Copy of a Copy of a Copy

I write a lot about reproduction, so my way into this book, as with so many things these days, was sperm. Every day, men produce millions of new sperm, derived from stem cells, which have to replenish themselves through cell division. That involves copying the entire genome: the body’s complete set of instructions, which is about 3 billion “letters” long. With each replication, there is a small but real chance of a copying error, known in the genetics world as a “mutation.” The older the dad, the more copying and the more mutations. Children born to older fathers are more likely to have health concerns such as schizophrenia or leukemia.

I did not know much about mutations when I heard about Roxanne Khamsi’s Beyond Inheritance, but I knew enough to know I wanted to know more. And there is so much to learn: from cancer to immunity to the heart, the effect of mutations on our health is profound.

When we think about genes and what they mean for our...