Silver dilution (Dapple)

Sample

• 30 to 40 - hair roots - envelope

• 5 mL - blood - K3 EDTA tube

Turnaround time

• 2 to 5  working days

Results description

The DNA test verifies the presence of the silver gene and presents results as one of the following:

• N/ – Negative for Silver - No evidence of the genetic variant for Silver. No risk to develop Multiple Congenital Ocular Abnormalities (MCOA) associated to Silver.
• Z/N - Heterozygous for Silver - The Black and Bay basic coat colour will be diluted by Silver. Black-based horses will be chocolate with flaxen mane and tail. Bay-based horses will have pigment on lower legs lightened and flaxen mane and tail. No effect on chestnut color. Moderate risk to develop MCOA.
• Z/ – Homozygous for Silver - Two copies of altered sequence detected. Black-based horses will be chocolate with flaxen mane and tail. Bay-based horses will have pigment on lower legs lightened and flaxen mane and tail. No effect on chestnut color, but will pass the variant on to 100% of offspring.  Higher risk to develop severe MCOA.

Additional information

The Silver dilution behaves as a coat colour dominant trait on bay and black base coat colours. While chestnut base colour is not affected by the Silver dilution and can pass the variant silently to the offspring. 

In short, the Silver dilution variant (Z) will only affect coat colour phenotype of black pigmented horses (E/e or E/E) and has no effect on red pigmented horses (e/e). 

In addition, the eye disorders associated to Silver genetic variant are incomplete autosomal dominant:  homozygous horses (with two copies of Z)  may be at higher risk of developing severe Multiple Congenital Ocular Abnormalities (MCOA), while heterozygous (with one copy of Z) may develop a milder form of MCOA.  

The effects of the silver dilution on coat colour gene can vary widely.

The agouti gene affects the coat colour by controlling the distribution of the black pigment whereas the Silver dilution variant dilutes areas of the black pigment.

Dilution by the Silver variant on a horse with a uniform black base typically involves lightening of the mane and tail and a dilution of the body to a chocolate color, often dappled as well.

A Bay horse carrying the Silver gene will usually have a lightened mane and tail, as well as lightened lower legs.

It is important to know that although a red horse (e/e) will not be diluted by the silver variant, it can be a carrier of the genetic variant and thus potentially pass the gene on to its offspring.

Silver dilution has been identified in a number of horse breeds including the Quarter horse, the Rocky Mountain horse, the Icelandic horse, Morgans, Shetland ponies and the Miniature horse.

References:

Brunberg, E., Andersson, L., Cothran, G., Sandberg, K., Mikko, S., Lindgren, G.: A missense mutation in PMEL17 is associated with the silver coat color in the horse. BMC Genetics 7:46, 2006.

Andersson, L.S., Wilbe, M., Viluma, A., Cothran, G., Ekesten, B., Ewart, S., Lindgren, G.: Equine Multiple Congenital Ocular Anomalies and Silver Coat Colour Result from the Pleiotropic Effects of Mutant PMEL. PLoS One 8:e75639, 2013.