I think that you have confused mutation with gene in your initial assertion.

There is probably only a few percent of variation between all of your 2^100 ancestors genomes (the majority of whom will be related), so protein coding regions of your genome and your MRCA are actually likely to be very similar.

Now if that MRCA had a specific mutation, the chances of your inheriting that specific mutation are probably low, but depend on that mutations prevalence in the population as well as whether the mutation is deleterious or not.

100 generations is very few in terms of changes within the genetic make up of a population under normal circumstances.