All of the following white patterns can occur on any base coat with any combination of dilutions, modifiers, and other white patterns. For example, it is completely possible to have a tobiano/sabino/splash/roan/palomino. However, while it is possible, not all breeds carry all white patterns and dilutions and possibly does not equate to a likely outcome.
All white patterns can also be present in minimal and maximal forms. Minimal can be very minimal indeed (think white spot on the bottom of a hoof for frame), and maximum expression can be almost completely white, especially when more than one pattern is present.
Tobiano is probably the best recognized white pattern. It is a dominant characteristic, meaning that if only one parent carries tobiano the foal will have a 50% chance of inheriting it. It tends to affect the legs, shoulders, and hips, and leave the barrel of the horse colored. It can cross the top line and where it does, it will cross at the hip and shoulder first. Tobiano alone is thought to leave the face solid, but very few horses carry tobiano without having one of the other white patterns as well.
There is only one form of Sabino that can be tested for, SB1, but many horses that exhibit phenotypical sabino markings test negative for SB1. It is not known at this time how many different genes may be expressed as what we call sabino. Sabino tends to affect the back legs of the horse first. It tends to leave the chin white, roaning along the edges of white markings, and sometimes scattered in the coat. Like any pattern, expression varies. At times markings as small as a star are blamed on sabino, although in truth it is difficult or impossible to tell what pattern a very minimal horse carries without testing. According to the current visual evidence, sabino does not cause blue eyes. Sabino, at least SB1, is an incomplete dominant, meaning that homozygous horses express much more white than heterozygous horses – hence the term “max white sabino” The only breed currently thought to lack sabino is the Icelandic Horse.
Splash, or splash white, is theorized to be an incomplete dominant gene. Splash tends to leave white markings with even, well-defined edges, almost as if the horse had been dipped in white paint, starting at the head and legs. Blue eyes are common with splash, as well as bottom heavy or apron blazes. Splash is associated with deafness, but not all splash horses are deaf. It is thought that deafness is caused when splash inhibits pigment production in the inner ear.
Frame is otherwise known as lethal white overo (LWO) syndrome. Frame tends to put color on the barrel and face of the horse, move horizontally, and leave the legs solid (although other patterns can add leg white). It also tends to leave white that is “splotchy,” with jagged edges. It is well known to cause blue eyes, but not to cause roaning in the coat. The name “Frame” comes from the pattern’s propensity to leave a “frame” of color along the outer edges of the barrel.
Frame (LWO) is lethal in its homozygous form, with the foals being born solid white and only surviving a few days. They are born without a functional digestive track, and unless euthanized, die in gastric distress. Because it is often hard to visually tell if a horse has frame (there have been cases of solid-colored horses testing positive), it is important to test breeding stock in all breeds that carry frame in order to avoid producing a lethal white foal.
Frame is an incomplete dominant characteristic and breeding frame to frame does not increase your chances of obtaining a frame foal; statistically, 25% of foals from frame-to-frame breedings will die, leaving 50% frame and 25% solid surviving foals. Breeding a negative frame to a frame will give 50% solid and 50% frame foals. By only breeding non-frame to frame you effectively reduce your chance of a dead foal by 25%. The confusion among some breeders is most likely the result of horses carrying multiple white patterns, such as sabino and splash, in addition to frame.
Roan is a white pattern that leaves white hairs scattered throughout the coat. The amount of white varies seasonally, with the greatest expression in the spring when they start to shed and the least during the winter when it can at times be difficult to tell the horse is roan. Roan typically leaves the legs below the knee, mane, tail, and head solid colored. It also tends to leave a characteristic inverted V on the lower legs where the roan pattern stops and the solid legs begin. Roan is a dominant characteristic.
Leopard Complex (LP) is believed to be the base gene for all Appaloosa patterns. It is believed to be a dominant characteristic, responsible for appaloosa roaning and necessary for other appaloosa patterns to exhibit.
In very minimal form, the horse many only show what are known as characteristics (sclera, mottled skin, and striped hooves on non-white legs) and even these can, at times, be so minimal as to escape notice. When LP is present along with a pattern gene, the horse will present with a blanket or leopard pattern. Horses that are homozygous for LP and also carry a pattern gene will have white areas without spots. A blanket without spots is known as a snow cap, and a leopard without spots is known as a few spot.
A fairly new study suggests that horses that are homozygous for LP are also night blind. The inheritance of the pattern is not fully understood, but it is thought to be controlled by at least two different genes. LP roaning (varnish) is progressive, meaning that the horse lightens as it ages. The difference between LP roaning and gray is that with LP roaning the bony areas and spots of the horse will stay dark so that the horse never turns completely white.
Rabicano adds white hairs in the coat, usually at the base of the tail and the flanks. In minimal form, it can express as a few white hairs at the base of the tail. With extreme expression, it can extend up to the shoulders and even the chest of the animal, and put white patches behind the ears. It can also form striping on the barrel and put white hairs into the tail, which is sometimes referred to as a skunktail or silvertail. The inheritance of rabicano is not understood.
Dominant white tends to resemble maximum sabino in expression, but inherits as a dominant gene and is believed to be lethal in utero when homozygous. To date, there have been 11 different mutations of dominant white discovered.