When rediscovered at the turn of the century, Mendel's laws were found to be applicable to humans, but from the beginning they were fraught with problems. Sex-linked traits and linked genes defied Mendel's rules. Later, other exceptions were found, including sporadic cases, non-penetrance, variable expressivity, and preferential parental transmission.
In this book, Harry Ostrer observes that some of these problems can be explained by incomplete ascertainment, typing errors and modifying genes. He then goes on to systematically explore the evidence for a number of newer genetic processes that were not foreseen by Mendel and his intellectual heirs, examining the molecular basis for these processes and their effects on transmission and phenotype. He shows that these non-Mendelian processes--gonadal and somatic mosaicism, sex-linked inheritance, mitochondrial transmission, genomic imprinting, accelerated rates of mutation, and viral infection--resolve many of the exceptions to Mendelian inheritance. He also provides a complete review of Mendelian genetics, as well as an overview of the structure and functions of genes, chromosomes, and their products. Thus the book presents a holistic view of human genetics.
In the last chapter, Ostrer grapples with the possibilities for identifying new genetic processes, and with genetic determinism--the view that a person's phenotype is fully subject to his or her genetic constitution. He contends that despite the large number of genetic combinations, phenotypes cannot be predicted precisely, even with sufficient computing power. Genetic processes are frequently modified by environmental exposure or they may be random or stochastic in theiroccurrence. Hence, there are innate limits to genetic determinism. Although prediction of phenotype based on genotype will improve in the future as all of the human genes are identified, such predictions will always remain imprecise.

Who are the Jews-a race, a people, a religious group? For over a century, non-Jews and Jews alike have tried to identify who they were-first applying the methods of physical anthropology and more recently of population genetics. In Legacy, Harry Ostrer, a medical geneticist and authority on the genetics of the Jewish people, explores not only the history of these efforts, but also the insights that genetics has provided about the histories of contemporary Jewish people. Much of the book is told through the lives of scientific pioneers. We meet Russian immigrant Maurice Fishberg; Australian Joseph Jacobs, the leading Jewish anthropologist in fin-de-siecle Europe; Chaim Sheba, a colorful Israeli geneticist and surgeon general of the Israeli Army; and Arthur Mourant, one of the foremost cataloguers of blood groups in the 20th century. As Ostrer describes their work and the work of others, he shows that to look over the genetics of Jewish groups, and to see the history of the Diaspora woven there, is truly a marvel. Here is what happened as the Jews migrated to new places and saw their numbers wax and wane, as they gained and lost adherents and thrived or were buffeted by famine, disease, wars, and persecution. Many of these groups-from North Africa, the Middle East, India-are little-known, and by telling their stories, Ostrer brings them to the forefront at a time when assimilation is literally changing the face of world Jewry. A fascinating blend of history, science, and biography, Legacy offers readers an entirely fresh perspective on the Jewish people and their history. It is as well a cutting-edge portrait of population genetics, a field which may soon take its place as a pillar of group identity alongside shared spirituality, shared social values, and a shared cultural legacy.