Well reliable, never mind slightly weird way of putting the question, and having considerable bioethic aspects is to start with this article on Huntington's disease aka Huntington's Chorea
https://en.wikipedia.org/wiki/Huntington's_disease#Genetic_mutation There are other diseases of similar causation, but this is the most common and most researched.
When it arises ex novo it is due to the replication machinery having made as some kind of a slip and duplicating a triplet. One or more copies of one amino acid are inserted into the sequence. Unfortunately once this has happened it is more likely to happen again. That is why you see this complex inheritance at the phenotypic level, with effects worsening down generations.
Maybe it is not too trivial to say at the start that if you find it, it means that having it is compatible with life, though not good health. The disease's relatively late onset, opposite too many other genetic diseases, is relevant to bioethics.
Note that this example is a single amino acid residue repeat. (I don't know whether there are examples of longer repeats that theoretically should exist - they are less likely to give functional proteins.)
A key word to use in searches is 'tandem repeats'.
You will certainly need for bioethics to be pretty conversant with tandem repeats that happen in DNA outside coding regions, so not affecting any proteins or phenotype. These are of variable length which has made them very useful, among other things, associating the DNA to individuals for forensic purposes, and also ancestry tracing.
https://en.wikipedia.org/wiki/Variable_number_tandem_repeat
If you have any conceptual difficulties in your further reading come back and hopefully someone here can help you.
