JACOB’S LADDER

A detailed history, not so much of the genome as of genetics itself.

Nature science writer Gee begins with a description of how a fertilized human egg develops into a person. As everyone has observed, that person will resemble both parents in various ways, but how are those traits inherited? Aristotle thought that the fetus was made largely out of menstrual blood; others thought that semen carried the entire reproductive package. The details of reproduction weren’t sorted out until the 17th century, when the newly invented microscope revealed the nature of sperm. Even two centuries later, a serious obstacle to the acceptance of Darwin’s theories was scientists’ incomplete understanding of just how heredity worked. Mendel’s work, which sorted out of the fundamentals of genetics, provided the missing link, and Thomas Hunt Morgan’s meticulous study of fruit flies proved that genes were more than just a metaphor. The discovery of DNA’s structure by Watson and Crick made the exact mechanism of inheritance clear once and for all. What happened next is not as well known, and here Gee comes into his own. He clearly and readably explains the “triplet code” by which DNA bases dictate the synthesis of enzymes, the key role of interactions between genes, and the presence of large amounts of “junk DNA” in the genome. He also deals with the evolutionary origins of various features of the human genome, which is within a percentage point of being indistinguishable from that of a mouse. Gee concludes with a look at the questions raised by gene therapy, cloning, and other techniques just coming over the horizon of practical application. Stunning progress, and yet the question of just what in the tangled double helix makes us human remains unanswered. The author hopes we find that answer before we go too far down unknown roads.

Well written and illuminating.