W20 is a dominant gene that makes any existing white on the horse, including markings, more extensive. The gene is found on the KIT locus with sabino, roan and tobiano.
| Genetic Panel |
| KIT |
| W20/W20 W20/n or n/W20 Rn/W20 or W20/Rn Sb/W20 or W20/Sb To/W20 or W20/To |
| Patterns | |
| Markings | Extensive Tobiano |
 |  |
| Extensive Overo | Extensive Splash |
 |  |
| Extensive Sabino | Extensive Roan |
 |  |
| Extensive Blanket | Extensive Snowflake |
 |  |
| Extensive Snowcap | Extensive Leopard |
 |  |
| Extensive Ghost | |
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