The human genome is a complete set of genetic instructions. It contains about three billion base pairs of DNA, organized into 23 pairs of chromosomes. Within that sequence are roughly 20,000 protein-coding genes, plus regulatory elements, non-coding RNAs, and vast stretches of DNA whose functions are still being studied. The genome is the blueprint for a human being, but it is not a simple one.

The genome was sequenced by the Human Genome Project, an international effort that began in 1990 and was declared complete in 2003. It cost about $2.7 billion. Today, a human genome can be sequenced for under $1,000 in a day. That drop in cost has transformed medicine and biology. Genomes are now sequenced for diagnosing rare diseases, guiding cancer treatment, tracking outbreaks, and studying human history.

The genome is not static. It accumulates mutations over time. Some are inherited. Others arise during life. Structural variations, like copy number changes and inversions, are common. The genome also interacts with the environment. Gene expression changes in response to diet, stress, and toxins. The sequence is fixed. The activity is not.

Genomes vary between individuals. On average, any two humans differ at about one in 1,000 base pairs. Those differences contribute to traits, disease risk, and drug response. Understanding them is the goal of genomics.