12:55 pm

Rangerthecowpony wrote:So double pearl on black gives dark brown points while bay has none at all? 😳😳 ive been doing pearls wrong this whole time

Not necessarily!! As in all coat colours there’s variance in the range of colour, in my time looking at references I generally find that black pearl pretty much always has darker legs but it’s kinda 50/50 for bay pearls. Bay pearls probably vary the most visually, of the pearl coats (at least in my experience). But they definitely can have darker leg points.

I’ll adding extra visual aids and real life references later so you can get an idea of the scope of colours they all come in!

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Equine Genetics Guide Pt 3: Cream by Arkinhallow

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