I wrote quite a bit but I am not sure why it isn't here... hmm let me write it again =). (if anyone saw that post in another thread, please let me know)
Firstly, let me remove the bio term since this mechanic is not really reflected in real life.
So let us call the thing that give us the stats boost or ability "Enhancement"
Each creature can have different slots for enhancement. For simplicity sake, let us use number. You can use different body parts as name for slot if you want =).
So say we have 10 slots for enhancement, different creatures can have different slot available to them.
For example, creature A may have Slot 1 3 5 7 9 while creature B have 2 4 6 8 10.
You can make the slot species based or individual based, that is up to you.
So now say we have 2 enhancement, one gives ATK+1, while the other gives ATK+30.
Since the first one is pretty weak, it can be allowed in all 10 enhancement slot.
The second one is pretty powerful, so we only allow it to occupy a specific slot, say 3.
So creature A can inherit the second ability since it has slot 3, while creature B can't
The breeding goes like this.
When 2 parents mate, you pool together all the Enhancement available for them.
Then you go through each enhancement (order to be discuss later), and for each enhancement, choose a slot to occupy, based on what is available and what is possible. So if the enhancement needs to be in slot 2 or 4 , but the creature has only slot 2 or if slot 4 is already occupied, the enhancement goes into slot 2.
The order to iterate the pool can be either be randomized or using a priority system.
On top of these, you can also have a unexpressed enhancement.
So for example, parent A have ATK+1 trait in slot 3, and an unexpressed ATK+30 trait in slot 3. When mated, the ATK+30 trait is also added to the pool of enhancements. This gives the child a chance to get the ATK+30 trait, even though both the parents doesn't expressed it. The player can deduce that one of the parent have it unexpressed, and can proceed to try to find out which one.
It is entirely up to you how much information you want to give your player =).
I personally think that this mechanic is too complicated, which is why I think is a bad idea. You can look at it and see how much of it is useful =D.
Remember, the best system is not when there is nothing to add, but nothing to remove.
This probably added too much to it.