Gaucher disease is caused by a chronic enzyme deficiency that is more common among Ashkenazi Jews than in the general population. A Jewish man has a sister afflicted with the disease. His parents and grandparents, and other three siblings are not affected. Discussions with his relatives in his wife's family reveal that the disease is not likely to be present in his wife's family, although some relatives recall that the brother of his wife's paternal grandmother suffered from a very similar disease. Diagram and analyze the relevant pedigree to determine

a. The genetic basis for inheriting this trait

b. The highest probability that if this couple has a child, the child will be affected