Introduction

Why does an elephant always give birth to a baby elephant, never anything else? Why does a mango seed only ever grow into a mango tree? And given that offspring resemble their parents, why aren't they identical — why do siblings sometimes look strikingly alike, and sometimes strikingly different?

These questions belong to genetics, the branch of biology that studies both inheritance (how traits pass from parents to offspring) and variation (how offspring differ from their parents). Long before genetics existed as a science, people already sensed that sexual reproduction was somehow the source of variation — as far back as 8000–1000 BCE, farmers and breeders were exploiting naturally occurring variation to selectively breed plants and animals for desirable traits. India's own Sahiwal cattle, bred from wild ancestral cows in Punjab, is a well-known example of this kind of artificial selection. What our ancestors lacked wasn't the practice — it was any scientific understanding of why it worked.