Most humans have 46 chromosomes in almost every cell of their body.

Chromosomes are long strands of DNA wrapped tightly around proteins. This DNA acts like an instruction manual for building and operating your body. In 1956, scientists Joe Hin Tjio and Albert Levan discovered the correct count of 46 chromosomes. Before this, experts thought humans had 48.The first 22 pairs of chromosomes are called autosomes. The 23rd pair determines a person's biological sex. Females usually have two X chromosomes, while males usually have one X and one Y chromosome. The Y chromosome is the smallest and carries the fewest instructions.If you stretched out the DNA from just one cell, it would be about 2 meters long. To fit inside a tiny cell, the DNA must coil up into these chromosome shapes. Most cells constantly divide to make new cells. During this process, the 46 chromosomes copy themselves so the new cell is identical.Sometimes a cell ends up with the wrong number of chromosomes. Having an extra copy of chromosome 21 results in Down syndrome. Another condition called Trisomy 18 happens when there are three copies of chromosome 18. These differences change how a person's body develops and grows.