The Silver dilution gene dilutes the black pigment but has no effect on the red pigment. The effect of the Silver dilution gene can vary greatly. The mane and tail are lightened to flaxen or silver gray, and may darken on some horses as they age. A black horse will be diluted to chocolate with a lightened mane and tail. A Bay horse with Silver dilution will usually have a lightened mane and tail, as well as lightened lower legs (places with black pigment). A horse can also carry mutations for other modifying genes which can further affect its coat colour.
The Coat Colour Silver dilution test (P784) tests for the genetic status of the PMEL17 gene. This gene has two variants (alleles). The dominant allele Z results in the dilution and the recessive allele N does not have an effect on the basic colour.
The same mutation responsible for the coat color Silver is also associated with Multiple Congenital Ocular Anomalies (MCOA) Syndrome, a wide range of ocular defects that occur in the anterior and posterior parts of the eye. The severity of the syndrome is dose related, so horses with 1 copy of allele Z have fewer severe signs than those with 2 copies of allele Z.
The Coat Colour Silver dilution test encloses the following results, in this scheme the results of the Coat Colour Silver dilution test are shown in combination with the possible results for the tests that determine the basic Coat Colour (Coat Colour Chestnut and Coat Colour Agouti test):
|
Result Silver dilution |
Result Chestnut + Agouti |
Coat Colour |
Description |
|---|---|---|---|
|
N/N |
e/e + A/A, A/a or a/a
|
Chestnut, Sorrel
|
Non-dilute. The basic colour chestnut/sorrel is not modified unless modified by other colour modifying genes. It can only pass on allele N to its offspring. |
|
N/N |
E/E or E/e + A/A or A/a |
Bay, Brown |
Non-dilute. The basic colour bay/brown is not modified unless modified by other colour modifying genes. It can only pass on allele N to its offspring. |
|
N/N |
E/E or E/e + a/a |
Black |
Non-dilute. The basic colour black is not modified unless modified by other colour modifying genes. It can only pass on allele N to its offspring. |
|
N/Z |
e/e + A/A, A/a or a/a
|
Chestnut, Sorrel
|
One copy of the dominant Z allele. The basic colour chestnut/sorrel is not modified unless modified by other colour modifying genes. It can pass on either allele N or Z to its offspring. |
|
N/Z |
E/E or E/e + A/A or A/a |
Silver dilution on Bay or Brown |
One copy of the dominant Z allele. The black pigment of bay/brown horses on lower legs is lightened and mane and tail are lightened to flaxen. The colour can be further modified by other colour modifying genes. It can pass on either allele N or Z to its offspring. |
|
N/Z |
E/E or E/e + a/a |
Chocolate |
One copy of the dominant Z allele. The basic colour black is diluted to chocolate with flaxen mane and tail. The colour can be further modified by other colour modifying genes. It can pass on either allele N or Z to its offspring. |
|
Z/Z |
e/e + A/A, A/a or a/a
|
Chestnut, Sorrel
|
Two copies of the dominant Z allele. The basic colour chestnut/sorrel is not modified unless modified by other colour modifying genes. It can only pass on allele Z to its offspring. |
|
Z/Z |
E/E or E/e + A/A or A/a |
Silver dilution on Bay or Brown |
Two copies of the dominant Z allele. The black pigment of bay/brown horses on lower legs is lightened and mane and tail are lightened to flaxen. The colour can be further modified by other colour modifying genes. It can only pass on allele Z to its offspring. |
|
Z/Z |
E/E or E/e + a/a |
Chocolate |
Two copies of the dominant Z allele. The basic colour black is diluted to chocolate with flaxen mane and tail. The colour can be further modified by other colour modifying genes. It can only pass on allele Z to its offspring. |
