The quirks of the English language are what make it both fun and difficult. One of those great quirks is the pairs of words that look like they should be opposites but that are actually synonymous, or nearly so, such as flammable and inflammable, press and depress, and ravel and unravel. Heritable and inheritable belong on that list, too, but as time goes on, they are becoming less synonymous.
Both words grew from the Latin hereditas “inheritance,” from heres “heir.” Both can mean “capable of being inherited.” Besides that Latin prefix in- “in” or “toward,” there really isn’t much surface difference between heritable and inheritable.
But the two words do seem to be differentiating over time. Heritable — like hereditary, which the dictionaries tag as a synonym — is almost always used in the context of genetics and the passing along of traits, and especially when dealing with mutations:
Since the two whole mammal tests are relatively expensive and time-consuming, they are usually reserved for testing chemicals highly suspected of causing heritable mutations. —Technologies for Detecting Heritable Mutations in Human Beings, U.S. Congress, Office of Technology Assessment, 1986
Inheritable is the word of choice when dealing with things passed on outside the body — property, royal titles, family wealth — as well as in computer programming:
What counted in the crystallization of aristocracy were more pragmatic interests, such as the grant of an inheritable title of duke or count by a Carolingian emperor, supplemented by grants of high titles by later monarchs. —The Last Knight, Norman F. Cantor, 2005
When no rule exists to set a property value in an element, the element may inherit that property from its parent element, if the property is inheritable. —Essential SUL Programming, Bullard, Smith, and Daconta, 2001
This differentiation is still in its early stages, though, so there is plenty of overlap in how writers and editors use heritable and inheritable.
Inheritable will likely be more comprehensible to a general audience as meaning “able to be inherited,” as opposed to the “able to be herited” reading of heritable. But in scientific disciplines, like genetics, for which readers are used to jargon that dips deeply into Latin and Greek, heritable offers little resistance.