You may have heard of Down syndrome, but what is it exactly?

It’s named after John Langdon Down, a British doctor who first described the condition way back in 1866. Still, despite all his research, he couldn’t figure out what caused it. In fact, it took almost 100 years for scientists to figure out what led to the condition. Turns out, it has to do with chromosomes.

Chromosomes contain the instructions — the DNA — your body uses to build itself. They are found in practically every cell in the body, and the body has trillions of cells. Most people have 46 chromosomes — 23 from the mother and a matching 23 from the father. People with Down syndrome have a third copy of one of these chromosomes giving them a total of 47.

Having those extra instructions leads to many of the characteristics we typically see in people with Down syndrome, like a certain eye shape or a larger space between their first or second toes. It’s also common for people with Down syndrome, or DS, to have heart issues and challenges with memory and learning. This can sometimes slow their development.

It’s important to remember having Down syndrome doesn’t define who a person is. It’s a characteristic, like having brown hair or being tall. Everyone with DS has his or her own hopes, fears, goals and dreams. Sometimes they need extra time and help to do something, but given the right support, people with Down syndrome can achieve big things.

Listen to our episode to learn more about how chromosomes work. Plus, you’ll meet ranchers with Down syndrome who look after cows, horses and chickens and you’ll get a pep talk from a young woman with Down syndrome whose job it is to help people with disabilities find work.

Plus: Our Moment of Um looks at why eggs go from clear to white when cooked.