Final answer:
The chances of getting Huntington's disease if both of your parents have it and are heterozygous is 75%. This includes a 25% chance of being homozygous affected and a 50% chance of being heterozygous affected.
Step-by-step explanation:
If both of your parents have Huntington's disease, which is an autosomal dominant disorder, the chances of inheriting the disease are higher than if only one parent has it. Each parent has a 50% chance of passing the affected gene to their offspring, so there are three possible scenarios. If both parents are heterozygous (Hh), then the theoretical possibility is that there is a 25% chance of an offspring being unaffected (hh), a 50% chance of being heterozygous like the parents (Hh), and a 25% chance of being homozygous (HH). If one parent is homozygous (HH) and the other is heterozygous (Hh), then all offspring will have at least one affected gene; thus, the chance of an afflicted child is 100%. If both parents are homozygous (HH), which is rare due to the lethal nature of the disease before reproductive age, all children will inherit the disease.
However, assuming both parents are heterozygous, we must consider each allele combination separately. Rather than add up the chances, we should use the multiplicative rule for independent events. Two heterozygous parents produce offspring with a genotype ratio of 1:2:1 (HH: Hh: hh). The probability of an offspring inheriting the affected gene from both heterozygous parents is 25% to be homozygous affected (HH), 50% to be heterozygous affected (Hh), and 25% to be homozygous unaffected (hh). Thus, the child has a 75% chance of having the disease (homozygous affected or heterozygous affected) and a 25% chance of being unaffected. This results in option 3) 75% being the correct response to the student's question.