She Has Her Mother's Laugh: What Heredity Is, Is Not, and May Become

by Carl Zimmer

Scientists began to compare the human genome to the genomes of other species, in order to learn on a molecular level how we evolved from common ancestors. They could examine the twenty thousand–odd genes that encode human proteins, one at a time, to learn about how they helped make a human and how they helped make us sick.

What did I really know about the people who had come before me? I could barely remember their names. How could I know anything about what I had inherited from them?

Women have two X chromosomes, she explained, and men have one X and one Y. A woman with a fragile X mutation on one copy of her X chromosome will be healthy, because her other X chromosome can compensate. Men have no backup. If I carried the mutation, it would have been obvious from when I was a baby.

Heredity is why we’re like our ancestors. Heredity is the inheritance of a gift, or of a curse. Heredity defines us through our biological past. It also gives us a chance at immortality by extending heredity into the future.

Each of us carries an amalgam of fragments of DNA, stitched together from some of our many ancestors. Each piece has its own ancestry, traveling a different path back through human history. A particular fragment may sometimes be cause for worry, but most of our DNA influences who were are—our appearance, our height, our penchants—in inconceivably subtle ways.

We’ve come to define heredity purely as the genes that parents pass down to their children. But heredity continues within us, as a single cell gives rise to a pedigree of trillions of cells that make up our entire bodies.

Father and son alike had lower jaws that jutted far forward, leaving their mouths to hang open. Their shared look was so distinctive that anatomists later named it after their dynasty: the Habsburg jaw.

A long head was a sign of nobility for the tribe, prompting parents to squeeze the skulls of newborns and wrap them in bandages. “Custom originally so acted that through force such a nature came into being,” Hippocrates said. Eventually, Longhead babies came into the world with their heads already stretched out.

Europeans developed a new explanation for the link between one generation and the next: They were joined by blood. Even today, Westerners still use the word blood to talk about kinship, as if the two were equivalent in some obvious way.

The label of race emerged around the time that Europeans began colonizing other parts of the world. They discovered more people to whom they could attach the label.

He hated doctors, like his father and grandfather before him. “My antipathy against their art is hereditary,” he said. Montaigne wondered if such an inclination could be inherited, along with diseases and physical traits. But how all of that could be carried from one generation to another in a seed, Montaigne could not begin to imagine.

Mercado believed it was necessary to understand how new lives develop. He argued that each part of the body—a hand, the heart, an eye—had its own distinctive shape, its own balance of humors, and its own particular function. In the bloodstream, Mercado claimed, the humors from each part of the body mixed together, and a mysterious formative power shaped them into seeds.

Mercado believed that this cycle of generation, combination, and development was well shielded from the outside world. The willy-nilly waves of chance could not reach the hidden seeds of human life and alter their hereditary traits.

But for the most part, a doctor could do little, because the stamp of heredity was sealed away from a physician’s reach.

Mercado urged instead that people with the same defect not marry, because their children would be at greatest risk of developing the same hereditary disease. All people should seek out a spouse as different from themselves in as many individual characteristics as possible.

“Such a knowledge of Nature and such ability to handle plant life would only be possible to an innately high genius,”

“to know, not second-hand, but first-hand, from Nature herself, what the rules of this exciting game of Life were.”