Let's say that we have a human society somewhere away from Earth which is psychologically ready for genetic transhumanism... perhaps humans just aren't coping with the environment, perhaps the local religion supports it, perhaps the local wildlife are a bit much to handle, or perhaps people just want their kids to have better bodies than theirs.
So, the geneticists whip up the DNA to insert into an embryo so that the baby will be born with the desired alterations. It's tested and approved for general release.
Now, the thing is, the parents paying for their child to receive this upgrade want their grandchildren and their descendants to inherit the augmentation... regardless of whether their child is male or female, and regardless of whether their child's reproductive partner is enhanced... without their grandchild having to be modified at the embryonic stage.
Conversely, those commissioning certain enhancements may not want the enhancements propagated to the next generation, except under specific circumstances... perhaps only if the reproductive partner also has the enhancement, or perhaps only if a particular environmental factor is present or absent.
So, the geneticists add an extra chromosome pair to the zygote, containing all the necessary genes for the enhancement. The added chromosomes are replicated along with the others as usual during mitosis (normal cell division), and have the desired effect upon the individual carrying them.
Now, the difference that allows both backwards-compatibility and selective non-transmission is only noticeable during meiosis (the cell divisions that produce haploid gametes).
Normally, before mitosis or meiosis occurs, a cell's DNA is replicated once. To prevent the creation of too many copies during mitosis or meiosis, a protein that binds to a start sequence on the chromosome is produced, and replication starts from there. The copies do not have the starter protein bound to them, and so are not themselves copied.
The difference is that the backwards-compatible chromosomes have an extra start-sequence that is different to the natural one. During mitosis, this is of no consequence. However, during meiosis, during DNA replication, another, different starter protein is produced that only binds to the extra different start sequence. This results in the two cells that result from the first meiotic division having four copies of the extra chromosomes rather than two. Then, during the second meiotic division, the paired chromosomes are split between the resultant cells. This results in the gametes being haploid with respect to the original chromosomes, but being diploid with respect to the new chromosomes.
So, whether the enhanced person's gamete is an egg or a sperm, on fertilisation with the gamete of an unenhanced person, the resultant zygote is fully diploid and enhanced, gaining both copies of the enhancement chromosome from their enhanced parent.
Now the trick is to not end up with extra copies of the enhancement chromosome when both parents are enhanced. This may be achieved by signal proteins on the surface of the gametes. When fertilisation occurs, if both egg and sperm are carrying the enhancement chromosome, they each have a male or female specific marker protein on their surface. If the sperm detects the female marker or the egg detects the male marker, a process similar to X-inactivation occurs to one of the enhancement chromosomes within that gamete, rendering that chromosome inactive. However, unlike X-inactivation, the inactivated enhancement chromosomes are destroyed shortly after fertilisation.
In order to transmit an enhancement chromosome only when reproducing with a similarly enhanced partner, meiosis is left unchanged, so that the resultant gametes are fully haploid. If the other gamete does not have the requisite marker, the enhancement chromosome is inactivated and destroyed. When both gametes have the enhancement chromosomes, the zygote should be properly diploid for all chromosomes.
When the enhancement chromosome must be transmitted only in the presence or absence of a particular environmental marker, that chromosome has only an alternate start sequence, and during meiosis, the alternate starter protein is produced only in the presence or absence of the marker, and the enhancement chromosome is destroyed in the absence of the alternate starter protein, resulting in unenhanced gametes. The alternate starter protein is always produced during mitosis.
So... is this feasible or would it have problems? Could it be improved?