#overo

The tobiano coat is caused by the TO/to gene, and the overo is caused by the OV/ov gene. In some cases, these two genes are codominant, resulting in a TO/OV gene, which creates the "tovero." I will explain the differences in these three coat-types in this lecture :3

Left: tobiano (chestnut + TO/TO) | Right: overo (black + OV/OV)

Tobiano patterns have rounded edges, and they cross the topline at least once (the topline is the neck, withers [shoulders], back, and rump) and they tend to cover all four legs. This pattern often pairs with softer facial markings, such as stars, stripes, snips, and blazes.

Overo patterns have jagged edges, and do not cross the topline. They also typically do not cover all four legs; they usually leave them bare. This pattern often pairs with bald faces, white faces, or bonnet blazes.

The exception to the rule: tovero (TO/OV or OV/TO)

Tovero (flaxen chestnut + TO/OV)

This is a classic example of a "tovero," because it mixes the bonnet face with the tobiano pattern, as well as the more jagged edges on the white markings, typically seen in an overo.

Medicine Hat tovero (black + OV/TO)

A "medicine hat" tovero refers to a pattern that is almost completely white, except for a few small patches of the basecoat, typically seen around the ears, dock (tail base), and hind extremities. This pattern often has a lip spot (lower lip and chin are pink/white) or bald face (completely white), with walleyes (blue eyes).

Frame tovero (black + OV/TO)

This is a classic example of "wait isn't that just an overo?" Nope, this horse is a tovero! This horse's pattern has the same white distribution as a tobiano, but it has jagged edges. The white parts also cross the topline, and this horse has a bald face.

OTHER LESSONS

solids

tobianos

overos

spots

roans

the overo coat comes from the gene OV/ov, and this pattern is different from the tobiano because a) it's jagged, and b) it does not cross the topline (the neck, back, and rump), and c) it tends not to cover all four legs

Left: minimal overo (bay + roan + ov/OV) | Middle: overo (black + OV/ov) | Right: maximal overo (chestnut + OV/OV)

"jagged frame" or "fancy frame" overo (black + OV/OV), not to be confused with "medicine hat"

OTHER LESSONS

solids

tobianos

spots

roans

misc

By Susan McClafferty

Also, how is embryonic lethal white (W/W) different? A quick search just brings me to overo lethal white, but embryonic lethal is in a coat genetics simulator I found.

(I can't really go into details because I don't know enough about embryonic development. People more knowledgeable in the suject are welcome to add to this or correct me.)

Lethal white syndrome: There's a gene that codes for a protein named EDNRB. This protein, when functional, enables the development of melanocytes (pigment cells) and also of neurons in an embryo's intestines. There's a mutation of the EDNRB gene that impairs the function of the protein. In its heterozygous form (O/o), it only causes less pigmentation (frame overo white spotting) and is benign. In its homozygous form (O/O), however, it causes a more drastic loss of function of EDNRB, resulting in no pigmentation at all (foal is completely white), and dysfunction of the intestines (can't move food along). O/O foals are born with an intestinal anomaly and die of colics a few hours or days after birth. To spare them the painful death, they are usually euthanised.

Steele