A’ight, let’s pop up some of the Abhuman background lore for you all to chew on, which includes quite a few details about heritability. (Again, let’s not put it on the Wiki just yet, as it’s subject to change until I’ve actually written the conversations with M’kyar in the chapter I’m working on now.)
Some Abhuman Lore
ABHUMANS
The earliest human civilization to make extensive use of Theurgy, in their case through self-transformation to adopt animal or part-animal form. Known to themselves as the Seracca, in Shayard called Abhumans or Werebeasts, in Karagon called Apanthropoi.
CORE BELIEFS
The Seracca understand themselves first and foremost as spirits, which are immaterial, infinitely changeable, and have no fixed nature. (This is a contrast with the northern human creeds, all of which believe in a material soul rather than an immaterial spirit.)
What is of value in and to a spirit is not its essence nor form but its accidents of history and (most of all) of relationships , which constitute its deepest identity. If physical form is lost/changed too quickly or completely, a spirit can get lost and forget its story and relationships.
Rituals and roles help Seracca to avoid losing themselves in change. Most of the roles are based in a guild (or Yega’a, in the Serac common tongue), while some are specific to a clan (or Gara’u). Rituals are based on age, gender, place, and season, as well as key life events; many rituals also vary by clan and guild.
From ancient times, the Seracca have revered animals, seeing them as spirits that pursued different courses through the world and discovered new glories. Through self-change, the Seracca seek to reclaim some of those animal perfections for humans, while retaining the human glories of intellect and conscious self-change.
BODIES
The Seracca believe that all animal species are changing their traits slowly over time, and that none has reached such a state of perfection that all change is for the worse. Thus they are open to trying new things, new combinations of traits in their own bodies. All will practice taking on some traits of the animal that defines their clan or Gara’u (even if the result is a less than perfectly efficient combination of traits), in order to better appreciate the fluidity of physical form and the persistence of spirit.
Many Seracca stay roughly human-sized, even if their Gara’u animal is larger or smaller. Change of body size leaves people defenseless for a lot longer, especially as their body has to adjust its natural system of temperature regulation to its new size. It’s no fun to spend years shivering or sweating, extremely cold or hot, and in chronic pain, until you complete the adjustment. Seracca that grow larger also have to spend more of their time sleeping and eating.
All Seracca are born bipedal and plantigrade, a legacy of their human origins. If they want to change to take the many advantages of being a digitigrade quadruped (much faster running, better agility, better balance, less weight on hips, less trackable and stealthier), it involves years of slow physical change, and thus of dependency on the rest of the Gara’u. They lose the benefits of bipedal plantigrade-ness (ability to pull, carry, remain upright for longer; skills at climbing, crossing varied terrain, and transverse movement) before they gain the benefits of digitigrade-ness.
Members of the Guild (Yega’a) of Seers tend to transform as thoroughly as possible into their clan’s model animal, ensuring only that they retain human intellect and the ability to communicate. The Scribes, who inevitably retain more humanoid traits, may play a key role in helping the rest of the Gara’u understand what an almost-completely transformed Seer is saying.
Most Seracca in the Warrior Yega’a tend to become digitigrade and ditch opposable thumbs as a hindrance to speed, relying on various body-weapons rather than retaining the ability to grasp tools and weapons. (Their lips are however often heavily developed to allow them to do some grasping with their mouths, including basic writing.) The Artisan, Builder, Scribe, and Merchant Yega’a tend to keep their thumbs, though some Merchants choose to grow large and fully quadripedal to carry their own cargo, and these may keep a vestigial thumb at best.
The Seracca who best understand a Change can choose to make it heritable, but mostly the Changes only affect an individual. The idea of using other people’s blood to fuel a Change is morally and religiously unthinkable. The Seracca believe all parts of an individual (including blood) partake of its immaterial spirit; you don’t want to contaminate your spirit with something else’s, or you’ll quickly lose touch with who you are.
As mentioned above, all Seracca are born with a heightened sense of smell. This allows them to smell among other things metals, diseases, insects, shifts in weather, emotions, fertility/pregnancy, and venoms (in creatures and in bites). They are also born with an innate sense of which way is north. Most Seracca choose to retain their human omnivorosity, with only some becoming exclusively herbivorous or carnivorous.
Seracca understand the aging process well enough to slow it; most have a natural lifespan of around 200 years, barring disease or accident. Many individuals never breed, as maintaining a functional reproductive system on top of all the other Changes is difficult. Those who do choose to breed will often space their offspring a decade or more apart.
Seracca can interbreed with each other, unless they’ve made additional heritable changes which limit them to breeding within their Gara’u. Many can still breed with non-Seracca humans, but their hybrid offspring are likely to be infertile without further self-change.
Note: Every time I use words like “plantigrade” or “digitigrade” I should immediately acknowledge my profound debt to @Laguz, who actually understands animals (unlike me). Batting around ideas with them over the past couple of years has immeasurably enriched my concept of the Abhumans. Thanks, Tea. 
The remaining naff bits should be credited to me alone.