Huntington’s disease is an inherited disorder that affects the nervous system. It is caused by a dominant allele. This means it can be passed on by just one parent if they have the disorder. The genetic diagram shows how this can happen.
Note that if you are doing the Foundation Tier paper you are expected to be able to interpret genetic diagrams. If you are doing the Higher Tier paper, you are expected to be able to draw genetic diagrams for any combination of dominant and recessive alleles.