The Overo coat pattern is a white spotting pattern with white patches on the side with a “frame” of colour surrounding the white. A horse can also carry mutations for other modifying genes which can further affect its coat colour. While Overo coloured horses are desirable, the mutation that causes the overo colour is linked to a fatal condition known as Overo Lethal White Syndrome or OLWS. A foal with OLWS is born all- white and dies of complications from intestinal tract abnormalities. The Coat Colour Overo test (P902) tests for a genetic factor that affects the function of the EDNRB gene. This gene has two variants (alleles). The allele O is semi-dominant. One copy of the O allele results in horses with overo coat pattern. Two copies of the O allele result in a lethal white foal (OLWS). The allele N is recessive and does not have an effect on the basic colour.
The Coat Colour Overo-factor test encloses the following results, in this scheme the results of the Coat Colour Overo-factor 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 Overo-factor |
Result Chestnut + Agouti |
Coat Colour |
Description |
|---|---|---|---|
|
N/N |
e/e + A/A, A/a or a/a
|
Chestnut, Sorrel
|
Not Overo. 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 |
Not Overo. 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 |
Not Overo. 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/O |
e/e + A/A, A/a or a/a
|
Chestnut/sorrel overo |
Overo pattern. One copy of the O allele. The horse has the desirable frame overo pattern unless modified by other colour modifying genes. It can pass on either allele N or O to its offspring. |
|
N/O |
E/E or E/e + A/A or A/a |
Brown/bay overo |
Overo pattern. One copy of the O allele. The horse has the desirable frame overo pattern unless modified by other colour modifying genes. It can pass on either allele N or O to its offspring. |
|
N/O |
E/E or E/e + a/a |
Black overo |
Overo pattern. One copy of the O allele. The horse has the desirable frame overo pattern unless modified by other colour modifying genes. It can pass on either allele N or O to its offspring. |
|
O/O |
Any result |
Lethal (OLWS) |
Foal with Overo Lethal White Syndrome (OLWS), lethal. |
