A chromosome is a tightly coiled package of DNA found inside the nucleus of almost every cell in your body. Think of DNA as an instruction manual for building and running you, and chromosomes as the way that manual gets organized and stored.

Humans have 46 chromosomes in most of their cells, arranged in 23 pairs. You inherited one chromosome in each pair from your mother and one from your father. These chromosomes contain genetic information that influences things like your eye color, hair texture, how tall you might grow, and thousands of other traits.

Under a microscope, chromosomes can look like tiny X-shaped structures, but they only take this shape when a cell is dividing. The rest of the time, they're unwound and spread throughout the nucleus like tangled thread.

Different species have different numbers of chromosomes: dogs have 78, cats have 38, and fruit flies have just 8. The discovery of chromosomes in the late 1800s helped scientists understand how traits pass from parents to children. Today, doctors can examine chromosomes to diagnose certain medical conditions, and scientists study them to understand heredity, evolution, and what makes each living thing unique.