Why Do Humans Have Such Different Blood Types?

Imagine that your body has a special code, like a secret message, written on the outside of your red blood cells. This code can be A, B, AB, or O. Depending on which code you have, others might not be able to help you if you need a blood transfusion. It’s kind of like having different types of keys that only fit certain locks. Some people are born with the same code as their parents; others get new combinations from both sides. This is why humans have such different blood types, it's all part of how our genes work together.