Now that we have decoded the human genome, why don't we improve it?

The question is at present theoretical but could well emerge as the hardest of all bioethical issues. Biologists routinely alter the genes of mice, with methods that are not yet acceptable for making inheritable changes in people, but one day genetic engineers may figure out how to apply safe patches to the human biological software.

Everyone would like to have children who are healthy, beautiful and gifted. But people vary widely in all these qualities, depending on their parents' genes, and the pure luck of the draw at conception when each child gets allotted a random selection of half the parental gene pool.

Most human genes exist in several different versions in the population: some of them are great to have, some so-so and some downright deleterious.

This month the Icelandic company Decode Genetics found three quite common versions of a gene called BMP-2, each of which considerably increases its owner's risk of osteoporosis and bone fracture.

Suppose it were possible to delete any bad version of...