If the mutation is not in the promoter, then both alleles should be expressed at the same level. Since it is an autosomal dominant disorder, expression of the mutant allele causes disease even though the normal allele is also expressed.

Dear Bhairavi,

It is not always true that only the mutant protein is synthesized in heterogeneous genotypes. I think it is the ability of the mutant protein to exert its adverse effects irrespective of the presence of the wild type protein. And such a behavior would make it dominant. For example, in prion diseases, which are primarily caused by misfolding and aggregation of the prion protein, autosomal dominant mutations increase the misfolding of the protein. In some cases, only the mutant protein misfolds while in another cases the mutant protein assists misfolding of the wild type protein as well. But in all cases, both the wild type and mutant proteins are expressed.

In cases, where only mutant protein is synthesized, I think there should be a feedback control driven by the mutant protein which inhibits synthesis of the wild type protein. But this is just a guess.  I would like to know if some mechanism is known for this.

Best,

Jogender

Important point to note here is about dominant negative mutations where the mutant protein impars the functioning of normal protein..

Autosomal dominant disorders means that the activity/function of the mutant protein dominates that of the wt protein, however, both proteins could still be synthesized.

Murphy's law?

Perhaps there is a correlation, but one reason you are observing the mutant is because it is not behaving as it should and therefore your attention is focused on it.  There are a lot of variations in the genome that are not deleterious even though they can lead to different amino acid sequences.  It is when the mutation is in a hot spot that the protein gives trouble, and these are typically in only a few places.  Mutations in inconsequential locations don't destroy the function of the protein, and, due to a lot of redundancies, many locations are relatively inconsequential.  Perhaps it might be better to rephrase the question as "why don't we see _more_ mutant proteins expressed"?