Huntington's disease is a dominant trait, which means that the individual must carry only one dominant allele to express it.
Let "H" represent the allele for Huntington's disease and "h" the normal allele.
If one of the parents has two alleles for Huntington's disease, then the genotype will be HH (this parent carries both dominant alleles for the disease)
The second parent is normal, since this disease is dominant, to be healthy the person must have two copies of the normal allele, so the genotype of this parent will be hh (homozygous recessive)
To determine the percentage of offspring that will have the disease you have to create a Punnet square to determine their genotypes:
All possible genotypes are carriers of the dominant allele "H", which means that 100% of the offspring will have the disease.
