gene
A gene is a short stretch of DNA that carries the instructions for one inherited feature — like the recipe for a single working part of you, often a particular protein. It is the basic unit of heredity: the thing a parent passes to a child that helps shape how that child turns out.
Think of your DNA as a very long cookbook. A gene is one recipe in it — say, the one for the pigment that colors your eyes. You inherit one copy from each parent, and together they help decide the result.
Remarkably, Gregor Mendel worked out that such units must exist in the 1860s, long before anyone knew DNA was the cookbook they were written in. He saw the pattern in his pea plants first; the molecule came much later.
One gene doesn't always equal one trait. Many traits (like height) are shaped by hundreds of genes working together, and a single gene can influence several things at once.