Underbelly

Underbelly is a common gene in which the fur along the underbelly or extremities is paler than elsewhere. It can cover the underbelly mane and tail mane, when present, but doesn't have to affect it entirely or at all. While it may have portions that are blurred, the majority of the marking should be fairly hard-edged, showing a clear distinction between it and the base coat or markings around it.

Underbelly is an optional two-toned marking, meaning that it can have two separate tones, though the paler of the two should always be closest to the underbelly or toes/tail tip - meaning you should not have a darker colour on the feet and a lighter colour above it

Direct Inheritance

Underbelly also allows for some direct inheritance in coloration. Example: if a parent has an underbelly marking that is yellow in hue, for instance if they are oat, their offspring may inherit that specific colour (colourpicked or nearabout) from the parent if they wish even if they are not oat. This allows for darker-than-basecoat coloration such as black underbelly but must be noted which parent it is inherited from.

An exception to this rule of direct inheritance is of pure genos - those who were generated via items being the most common source. These may inherit the specific colour from the underbelly marking of any applicable ancestor visible in their tree, even if it is not a direct parent.

Clarification: Holes

Something we see a lot in the approval queue is misunderstandings over the "no holes" rule as well as the "extremities first" spreading rule. This means that feet, face, and tailtip (etc) should not have holes where the marking isn't present, unless those holes are connected to the larger area of marking-absence on the back.