What are the chances that I have Huntingtons disease? (genetic disease)?

You have a 75% chance of having Huntington's disease. If your brother has tested negative for Huntington's disease, then your mother was a heterozygous carrier for the allele. This means that the children of your mother and father could be 25% HH, 50% Hh, and 25% hh. Your parents are heterozygous carriers of Huntington's disease since your brother does not have it, it is possible to have the HH genotype, and it is a dominant trait. Your brother must have the hh recessive genotype.