Cr-locus – Cream Dilution (Cremello, Pearl & Snowdrop)

Cr-locus – Cream Dilution (Cremello, Pearl & Snowdrop)

Coat colour in horses is controlled by a wide range of different loci working together. Modifications that can occur include dilutions of the base coat colour. There are multiple genes known to have this effect and one of them is the solute carrier family 45, member 2 gene (SLC45A2, previously known as MATP). This gene is thought to disrupt the distribution of pigment, which leads to a dilution of the coat colour.

There are several known mutations on this gene, of which three are discussed here. The first one is relatively common and leads to a Cream dilution (Cr). A rarer mutation in the SLC45A2 gene is the Pearl dilution (Prl) or Barlink Factor. The third variant, also rare, has been found in Gypsy Vanners, Tinkers and Irish Cobs, and is called Snowdrop (Csno).

Cream

The cream gene is inherited in an autosomal incomplete dominant manner. A single copy of the mutated Cr allele is enough to dilute the coat colour, but horses with two copies are even more diluted. Because there is a clear difference in appearance between horses with one copy (heterozygous) and those with two copies (homozygous), the inheritance pattern is classified as incomplete dominant.
Horses carrying one Cr allele show diluted coat colours, depending on their base colour. This results in Palomino (from chestnut), Buckskin (from bay or brown), or Smoky Black (from black). In Smoky Black horses, the dilution effect may be very subtle and hard to distinguish.
Horses with two Cr alleles are known as double-dilutes and exhibit much more dramatic dilution. This produces Cremello (from chestnut), Perlino (from bay or brown), or Smoky Cream (from black), depending on the base colour. These horses typically have rosy-pink skin and pale blue eyes, and are sometimes referred to as “blue-eyed creams.”

Pearl

Pearl is inherited in an autosomal recessive manner. Meaning that an animal needs two copies of the mutant allele (Prl/Prl) to display a diluted coat colour, often with a noticeable pale or “frosted” appearance. The dilution lightens the base coat colour in a way that is distinct from the Cream dilution. A bay horse appear to have a golden hue (“Pearl or Frosted Bay”), while a chestnut horse appear to have a creamy shade (“Pearl chestnut”). The eyes, skin around the eyes and muzzles, mane and tail can also show lighter colours.

Pearl x Cream

The Pearl dilution is known to interact with the Cream gene, producing phenotypes often referred to as “pseudo-double dilutes,” such as “pseudo-cremello” or “pseudo-smoky cream.” These horses typically have pale, diluted skin and blue or green eyes, resembling true double-dilutes in appearance.

Snowdrop

Snowdrop is also inherited in an autosomal recessive manner, meaning that an animal needs two copies of the mutated allele (Csno/Csno) to express the dilution. This snowdrop phenotype is similar to double diluted cream horses which includes pink skin, blue eyes and a pale colored coat.

Inheritance Cream and Pearl combined

*See additional information

Inheritance Snowdrop

*See additional information

Additional information

These three mutations are all located on the same gene but in a different position. As a result, the mutations have a more or less similar effect. Although combinations of these mutations are rare, it is known that some variants can influence each other and produce so-called ‘pseudo-double dilutes”. At CombiBreed, we therefore decided to combine the tests of the Pearl and Cream variant. Both variants are tested and produce a combined result in the reports. The Snowdrop variant is extremely rare and the effect of combinations with other variants are still unknown. Therefore, the Snowdrop variant is till tested individually. Phenotypes of all variants can look similar and therefore DNA testing can provide a definitive answer on which dilution variant is present.

Relevant tests

• P384
• P713
• P783
• P333