She Has Her Mother's Laugh

The Early Science of Heredity

Heredity begins with the observation that children resemble their parents while remaining unique. This truth is the foundation of genetics, a field explaining the biological links between generations. Carl Zimmer examines how these traits shape identity, tracing the path from early curiosity to modern medical breakthroughs. Understanding these connections is essential to knowing what it means to be human. The history of heredity is not only a story of progress; while it has led to life-saving discoveries, it has also been used to support harmful beliefs. Concepts of inheritance once fueled eugenics and racial ideologies that caused significant suffering. These misuses show how biology can be weaponized when mixed with prejudice, turning a search for knowledge into a tool for exclusion.

Modern research provides a deeper look into how humans change. It explains why recent generations are often taller and score higher on intelligence tests than those before them. It also reveals facts about ancient ancestors, such as the persistence of Neanderthal DNA in modern people. As genetic technology evolves, it offers the power to influence the future. The ability to understand and potentially change the traits passed down brings both hope and ethical challenges. This field is moving beyond biology to impact every part of society, including politics and how people view their own family history.

The transition of heredity from a legal concept to a biological science began in the fields and pastures of the eighteenth and nineteenth centuries. In the mid-1700s, English farmer Robert Bakewell revolutionized animal husbandry by practicing "in-and-in breeding" to fix desirable traits in his sheep, proving that heredity could be steered toward specific goals. This spirit of inquiry spread to plant breeding, where researchers like Thomas Andrew Knight in England and a society of breeders in Moravia sought to uncover the "genetic rules of nature." This intellectual environment provided the foundation for Gregor Mendel, an Augustinian friar who, through massive experiments with pea plants, discovered that inheritance followed predictable mathematical ratios. He proposed that every plant carried a pair of antagonistic elements—what we now call genes—and that some were dominant while others were recessive.

While Mendel worked in obscurity, Charles Darwin grappled with the same mysteries for his theory of evolution. He proposed a hypothesis called pangenesis, imagining that tiny particles called gemmules collected in reproductive organs, allowing for the inheritance of acquired traits. However, Darwin’s cousin, Francis Galton, and German biologist August Weismann dismantled this idea. Galton’s experiments with rabbit blood transfusions failed to show any transfer of traits, while Weismann’s germ-line theory proposed a fundamental barrier between reproductive cells (sperm and eggs) and the rest of the body’s somatic cells. This meant changes to the body during a lifetime could not be passed on. By the turn of the twentieth century, the rediscovery of Mendel’s work sparked a revolution, leading to the birth of the field of genetics. American horticulturalist Luther Burbank, the "wizard of Santa Rosa," bridged the old world of intuitive breeding and the new science, creating thousands of new plant varieties like the Burbank potato, even as he clung to outdated theories.