In A Nutshell
You have heard that your nose is just like your dad's, but your eyes are like your mom's. And, you're probably tired of your grandma saying how much you remind her of her brother when you smile. But, why do they keep saying these things? It all comes down to heredity: kind of like an inheritance, but rather than money or land, you inherit genes. You share 50% of your genes with your dad, 50% with your mom, and up to 12.5% with your grandmother's brother. Yet, you are a unique human being, unlike any other in the world. Unless you have an identical twin sibling—and even identical twins are rarely truly identical—the chances of someone else turning up the same way you did are practically 0.
A large part of how an organism looks, functions, and behaves is determined by its genotype, a sort of genetic "blueprint," much like the ones you would have for a building. This collection of genes, in conjunction with the environment, determines everything about all living organisms. Specific rules determine how heredity works, and these principles are the basis of evolutionary theory, the development of transgenic crops, and, of course, lots of the forensic science you love on CSI.