a. Yes, human pedigrees are inherited as autosomal dominant disorder in Huntington's disease. The phenotype appears in every generation of the pedigree, fathers and mothers transmit the disease to their sons and daughters. Because of the late onset of the disease, the affected individuals could not be shown in generation IV. Thus, it is not shown and generation III develop the disease later.
b. Susan is in the fifth generation. Her great grandmother whose mother had the disease is about 75 years old now is not having the disease. As this is autosomal dominant disorder, 100% of the people who have this allele will develop the disease. So, there is no chance of Susan having the disease.
Alan's great grandfather died of Huntington's disease. His father is now almost 50 years old and has not developed it yet. According to the graph in figure 2-24, 20% of people with that allele develop the disease by the age of 50. The probability of his grandfather receiving allele from his father is 1/2. So, the chances, for his grandfather to get the disease are 10%.
The probability of Alan's father getting the disease from his father is 1/2.
The probability of Alan getting the disease from his father is 1/2. So, the probability of Alan getting the disease is 1/10 X 1/2X1/2=1/40.
If his grandfather has developed that disease, then the probability of John having the disease is 1/2x1/2 = 1/4.