How is alzheimer's inherited?

Depends. Early onset illness is inherited at a high rate but the more common late onset alzheimers has a number of genes taht increase risk a small amount each.
Not yet known. Many details about the genetics of ad are known as colleagues indicate, it remains an area of research and competing hypotheses (see http://www.Alzheimersblog.Org/tag/genetic/?Gclid=clddgfsv5lucfy07ogodkhmazw). Like most human conditions many genes appear to influence ad. This yields a "risk" model, where certain conditions elevate the likelihood of offspring having ad without guaranteeing it.
Autosomal Dom. In 2-5 % of cases there is a familial autosomal dominant pattern, associated with three more common mutations. Amyoid precursor protein, presenilin i, and presenilin ii. The disease presents often in the 30's and 40's in these groupsl.