A man has a heritable disease, which his father suffers from, along with his son. His father’s symptoms are comparatively light, where his sons are profound and seen at a much earlier age. What is a good explanation for the mechanism that is causing this inheritance pattern?

• trinucleotide repeats are added every generation to a region of DNA
• each successive generation gains a new mutation in a different loci on the chromosomes
• the mothers do not have the diseases badly because they are females, and are protected from X linked recessive phenotypes because of an XX karyotype
• DNA that has gained a mutation is much easier to mutate each successive generation