I think it is planed, but I am not sure about the extend of it.
It is a very complex thing.
Different species for example have different gen-sequences for the stop-sequence. If you merge 2 dissimilar species, the result can lack the ability to use either sequence-variant and lose the ability to process half the genome. No mixing of alien-species.
This doesn't just apply to the stop-sequence, but also start, multipliers, etc.
If anything is mixed it has to be done with genetic engineering or be very very improbable. The result would probably sterile or only able to reproduce with one of it's parent species.
There could be one gen responsible for multiple relative proportions (middle finger to hand width, eye-iris texture to personality traits, etc).
If you change one, it also changes something else.
Perhaps you could change the DNA of some part when it's separated from the rest, having one part brother, one part sister. Even IRL there are individuals which have different DNA for different body regions, but it's unlikely as it origins -in one case at least- from different off-springs merged in an early state of pregnancy.
I heard about that first, in a criminal movie where they made a story of the case in which the sperms had a different DNA than the mouth from which DNA-tests were taken. But I think it is not entirely fictional (one girl has 2 heads, originating from a later-stage merging of twins, but that comes with a lot of problems. Companies could be unsure whether to pay once or twice! Maybe twice in an intellectual job?)
I think that some aspects of it should be realistic:
- Bastards of alien-differentAlien-combinations should mainly belong to one instead of both species.
- One piece being responsible for many Aspects.
- Variable choices that can dominate each other.
- Gens that disable when another is dominant or doesn't exist (male, female).
- …
It should not be hyper-realistic (so that you can copy it 1-to-1 into reality and get a living animal) but at least offer many easier-to-implement quirks to keep it not entirely unrelated either.
You should have some freedom or even the ability to unlink 2 characteristics from each other, but there should always be a penalty : less fertility, de-buffs in abilities, …
This penalty should occur in the majority (maybe 9/10) cases to remind players of the dangers of this subject.